diff --git a/src/generators/Generator.ts b/src/generators/Generator.ts
index 002e52bbf9..7798cf3c86 100644
--- a/src/generators/Generator.ts
+++ b/src/generators/Generator.ts
@@ -125,7 +125,8 @@ export default class Generator {
 	) {
 		this.addSourcemapLocations(expression);
 
-		const usedContexts: string[] = [];
+		const usedContexts = new Set();
+		const usedIndexes = new Set();
 
 		const { code, helpers } = this;
 		const { contexts, indexes } = block;
@@ -168,12 +169,13 @@ export default class Generator {
 							);
 						}
 
-						if (!~usedContexts.indexOf(name)) usedContexts.push(name);
+						usedContexts.add(name);
 					} else if (helpers.has(name)) {
 						code.prependRight(node.start, `${self.alias('template')}.helpers.`);
 					} else if (indexes.has(name)) {
 						const context = indexes.get(name);
-						if (!~usedContexts.indexOf(context)) usedContexts.push(context);
+						usedContexts.add(context); // TODO is this right?
+						usedIndexes.add(name);
 					} else {
 						// handle shorthand properties
 						if (parent && parent.type === 'Property' && parent.shorthand) {
@@ -193,7 +195,7 @@ export default class Generator {
 							code.prependRight(node.start, `state.`);
 						}
 
-						if (!~usedContexts.indexOf('state')) usedContexts.push('state');
+						usedContexts.add('state');
 					}
 
 					this.skip();
@@ -221,6 +223,7 @@ export default class Generator {
 		return {
 			dependencies: Array.from(dependencies),
 			contexts: usedContexts,
+			indexes: usedIndexes,
 			snippet: `[✂${expression.start}-${expression.end}✂]`,
 		};
 	}
diff --git a/src/generators/dom/Block.ts b/src/generators/dom/Block.ts
index f851727276..0cfce0924a 100644
--- a/src/generators/dom/Block.ts
+++ b/src/generators/dom/Block.ts
@@ -12,6 +12,7 @@ export interface BlockOptions {
 	key?: string;
 	contexts?: Map<string, string>;
 	indexes?: Map<string, string>;
+	changeableIndexes?: Map<string, boolean>;
 	contextDependencies?: Map<string, string[]>;
 	params?: string[];
 	indexNames?: Map<string, string>;
@@ -32,6 +33,7 @@ export default class Block {
 
 	contexts: Map<string, string>;
 	indexes: Map<string, string>;
+	changeableIndexes: Map<string, boolean>;
 	contextDependencies: Map<string, string[]>;
 	dependencies: Set<string>;
 	params: string[];
@@ -77,6 +79,7 @@ export default class Block {
 
 		this.contexts = options.contexts;
 		this.indexes = options.indexes;
+		this.changeableIndexes = options.changeableIndexes;
 		this.contextDependencies = options.contextDependencies;
 		this.dependencies = new Set();
 
@@ -121,8 +124,7 @@ export default class Block {
 		name: string,
 		renderStatement: string,
 		claimStatement: string,
-		parentNode: string,
-		needsIdentifier = false
+		parentNode: string
 	) {
 		const isToplevel = !parentNode;
 
@@ -195,7 +197,7 @@ export default class Block {
 		let outroing;
 		const hasOutros = !this.builders.outro.isEmpty();
 		if (hasOutros) {
-			outroing = this.getUniqueName('outroing');
+			outroing = this.alias('outroing');
 			this.addVariable(outroing);
 		}
 
diff --git a/src/generators/dom/index.ts b/src/generators/dom/index.ts
index 2ffae2ee2f..2513623c49 100644
--- a/src/generators/dom/index.ts
+++ b/src/generators/dom/index.ts
@@ -74,10 +74,9 @@ export default function dom(
 	});
 
 	const builder = new CodeBuilder();
+	const computationBuilder = new CodeBuilder();
 
 	if (computations.length) {
-		const computationBuilder = new CodeBuilder();
-
 		computations.forEach(({ key, deps }) => {
 			if (generator.readonly.has(key)) {
 				// <:Window> bindings
@@ -88,48 +87,16 @@ export default function dom(
 
 			generator.readonly.add(key);
 
-			const condition = `isInitial || ${deps
-				.map(
-					dep =>
-						`( '${dep}' in newState && @differs( state.${dep}, oldState.${dep} ) )`
-				)
-				.join(' || ')}`;
-			const statement = `state.${key} = newState.${key} = @template.computed.${key}( ${deps
+			const condition = `isInitial || ${deps.map(dep => `changed.${dep}`).join(' || ')}`;
+
+			const statement = `if ( @differs( ( state.${key} = @template.computed.${key}( ${deps
 				.map(dep => `state.${dep}`)
-				.join(', ')} );`;
+				.join(', ')} ) ), oldState.${key} ) ) changed.${key} = true;`;
 
-			computationBuilder.addConditionalLine(condition, statement);
+			computationBuilder.addConditional(condition, statement);
 		});
-
-		builder.addBlock(deindent`
-			function @recompute ( state, newState, oldState, isInitial ) {
-				${computationBuilder}
-			}
-		`);
 	}
 
-	const _set = deindent`
-		${options.dev &&
-			deindent`
-			if ( typeof newState !== 'object' ) {
-				throw new Error( 'Component .set was called without an object of data key-values to update.' );
-			}
-
-			${Array.from(generator.readonly).map(
-				prop =>
-					`if ( '${prop}' in newState && !this._updatingReadonlyProperty ) throw new Error( "Cannot set read-only property '${prop}'" );`
-			)}
-		`}
-
-		var oldState = this._state;
-		this._state = @assign( {}, oldState, newState );
-		${computations.length &&
-			`@recompute( this._state, newState, oldState, false )`}
-		@dispatchObservers( this, this._observers.pre, newState, oldState );
-		${block.hasUpdateMethod && `this._fragment.update( newState, this._state );`}
-		@dispatchObservers( this, this._observers.post, newState, oldState );
-	`;
-
 	if (hasJs) {
 		builder.addBlock(`[✂${parsed.js.content.start}-${parsed.js.content.end}✂]`);
 	}
@@ -174,7 +141,7 @@ export default function dom(
 		? `@proto `
 		: deindent`
 		{
-			${['destroy', 'get', 'fire', 'observe', 'on', 'set', 'teardown']
+			${['destroy', 'get', 'fire', 'observe', 'on', 'set', '_set', 'teardown']
 				.map(n => `${n}: @${n === 'teardown' ? 'destroy' : n}`)
 				.join(',\n')}
 		}`;
@@ -190,7 +157,7 @@ export default function dom(
 				? `@assign( @template.data(), options.data )`
 				: `options.data || {}`};
 			${generator.metaBindings}
-			${computations.length && `@recompute( this._state, this._state, {}, true );`}
+			${computations.length && `this._recompute( {}, this._state, {}, true );`}
 			${options.dev &&
 				Array.from(generator.expectedProperties).map(
 					prop =>
@@ -259,9 +226,20 @@ export default function dom(
 
 		@assign( ${prototypeBase}, ${proto});
 
-		${name}.prototype._set = function _set ( newState ) {
-			${_set}
-		};
+		${options.dev && deindent`
+			${name}.prototype._checkReadOnly = function _checkReadOnly ( newState ) {
+				${Array.from(generator.readonly).map(
+					prop =>
+						`if ( '${prop}' in newState && !this._updatingReadonlyProperty ) throw new Error( "Cannot set read-only property '${prop}'" );`
+				)}
+			};
+		`}
+
+		${computations.length ? deindent`
+			${name}.prototype._recompute = function _recompute ( changed, state, oldState, isInitial ) {
+				${computationBuilder}
+			}
+		` : (!sharedPath && `${name}.prototype._recompute = @noop;`)}
 
 		${templateProperties.setup && `@template.setup( ${name} );`}
 	`);
diff --git a/src/generators/dom/preprocess.ts b/src/generators/dom/preprocess.ts
index 1811bc762f..9b784bc558 100644
--- a/src/generators/dom/preprocess.ts
+++ b/src/generators/dom/preprocess.ts
@@ -189,7 +189,10 @@ const preprocessors = {
 		contexts.set(node.context, context);
 
 		const indexes = new Map(block.indexes);
-		if (node.index) indexes.set(indexName, node.context);
+		if (node.index) indexes.set(node.index, node.context);
+
+		const changeableIndexes = new Map(block.changeableIndexes);
+		if (node.index) changeableIndexes.set(node.index, node.key);
 
 		const contextDependencies = new Map(block.contextDependencies);
 		contextDependencies.set(node.context, dependencies);
@@ -203,6 +206,7 @@ const preprocessors = {
 			contextDependencies,
 			contexts,
 			indexes,
+			changeableIndexes,
 
 			listName,
 			indexName,
@@ -274,6 +278,11 @@ const preprocessors = {
 						}
 					}
 				});
+			} else if (attribute.type === 'EventHandler' && attribute.expression) {
+				attribute.expression.arguments.forEach((arg: Node) => {
+					const dependencies = block.findDependencies(arg);
+					block.addDependencies(dependencies);
+				});
 			} else if (attribute.type === 'Binding') {
 				const dependencies = block.findDependencies(attribute.value);
 				block.addDependencies(dependencies);
@@ -444,6 +453,7 @@ export default function preprocess(
 
 		contexts: new Map(),
 		indexes: new Map(),
+		changeableIndexes: new Map(),
 		contextDependencies: new Map(),
 
 		params: ['state'],
@@ -462,7 +472,7 @@ export default function preprocess(
 
 	generator.blocks.push(block);
 	preprocessChildren(generator, block, state, node, false, [], true, null);
-	block.hasUpdateMethod = block.dependencies.size > 0;
+	block.hasUpdateMethod = true;
 
 	return { block, state };
 }
diff --git a/src/generators/dom/visitors/Component/Binding.ts b/src/generators/dom/visitors/Component/Binding.ts
index e87ab0bff1..75cdd637e3 100644
--- a/src/generators/dom/visitors/Component/Binding.ts
+++ b/src/generators/dom/visitors/Component/Binding.ts
@@ -89,7 +89,7 @@ export default function visitBinding(
 
 	local.update.addBlock(deindent`
 		if ( !${updating} && ${dependencies
-		.map(dependency => `'${dependency}' in changed`)
+		.map(dependency => `changed.${dependency}`)
 		.join(' || ')} ) {
 			${updating} = true;
 			${local.name}._set({ ${attribute.name}: ${snippet} });
diff --git a/src/generators/dom/visitors/Component/Component.ts b/src/generators/dom/visitors/Component/Component.ts
index a9fff4ee39..0e966cdc0f 100644
--- a/src/generators/dom/visitors/Component/Component.ts
+++ b/src/generators/dom/visitors/Component/Component.ts
@@ -195,10 +195,8 @@ export default function visitComponent(
 			if (attribute.dependencies.length) {
 				return deindent`
 					if ( ${attribute.dependencies
-						.map(dependency => `'${dependency}' in changed`)
-						.join(
-							'||'
-						)} ) ${name}_changes.${attribute.name} = ${attribute.value};
+						.map(dependency => `changed.${dependency}`)
+						.join(' || ')} ) ${name}_changes.${attribute.name} = ${attribute.value};
 				`;
 			}
 
@@ -212,7 +210,7 @@ export default function visitComponent(
 
 			${updates.join('\n')}
 
-			if ( Object.keys( ${name}_changes ).length ) ${name}._set( ${name}_changes );
+			${name}._set( ${name}_changes );
 		`);
 	}
 
diff --git a/src/generators/dom/visitors/EachBlock.ts b/src/generators/dom/visitors/EachBlock.ts
index 2ad8117c36..1c8a9c9346 100644
--- a/src/generators/dom/visitors/EachBlock.ts
+++ b/src/generators/dom/visitors/EachBlock.ts
@@ -57,8 +57,7 @@ export default function visitEachBlock(
 			anchor,
 			`@createComment()`,
 			`@createComment()`,
-			state.parentNode,
-			true
+			state.parentNode
 		);
 	} else if (node.next) {
 		node.next.usedAsAnchor = true;
@@ -172,8 +171,7 @@ function keyed(
 			node._block.first,
 			`@createComment()`,
 			`@createComment()`,
-			null,
-			true
+			null
 		);
 	}
 
@@ -411,7 +409,7 @@ function unkeyed(
 
 	// TODO do this for keyed blocks as well
 	const condition = Array.from(allDependencies)
-		.map(dependency => `'${dependency}' in changed`)
+		.map(dependency => `changed.${dependency}`)
 		.join(' || ');
 
 	const parentNode = state.parentNode || `${anchor}.parentNode`;
diff --git a/src/generators/dom/visitors/Element/Attribute.ts b/src/generators/dom/visitors/Element/Attribute.ts
index b03c379482..5ee1afdea7 100644
--- a/src/generators/dom/visitors/Element/Attribute.ts
+++ b/src/generators/dom/visitors/Element/Attribute.ts
@@ -47,10 +47,28 @@ export default function visitAttribute(
 	if (isDynamic) {
 		let value;
 
+		const allDependencies = new Set();
+		let shouldCache;
+		let hasChangeableIndex;
+
+		// TODO some of this code is repeated in Tag.ts — would be good to
+		// DRY it out if that's possible without introducing crazy indirection
 		if (attribute.value.length === 1) {
 			// single {{tag}} — may be a non-string
-			const { snippet } = block.contextualise(attribute.value[0].expression);
+			const { expression } = attribute.value[0];
+			const { snippet, dependencies, indexes } = block.contextualise(expression);
 			value = snippet;
+			dependencies.forEach(d => {
+				allDependencies.add(d);
+			});
+
+			hasChangeableIndex = Array.from(indexes).some(index => block.changeableIndexes.get(index));
+
+			shouldCache = (
+				expression.type !== 'Identifier' ||
+				block.contexts.has(expression.name) ||
+				hasChangeableIndex
+			);
 		} else {
 			// '{{foo}} {{bar}}' — treat as string concatenation
 			value =
@@ -60,22 +78,35 @@ export default function visitAttribute(
 						if (chunk.type === 'Text') {
 							return stringify(chunk.data);
 						} else {
-							const { snippet } = block.contextualise(chunk.expression);
+							const { snippet, dependencies, indexes } = block.contextualise(chunk.expression);
+
+							if (Array.from(indexes).some(index => block.changeableIndexes.get(index))) {
+								hasChangeableIndex = true;
+							}
+
+							dependencies.forEach(d => {
+								allDependencies.add(d);
+							});
+
 							return `( ${snippet} )`;
 						}
 					})
 					.join(' + ');
+
+			shouldCache = true;
 		}
 
-		const last = block.getUniqueName(
+		const isSelectValueAttribute =
+			name === 'value' && state.parentNodeName === 'select';
+
+		const last = (shouldCache || isSelectValueAttribute) && block.getUniqueName(
 			`${state.parentNode}_${name.replace(/[^a-zA-Z_$]/g, '_')}_value`
 		);
-		block.addVariable(last);
 
-		const isSelectValueAttribute =
-			name === 'value' && state.parentNodeName === 'select';
+		if (shouldCache || isSelectValueAttribute) block.addVariable(last);
 
 		let updater;
+		const init = shouldCache ? `${last} = ${value}` : value;
 
 		if (isSelectValueAttribute) {
 			// annoying special case
@@ -104,27 +135,42 @@ export default function visitAttribute(
 				}
 			`;
 
-			block.builders.hydrate.addLine(deindent`
-				${last} = ${value}
+			block.builders.hydrate.addBlock(deindent`
+				${last} = ${value};
 				${updater}
 			`);
+
+			block.builders.update.addLine(`${last} = ${value};`);
 		} else if (propertyName) {
 			block.builders.hydrate.addLine(
-				`${state.parentNode}.${propertyName} = ${last} = ${value};`
+				`${state.parentNode}.${propertyName} = ${init};`
 			);
-			updater = `${state.parentNode}.${propertyName} = ${last};`;
+			updater = `${state.parentNode}.${propertyName} = ${shouldCache || isSelectValueAttribute ? last : value};`;
 		} else {
 			block.builders.hydrate.addLine(
-				`${method}( ${state.parentNode}, '${name}', ${last} = ${value} );`
+				`${method}( ${state.parentNode}, '${name}', ${init} );`
 			);
-			updater = `${method}( ${state.parentNode}, '${name}', ${last} );`;
+			updater = `${method}( ${state.parentNode}, '${name}', ${shouldCache || isSelectValueAttribute ? last : value} );`;
 		}
 
-		block.builders.update.addBlock(deindent`
-			if ( ${last} !== ( ${last} = ${value} ) ) {
-				${updater}
-			}
-		`);
+		if (allDependencies.size || hasChangeableIndex || isSelectValueAttribute) {
+			const dependencies = Array.from(allDependencies);
+			const changedCheck = (
+				( block.hasOutroMethod ? `#outroing || ` : '' ) +
+				dependencies.map(dependency => `changed.${dependency}`).join(' || ')
+			);
+
+			const updateCachedValue = `${last} !== ( ${last} = ${value} )`;
+
+			const condition = shouldCache ?
+				( dependencies.length ? `( ${changedCheck} ) && ${updateCachedValue}` : updateCachedValue ) :
+				changedCheck;
+
+			block.builders.update.addConditional(
+				condition,
+				updater
+			);
+		}
 	} else {
 		const value = attribute.value === true
 			? 'true'
diff --git a/src/generators/dom/visitors/Element/meta/Window.ts b/src/generators/dom/visitors/Element/meta/Window.ts
index d6c848c342..8e15ee2512 100644
--- a/src/generators/dom/visitors/Element/meta/Window.ts
+++ b/src/generators/dom/visitors/Element/meta/Window.ts
@@ -38,8 +38,8 @@ export default function visitWindow(
 			let usesState = false;
 
 			attribute.expression.arguments.forEach((arg: Node) => {
-				const { contexts } = block.contextualise(arg, null, true);
-				if (contexts.length) usesState = true;
+				const { dependencies } = block.contextualise(arg, null, true);
+				if (dependencies.length) usesState = true;
 			});
 
 			const flattened = flattenReference(attribute.expression.callee);
diff --git a/src/generators/dom/visitors/IfBlock.ts b/src/generators/dom/visitors/IfBlock.ts
index dfb6b3bfcd..193edd872b 100644
--- a/src/generators/dom/visitors/IfBlock.ts
+++ b/src/generators/dom/visitors/IfBlock.ts
@@ -119,8 +119,7 @@ export default function visitIfBlock(
 			anchor,
 			`@createComment()`,
 			`@createComment()`,
-			state.parentNode,
-			true
+			state.parentNode
 		);
 	} else if (node.next) {
 		node.next.usedAsAnchor = true;
diff --git a/src/generators/dom/visitors/MustacheTag.ts b/src/generators/dom/visitors/MustacheTag.ts
index 3ef015a9fd..3c03c51167 100644
--- a/src/generators/dom/visitors/MustacheTag.ts
+++ b/src/generators/dom/visitors/MustacheTag.ts
@@ -1,4 +1,5 @@
 import deindent from '../../../utils/deindent';
+import visitTag from './shared/Tag';
 import { DomGenerator } from '../index';
 import Block from '../Block';
 import { Node } from '../../../interfaces';
@@ -10,25 +11,21 @@ export default function visitMustacheTag(
 	state: State,
 	node: Node
 ) {
-	const name = node._state.name;
-	const value = block.getUniqueName(`${name}_value`);
+	const { name } = node._state;
 
-	const { snippet } = block.contextualise(node.expression);
+	const { init } = visitTag(
+		generator,
+		block,
+		state,
+		node,
+		name,
+		value => `${name}.data = ${value};`
+	);
 
-	block.addVariable(value);
 	block.addElement(
 		name,
-		`@createText( ${value} = ${snippet} )`,
-		generator.hydratable
-			? `@claimText( ${state.parentNodes}, ${value} = ${snippet} )`
-			: '',
-		state.parentNode,
-		true
+		`@createText( ${init} )`,
+		`@claimText( ${state.parentNodes}, ${init} )`,
+		state.parentNode
 	);
-
-	block.builders.update.addBlock(deindent`
-		if ( ${value} !== ( ${value} = ${snippet} ) ) {
-			${name}.data = ${value};
-		}
-	`);
 }
diff --git a/src/generators/dom/visitors/RawMustacheTag.ts b/src/generators/dom/visitors/RawMustacheTag.ts
index 41c3af9aa4..02facfcd24 100644
--- a/src/generators/dom/visitors/RawMustacheTag.ts
+++ b/src/generators/dom/visitors/RawMustacheTag.ts
@@ -1,4 +1,6 @@
 import deindent from '../../../utils/deindent';
+import addUpdateBlock from './shared/addUpdateBlock';
+import visitTag from './shared/Tag';
 import { DomGenerator } from '../index';
 import Block from '../Block';
 import { Node } from '../../../interfaces';
@@ -12,12 +14,19 @@ export default function visitRawMustacheTag(
 ) {
 	const name = node._state.basename;
 	const before = node._state.name;
-	const value = block.getUniqueName(`${name}_value`);
 	const after = block.getUniqueName(`${name}_after`);
 
-	const { snippet } = block.contextualise(node.expression);
-
-	block.addVariable(value);
+	const { init } = visitTag(
+		generator,
+		block,
+		state,
+		node,
+		name,
+		value => deindent`
+			@detachBetween( ${before}, ${after} );
+			${before}.insertAdjacentHTML( 'afterend', ${value} );
+		`
+	);
 
 	// we would have used comments here, but the `insertAdjacentHTML` api only
 	// exists for `Element`s.
@@ -25,30 +34,16 @@ export default function visitRawMustacheTag(
 		before,
 		`@createElement( 'noscript' )`,
 		`@createElement( 'noscript' )`,
-		state.parentNode,
-		true
+		state.parentNode
 	);
+
 	block.addElement(
 		after,
 		`@createElement( 'noscript' )`,
 		`@createElement( 'noscript' )`,
-		state.parentNode,
-		true
+		state.parentNode
 	);
 
-	const isToplevel = !state.parentNode;
-
-	const mountStatement = `${before}.insertAdjacentHTML( 'afterend', ${value} = ${snippet} );`;
-	const detachStatement = `@detachBetween( ${before}, ${after} );`;
-
-	block.builders.mount.addLine(mountStatement);
-
-	block.builders.update.addBlock(deindent`
-		if ( ${value} !== ( ${value} = ${snippet} ) ) {
-			${detachStatement}
-			${mountStatement}
-		}
-	`);
-
-	block.builders.detachRaw.addBlock(detachStatement);
-}
+	block.builders.mount.addLine(`${before}.insertAdjacentHTML( 'afterend', ${init} );`);
+	block.builders.detachRaw.addBlock(`@detachBetween( ${before}, ${after} );`);
+}
\ No newline at end of file
diff --git a/src/generators/dom/visitors/Text.ts b/src/generators/dom/visitors/Text.ts
index 49135a129e..796bf9e309 100644
--- a/src/generators/dom/visitors/Text.ts
+++ b/src/generators/dom/visitors/Text.ts
@@ -14,10 +14,7 @@ export default function visitText(
 	block.addElement(
 		node._state.name,
 		`@createText( ${stringify(node.data)} )`,
-		generator.hydratable
-			? `@claimText( ${state.parentNodes}, ${stringify(node.data)} )`
-			: '',
-		state.parentNode,
-		node.usedAsAnchor
+		`@claimText( ${state.parentNodes}, ${stringify(node.data)} )`,
+		state.parentNode
 	);
 }
diff --git a/src/generators/dom/visitors/shared/Tag.ts b/src/generators/dom/visitors/shared/Tag.ts
new file mode 100644
index 0000000000..f15d72a5ba
--- /dev/null
+++ b/src/generators/dom/visitors/shared/Tag.ts
@@ -0,0 +1,49 @@
+import deindent from '../../../../utils/deindent';
+import { DomGenerator } from '../../index';
+import Block from '../../Block';
+import { Node } from '../../../../interfaces';
+import { State } from '../../interfaces';
+
+export default function visitTag(
+	generator: DomGenerator,
+	block: Block,
+	state: State,
+	node: Node,
+	name: string,
+	update: (value: string) => string
+) {
+	const { dependencies, indexes, snippet } = block.contextualise(node.expression);
+
+	const hasChangeableIndex = Array.from(indexes).some(index => block.changeableIndexes.get(index));
+
+	const shouldCache = (
+		node.expression.type !== 'Identifier' ||
+		block.contexts.has(node.expression.name) ||
+		hasChangeableIndex
+	);
+
+	const value = shouldCache && block.getUniqueName(`${name}_value`);
+	const init = shouldCache ? value : snippet;
+
+	if (shouldCache) block.addVariable(value, snippet);
+
+	if (dependencies.length || hasChangeableIndex) {
+		const changedCheck = (
+			( block.hasOutroMethod ? `#outroing || ` : '' ) +
+			dependencies.map(dependency => `changed.${dependency}`).join(' || ')
+		);
+
+		const updateCachedValue = `${value} !== ( ${value} = ${snippet} )`;
+
+		const condition = shouldCache ?
+			( dependencies.length ? `( ${changedCheck} ) && ${updateCachedValue}` : updateCachedValue ) :
+			changedCheck;
+
+		block.builders.update.addConditional(
+			condition,
+			update(shouldCache ? value : snippet)
+		);
+	}
+
+	return { init };
+}
diff --git a/src/shared/index.js b/src/shared/index.js
index f198a4ef95..901fbfdb3b 100644
--- a/src/shared/index.js
+++ b/src/shared/index.js
@@ -26,25 +26,23 @@ export function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-export function dispatchObservers(component, group, newState, oldState) {
+export function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -133,6 +131,34 @@ export function set(newState) {
 	this._root._lock = false;
 }
 
+export function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
+export function _setDev(newState) {
+	if (typeof newState !== 'object') {
+		throw new Error(
+			'Component .set was called without an object of data key-values to update.'
+		);
+	}
+
+	this._checkReadOnly(newState);
+	_set.call(this, newState);
+}
+
 export function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -144,7 +170,9 @@ export var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 export var protoDev = {
@@ -154,5 +182,7 @@ export var protoDev = {
 	observe: observeDev,
 	on: onDev,
 	set: set,
-	teardown: destroyDev
+	teardown: destroyDev,
+	_recompute: noop,
+	_set: _setDev
 };
diff --git a/src/utils/CodeBuilder.ts b/src/utils/CodeBuilder.ts
index 92c88ea483..bdbcc3fcb6 100644
--- a/src/utils/CodeBuilder.ts
+++ b/src/utils/CodeBuilder.ts
@@ -21,15 +21,17 @@ export default class CodeBuilder {
 		this.lastCondition = null;
 	}
 
-	addConditionalLine(condition: string, line: string) {
+	addConditional(condition: string, body: string) {
+		body = body.replace(/^/gm, '\t');
+
 		if (condition === this.lastCondition) {
-			this.result += `\n\t${line}`;
+			this.result += `\n${body}`;
 		} else {
 			if (this.lastCondition) {
-				this.result += `\n}\n\n`;
+				this.result += `\n}`;
 			}
 
-			this.result += `if ( ${condition} ) {\n\t${line}`;
+			this.result += `${this.last === ChunkType.Block ? '\n\n' : '\n'}if ( ${condition} ) {\n${body}`;
 			this.lastCondition = condition;
 		}
 
diff --git a/test/helpers.js b/test/helpers.js
index f4be292ef4..e3343d4aa8 100644
--- a/test/helpers.js
+++ b/test/helpers.js
@@ -125,7 +125,7 @@ export function normalizeHtml(window, html) {
 		.replace(/>[\s\r\n]+</g, '><')
 		.trim();
 	cleanChildren(node, '');
-	return node.innerHTML;
+	return node.innerHTML.replace(/<\/?noscript\/?>/g, '');
 }
 
 export function setupHtmlEqual() {
diff --git a/test/js/index.js b/test/js/index.js
index d6497b6b75..737a76e700 100644
--- a/test/js/index.js
+++ b/test/js/index.js
@@ -52,7 +52,7 @@ describe("js", () => {
 					}
 				]
 			}).then(bundle => {
-				return bundle.generate({ format: "es" })
+				return bundle.generate({ format: "es" });
 			}).then(({ code }) => {
 				fs.writeFileSync(`${dir}/_actual-bundle.js`, code);
 
@@ -65,6 +65,9 @@ describe("js", () => {
 					code.trim().replace(/^\s+$/gm, ""),
 					expectedBundle.trim().replace(/^\s+$/gm, "")
 				);
+			}).catch(err => {
+				console.error(err.loc);
+				throw err;
 			});
 		});
 	});
diff --git a/test/js/samples/collapses-text-around-comments/expected-bundle.js b/test/js/samples/collapses-text-around-comments/expected-bundle.js
index e52f943862..10a7a08930 100644
--- a/test/js/samples/collapses-text-around-comments/expected-bundle.js
+++ b/test/js/samples/collapses-text-around-comments/expected-bundle.js
@@ -52,25 +52,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -134,6 +132,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -145,7 +160,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 var template = (function () {
@@ -168,12 +185,12 @@ function add_css () {
 }
 
 function create_main_fragment ( state, component ) {
-	var p, text_value, text;
+	var p, text;
 
 	return {
 		create: function () {
 			p = createElement( 'p' );
-			text = createText( text_value = state.foo );
+			text = createText( state.foo );
 			this.hydrate();
 		},
 
@@ -187,8 +204,8 @@ function create_main_fragment ( state, component ) {
 		},
 
 		update: function ( changed, state ) {
-			if ( text_value !== ( text_value = state.foo ) ) {
-				text.data = text_value;
+			if ( changed.foo ) {
+				text.data = state.foo;
 			}
 		},
 
@@ -226,12 +243,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/collapses-text-around-comments/expected.js b/test/js/samples/collapses-text-around-comments/expected.js
index 5a3e3d52f6..60a7180f1d 100644
--- a/test/js/samples/collapses-text-around-comments/expected.js
+++ b/test/js/samples/collapses-text-around-comments/expected.js
@@ -1,4 +1,4 @@
-import { appendNode, assign, createElement, createText, detachNode, dispatchObservers, insertNode, noop, proto, setAttribute } from "svelte/shared.js";
+import { appendNode, assign, createElement, createText, detachNode, insertNode, noop, proto, setAttribute } from "svelte/shared.js";
 
 var template = (function () {
 	return {
@@ -20,12 +20,12 @@ function add_css () {
 }
 
 function create_main_fragment ( state, component ) {
-	var p, text_value, text;
+	var p, text;
 
 	return {
 		create: function () {
 			p = createElement( 'p' );
-			text = createText( text_value = state.foo );
+			text = createText( state.foo );
 			this.hydrate();
 		},
 
@@ -39,8 +39,8 @@ function create_main_fragment ( state, component ) {
 		},
 
 		update: function ( changed, state ) {
-			if ( text_value !== ( text_value = state.foo ) ) {
-				text.data = text_value;
+			if ( changed.foo ) {
+				text.data = state.foo;
 			}
 		},
 
@@ -78,12 +78,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/computed-collapsed-if/expected-bundle.js b/test/js/samples/computed-collapsed-if/expected-bundle.js
index 0da5cdf32c..20fe6ef0fa 100644
--- a/test/js/samples/computed-collapsed-if/expected-bundle.js
+++ b/test/js/samples/computed-collapsed-if/expected-bundle.js
@@ -28,25 +28,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -110,6 +108,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -121,16 +136,11 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
-function recompute ( state, newState, oldState, isInitial ) {
-	if ( isInitial || ( 'x' in newState && differs( state.x, oldState.x ) ) ) {
-		state.a = newState.a = template.computed.a( state.x );
-		state.b = newState.b = template.computed.b( state.x );
-	}
-}
-
 var template = (function () {
 	return {
 		computed: {
@@ -147,6 +157,8 @@ function create_main_fragment ( state, component ) {
 
 		mount: noop,
 
+		update: noop,
+
 		unmount: noop,
 
 		destroy: noop
@@ -156,7 +168,7 @@ function create_main_fragment ( state, component ) {
 function SvelteComponent ( options ) {
 	options = options || {};
 	this._state = options.data || {};
-	recompute( this._state, this._state, {}, true );
+	this._recompute( {}, this._state, {}, true );
 
 	this._observers = {
 		pre: Object.create( null ),
@@ -178,12 +190,11 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	recompute( this._state, newState, oldState, false );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
+SvelteComponent.prototype._recompute = function _recompute ( changed, state, oldState, isInitial ) {
+	if ( isInitial || changed.x ) {
+		if ( differs( ( state.a = template.computed.a( state.x ) ), oldState.a ) ) changed.a = true;
+		if ( differs( ( state.b = template.computed.b( state.x ) ), oldState.b ) ) changed.b = true;
+	}
 };
 
 export default SvelteComponent;
diff --git a/test/js/samples/computed-collapsed-if/expected.js b/test/js/samples/computed-collapsed-if/expected.js
index a17e730122..6318b3b5cf 100644
--- a/test/js/samples/computed-collapsed-if/expected.js
+++ b/test/js/samples/computed-collapsed-if/expected.js
@@ -1,11 +1,4 @@
-import { assign, differs, dispatchObservers, noop, proto } from "svelte/shared.js";
-
-function recompute ( state, newState, oldState, isInitial ) {
-	if ( isInitial || ( 'x' in newState && differs( state.x, oldState.x ) ) ) {
-		state.a = newState.a = template.computed.a( state.x );
-		state.b = newState.b = template.computed.b( state.x );
-	}
-}
+import { assign, differs, noop, proto } from "svelte/shared.js";
 
 var template = (function () {
 	return {
@@ -23,6 +16,8 @@ function create_main_fragment ( state, component ) {
 
 		mount: noop,
 
+		update: noop,
+
 		unmount: noop,
 
 		destroy: noop
@@ -32,7 +27,7 @@ function create_main_fragment ( state, component ) {
 function SvelteComponent ( options ) {
 	options = options || {};
 	this._state = options.data || {};
-	recompute( this._state, this._state, {}, true );
+	this._recompute( {}, this._state, {}, true );
 
 	this._observers = {
 		pre: Object.create( null ),
@@ -54,12 +49,11 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	recompute( this._state, newState, oldState, false )
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
+SvelteComponent.prototype._recompute = function _recompute ( changed, state, oldState, isInitial ) {
+	if ( isInitial || changed.x ) {
+		if ( differs( ( state.a = template.computed.a( state.x ) ), oldState.a ) ) changed.a = true;
+		if ( differs( ( state.b = template.computed.b( state.x ) ), oldState.b ) ) changed.b = true;
+	}
+}
 
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/css-media-query/expected-bundle.js b/test/js/samples/css-media-query/expected-bundle.js
index 3324ebf9be..868ca03e71 100644
--- a/test/js/samples/css-media-query/expected-bundle.js
+++ b/test/js/samples/css-media-query/expected-bundle.js
@@ -48,25 +48,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -130,6 +128,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -141,7 +156,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 function encapsulateStyles ( node ) {
@@ -172,6 +189,8 @@ function create_main_fragment ( state, component ) {
 			insertNode( div, target, anchor );
 		},
 
+		update: noop,
+
 		unmount: function () {
 			detachNode( div );
 		},
@@ -206,11 +225,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/css-media-query/expected.js b/test/js/samples/css-media-query/expected.js
index 72338bb55b..8fb4bde21a 100644
--- a/test/js/samples/css-media-query/expected.js
+++ b/test/js/samples/css-media-query/expected.js
@@ -1,4 +1,4 @@
-import { appendNode, assign, createElement, detachNode, dispatchObservers, insertNode, noop, proto, setAttribute } from "svelte/shared.js";
+import { appendNode, assign, createElement, detachNode, insertNode, noop, proto, setAttribute } from "svelte/shared.js";
 
 function encapsulateStyles ( node ) {
 	setAttribute( node, 'svelte-2363328337', '' );
@@ -28,6 +28,8 @@ function create_main_fragment ( state, component ) {
 			insertNode( div, target, anchor );
 		},
 
+		update: noop,
+
 		unmount: function () {
 			detachNode( div );
 		},
@@ -62,11 +64,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/each-block-changed-check/expected-bundle.js b/test/js/samples/each-block-changed-check/expected-bundle.js
index aac78b9adb..2d14d12151 100644
--- a/test/js/samples/each-block-changed-check/expected-bundle.js
+++ b/test/js/samples/each-block-changed-check/expected-bundle.js
@@ -61,25 +61,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -143,6 +141,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -154,11 +169,13 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 function create_main_fragment ( state, component ) {
-	var text, p, text_1_value, text_1;
+	var text, p, text_1;
 
 	var each_block_value = state.comments;
 
@@ -176,7 +193,7 @@ function create_main_fragment ( state, component ) {
 
 			text = createText( "\n\n" );
 			p = createElement( 'p' );
-			text_1 = createText( text_1_value = state.foo );
+			text_1 = createText( state.foo );
 		},
 
 		mount: function ( target, anchor ) {
@@ -192,7 +209,7 @@ function create_main_fragment ( state, component ) {
 		update: function ( changed, state ) {
 			var each_block_value = state.comments;
 
-			if ( 'comments' in changed || 'elapsed' in changed || 'time' in changed ) {
+			if ( changed.comments || changed.elapsed || changed.time ) {
 				for ( var i = 0; i < each_block_value.length; i += 1 ) {
 					if ( each_block_iterations[i] ) {
 						each_block_iterations[i].update( changed, state, each_block_value, each_block_value[i], i );
@@ -210,8 +227,8 @@ function create_main_fragment ( state, component ) {
 				each_block_iterations.length = each_block_value.length;
 			}
 
-			if ( text_1_value !== ( text_1_value = state.foo ) ) {
-				text_1.data = text_1_value;
+			if ( changed.foo ) {
+				text_1.data = state.foo;
 			}
 		},
 
@@ -231,18 +248,18 @@ function create_main_fragment ( state, component ) {
 }
 
 function create_each_block ( state, each_block_value, comment, i, component ) {
-	var div, strong, text_value, text, text_1, span, text_2_value, text_2, text_3, text_4_value, text_4, text_5, text_6, raw_value, raw_before, raw_after;
+	var div, strong, text, text_1, span, text_2_value = comment.author, text_2, text_3, text_4_value = state.elapsed(comment.time, state.time), text_4, text_5, text_6, raw_value = comment.html, raw_before, raw_after;
 
 	return {
 		create: function () {
 			div = createElement( 'div' );
 			strong = createElement( 'strong' );
-			text = createText( text_value = i );
+			text = createText( i );
 			text_1 = createText( "\n\n\t\t" );
 			span = createElement( 'span' );
-			text_2 = createText( text_2_value = comment.author );
+			text_2 = createText( text_2_value );
 			text_3 = createText( " wrote " );
-			text_4 = createText( text_4_value = state.elapsed(comment.time, state.time) );
+			text_4 = createText( text_4_value );
 			text_5 = createText( " ago:" );
 			text_6 = createText( "\n\n\t\t" );
 			raw_before = createElement( 'noscript' );
@@ -268,25 +285,21 @@ function create_each_block ( state, each_block_value, comment, i, component ) {
 			appendNode( text_6, div );
 			appendNode( raw_before, div );
 			appendNode( raw_after, div );
-			raw_before.insertAdjacentHTML( 'afterend', raw_value = comment.html );
+			raw_before.insertAdjacentHTML( 'afterend', raw_value );
 		},
 
 		update: function ( changed, state, each_block_value, comment, i ) {
-			if ( text_value !== ( text_value = i ) ) {
-				text.data = text_value;
-			}
-
-			if ( text_2_value !== ( text_2_value = comment.author ) ) {
+			if ( ( changed.comments ) && text_2_value !== ( text_2_value = comment.author ) ) {
 				text_2.data = text_2_value;
 			}
 
-			if ( text_4_value !== ( text_4_value = state.elapsed(comment.time, state.time) ) ) {
+			if ( ( changed.elapsed || changed.comments || changed.time ) && text_4_value !== ( text_4_value = state.elapsed(comment.time, state.time) ) ) {
 				text_4.data = text_4_value;
 			}
 
-			if ( raw_value !== ( raw_value = comment.html ) ) {
+			if ( ( changed.comments ) && raw_value !== ( raw_value = comment.html ) ) {
 				detachBetween( raw_before, raw_after );
-				raw_before.insertAdjacentHTML( 'afterend', raw_value = comment.html );
+				raw_before.insertAdjacentHTML( 'afterend', raw_value );
 			}
 		},
 
@@ -324,12 +337,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/each-block-changed-check/expected.js b/test/js/samples/each-block-changed-check/expected.js
index e7683e0743..9bfc875e78 100644
--- a/test/js/samples/each-block-changed-check/expected.js
+++ b/test/js/samples/each-block-changed-check/expected.js
@@ -1,7 +1,7 @@
-import { appendNode, assign, createElement, createText, destroyEach, detachBetween, detachNode, dispatchObservers, insertNode, noop, proto } from "svelte/shared.js";
+import { appendNode, assign, createElement, createText, destroyEach, detachBetween, detachNode, insertNode, noop, proto } from "svelte/shared.js";
 
 function create_main_fragment ( state, component ) {
-	var text, p, text_1_value, text_1;
+	var text, p, text_1;
 
 	var each_block_value = state.comments;
 
@@ -19,7 +19,7 @@ function create_main_fragment ( state, component ) {
 
 			text = createText( "\n\n" );
 			p = createElement( 'p' );
-			text_1 = createText( text_1_value = state.foo );
+			text_1 = createText( state.foo );
 		},
 
 		mount: function ( target, anchor ) {
@@ -35,7 +35,7 @@ function create_main_fragment ( state, component ) {
 		update: function ( changed, state ) {
 			var each_block_value = state.comments;
 
-			if ( 'comments' in changed || 'elapsed' in changed || 'time' in changed ) {
+			if ( changed.comments || changed.elapsed || changed.time ) {
 				for ( var i = 0; i < each_block_value.length; i += 1 ) {
 					if ( each_block_iterations[i] ) {
 						each_block_iterations[i].update( changed, state, each_block_value, each_block_value[i], i );
@@ -53,8 +53,8 @@ function create_main_fragment ( state, component ) {
 				each_block_iterations.length = each_block_value.length;
 			}
 
-			if ( text_1_value !== ( text_1_value = state.foo ) ) {
-				text_1.data = text_1_value;
+			if ( changed.foo ) {
+				text_1.data = state.foo;
 			}
 		},
 
@@ -74,18 +74,18 @@ function create_main_fragment ( state, component ) {
 }
 
 function create_each_block ( state, each_block_value, comment, i, component ) {
-	var div, strong, text_value, text, text_1, span, text_2_value, text_2, text_3, text_4_value, text_4, text_5, text_6, raw_value, raw_before, raw_after;
+	var div, strong, text, text_1, span, text_2_value = comment.author, text_2, text_3, text_4_value = state.elapsed(comment.time, state.time), text_4, text_5, text_6, raw_value = comment.html, raw_before, raw_after;
 
 	return {
 		create: function () {
 			div = createElement( 'div' );
 			strong = createElement( 'strong' );
-			text = createText( text_value = i );
+			text = createText( i );
 			text_1 = createText( "\n\n\t\t" );
 			span = createElement( 'span' );
-			text_2 = createText( text_2_value = comment.author );
+			text_2 = createText( text_2_value );
 			text_3 = createText( " wrote " );
-			text_4 = createText( text_4_value = state.elapsed(comment.time, state.time) );
+			text_4 = createText( text_4_value );
 			text_5 = createText( " ago:" );
 			text_6 = createText( "\n\n\t\t" );
 			raw_before = createElement( 'noscript' );
@@ -111,25 +111,21 @@ function create_each_block ( state, each_block_value, comment, i, component ) {
 			appendNode( text_6, div );
 			appendNode( raw_before, div );
 			appendNode( raw_after, div );
-			raw_before.insertAdjacentHTML( 'afterend', raw_value = comment.html );
+			raw_before.insertAdjacentHTML( 'afterend', raw_value );
 		},
 
 		update: function ( changed, state, each_block_value, comment, i ) {
-			if ( text_value !== ( text_value = i ) ) {
-				text.data = text_value;
-			}
-
-			if ( text_2_value !== ( text_2_value = comment.author ) ) {
+			if ( ( changed.comments ) && text_2_value !== ( text_2_value = comment.author ) ) {
 				text_2.data = text_2_value;
 			}
 
-			if ( text_4_value !== ( text_4_value = state.elapsed(comment.time, state.time) ) ) {
+			if ( ( changed.elapsed || changed.comments || changed.time ) && text_4_value !== ( text_4_value = state.elapsed(comment.time, state.time) ) ) {
 				text_4.data = text_4_value;
 			}
 
-			if ( raw_value !== ( raw_value = comment.html ) ) {
+			if ( ( changed.comments ) && raw_value !== ( raw_value = comment.html ) ) {
 				detachBetween( raw_before, raw_after );
-				raw_before.insertAdjacentHTML( 'afterend', raw_value = comment.html );
+				raw_before.insertAdjacentHTML( 'afterend', raw_value );
 			}
 		},
 
@@ -167,12 +163,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/event-handlers-custom/expected-bundle.js b/test/js/samples/event-handlers-custom/expected-bundle.js
index ed200a7b4c..97090f092d 100644
--- a/test/js/samples/event-handlers-custom/expected-bundle.js
+++ b/test/js/samples/event-handlers-custom/expected-bundle.js
@@ -48,25 +48,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -130,6 +128,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -141,7 +156,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 var template = (function () {
@@ -181,6 +198,8 @@ function create_main_fragment ( state, component ) {
 			appendNode( text, button );
 		},
 
+		update: noop,
+
 		unmount: function () {
 			detachNode( button );
 		},
@@ -215,11 +234,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, template.methods, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/event-handlers-custom/expected.js b/test/js/samples/event-handlers-custom/expected.js
index 9d5e771bdd..54e9111aff 100644
--- a/test/js/samples/event-handlers-custom/expected.js
+++ b/test/js/samples/event-handlers-custom/expected.js
@@ -1,4 +1,4 @@
-import { appendNode, assign, createElement, createText, detachNode, dispatchObservers, insertNode, proto } from "svelte/shared.js";
+import { appendNode, assign, createElement, createText, detachNode, insertNode, noop, proto } from "svelte/shared.js";
 
 var template = (function () {
 	return {
@@ -37,6 +37,8 @@ function create_main_fragment ( state, component ) {
 			appendNode( text, button );
 		},
 
+		update: noop,
+
 		unmount: function () {
 			detachNode( button );
 		},
@@ -71,11 +73,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, template.methods, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/if-block-no-update/expected-bundle.js b/test/js/samples/if-block-no-update/expected-bundle.js
index 273ce81fc1..a36be53f93 100644
--- a/test/js/samples/if-block-no-update/expected-bundle.js
+++ b/test/js/samples/if-block-no-update/expected-bundle.js
@@ -52,25 +52,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -134,6 +132,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -145,7 +160,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 function create_main_fragment ( state, component ) {
@@ -259,12 +276,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/if-block-no-update/expected.js b/test/js/samples/if-block-no-update/expected.js
index 8d30b66a03..025e0a0e43 100644
--- a/test/js/samples/if-block-no-update/expected.js
+++ b/test/js/samples/if-block-no-update/expected.js
@@ -1,4 +1,4 @@
-import { appendNode, assign, createComment, createElement, createText, detachNode, dispatchObservers, insertNode, noop, proto } from "svelte/shared.js";
+import { appendNode, assign, createComment, createElement, createText, detachNode, insertNode, noop, proto } from "svelte/shared.js";
 
 function create_main_fragment ( state, component ) {
 	var if_block_anchor;
@@ -111,12 +111,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/if-block-simple/expected-bundle.js b/test/js/samples/if-block-simple/expected-bundle.js
index 3b57117283..c16dbcaaa3 100644
--- a/test/js/samples/if-block-simple/expected-bundle.js
+++ b/test/js/samples/if-block-simple/expected-bundle.js
@@ -52,25 +52,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -134,6 +132,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -145,7 +160,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 function create_main_fragment ( state, component ) {
@@ -235,12 +252,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/if-block-simple/expected.js b/test/js/samples/if-block-simple/expected.js
index db04589093..2cc3be36f9 100644
--- a/test/js/samples/if-block-simple/expected.js
+++ b/test/js/samples/if-block-simple/expected.js
@@ -1,4 +1,4 @@
-import { appendNode, assign, createComment, createElement, createText, detachNode, dispatchObservers, insertNode, noop, proto } from "svelte/shared.js";
+import { appendNode, assign, createComment, createElement, createText, detachNode, insertNode, noop, proto } from "svelte/shared.js";
 
 function create_main_fragment ( state, component ) {
 	var if_block_anchor;
@@ -87,12 +87,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/non-imported-component/expected-bundle.js b/test/js/samples/non-imported-component/expected-bundle.js
index b40f56d109..09576689bc 100644
--- a/test/js/samples/non-imported-component/expected-bundle.js
+++ b/test/js/samples/non-imported-component/expected-bundle.js
@@ -42,25 +42,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -124,6 +122,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -135,7 +150,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 var template = (function () {
@@ -170,6 +187,8 @@ function create_main_fragment ( state, component ) {
 			nonimported._fragment.mount( target, anchor );
 		},
 
+		update: noop,
+
 		unmount: function () {
 			imported._fragment.unmount();
 			detachNode( text );
@@ -221,11 +240,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/non-imported-component/expected.js b/test/js/samples/non-imported-component/expected.js
index 2382e04ab3..ce86538e00 100644
--- a/test/js/samples/non-imported-component/expected.js
+++ b/test/js/samples/non-imported-component/expected.js
@@ -1,6 +1,6 @@
 import Imported from 'Imported.html';
 
-import { assign, callAll, createText, detachNode, dispatchObservers, insertNode, proto } from "svelte/shared.js";
+import { assign, callAll, createText, detachNode, insertNode, noop, proto } from "svelte/shared.js";
 
 var template = (function () {
 	return {
@@ -34,6 +34,8 @@ function create_main_fragment ( state, component ) {
 			nonimported._fragment.mount( target, anchor );
 		},
 
+		update: noop,
+
 		unmount: function () {
 			imported._fragment.unmount();
 			detachNode( text );
@@ -85,11 +87,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/onrender-onteardown-rewritten/expected-bundle.js b/test/js/samples/onrender-onteardown-rewritten/expected-bundle.js
index a754118158..227ac71e7c 100644
--- a/test/js/samples/onrender-onteardown-rewritten/expected-bundle.js
+++ b/test/js/samples/onrender-onteardown-rewritten/expected-bundle.js
@@ -28,25 +28,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -110,6 +108,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -121,7 +136,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 var template = (function () {
@@ -139,6 +156,8 @@ function create_main_fragment ( state, component ) {
 
 		mount: noop,
 
+		update: noop,
+
 		unmount: noop,
 
 		destroy: noop
@@ -182,11 +201,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/onrender-onteardown-rewritten/expected.js b/test/js/samples/onrender-onteardown-rewritten/expected.js
index bd473fc2d7..a5125745c6 100644
--- a/test/js/samples/onrender-onteardown-rewritten/expected.js
+++ b/test/js/samples/onrender-onteardown-rewritten/expected.js
@@ -1,4 +1,4 @@
-import { assign, callAll, dispatchObservers, noop, proto } from "svelte/shared.js";
+import { assign, callAll, noop, proto } from "svelte/shared.js";
 
 var template = (function () {
 	return {
@@ -15,6 +15,8 @@ function create_main_fragment ( state, component ) {
 
 		mount: noop,
 
+		update: noop,
+
 		unmount: noop,
 
 		destroy: noop
@@ -58,11 +60,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/setup-method/expected-bundle.js b/test/js/samples/setup-method/expected-bundle.js
index 006152cea5..0ecdbafa5b 100644
--- a/test/js/samples/setup-method/expected-bundle.js
+++ b/test/js/samples/setup-method/expected-bundle.js
@@ -28,25 +28,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -110,6 +108,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -121,7 +136,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 var template = (function () {
@@ -150,6 +167,8 @@ function create_main_fragment ( state, component ) {
 
 		mount: noop,
 
+		update: noop,
+
 		unmount: noop,
 
 		destroy: noop
@@ -180,13 +199,6 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, template.methods, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 template.setup( SvelteComponent );
 
 export default SvelteComponent;
diff --git a/test/js/samples/setup-method/expected.js b/test/js/samples/setup-method/expected.js
index 85f0d5c393..bf9da033ad 100644
--- a/test/js/samples/setup-method/expected.js
+++ b/test/js/samples/setup-method/expected.js
@@ -1,4 +1,4 @@
-import { assign, dispatchObservers, noop, proto } from "svelte/shared.js";
+import { assign, noop, proto } from "svelte/shared.js";
 
 var template = (function () {
 	return {
@@ -26,6 +26,8 @@ function create_main_fragment ( state, component ) {
 
 		mount: noop,
 
+		update: noop,
+
 		unmount: noop,
 
 		destroy: noop
@@ -56,13 +58,6 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, template.methods, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 template.setup( SvelteComponent );
 
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/use-elements-as-anchors/expected-bundle.js b/test/js/samples/use-elements-as-anchors/expected-bundle.js
index 502e08b22e..33f1b49df3 100644
--- a/test/js/samples/use-elements-as-anchors/expected-bundle.js
+++ b/test/js/samples/use-elements-as-anchors/expected-bundle.js
@@ -52,25 +52,23 @@ function differs(a, b) {
 	return a !== b || ((a && typeof a === 'object') || typeof a === 'function');
 }
 
-function dispatchObservers(component, group, newState, oldState) {
+function dispatchObservers(component, group, changed, newState, oldState) {
 	for (var key in group) {
-		if (!(key in newState)) continue;
+		if (!changed[key]) continue;
 
 		var newValue = newState[key];
 		var oldValue = oldState[key];
 
-		if (differs(newValue, oldValue)) {
-			var callbacks = group[key];
-			if (!callbacks) continue;
+		var callbacks = group[key];
+		if (!callbacks) continue;
 
-			for (var i = 0; i < callbacks.length; i += 1) {
-				var callback = callbacks[i];
-				if (callback.__calling) continue;
+		for (var i = 0; i < callbacks.length; i += 1) {
+			var callback = callbacks[i];
+			if (callback.__calling) continue;
 
-				callback.__calling = true;
-				callback.call(component, newValue, oldValue);
-				callback.__calling = false;
-			}
+			callback.__calling = true;
+			callback.call(component, newValue, oldValue);
+			callback.__calling = false;
 		}
 	}
 }
@@ -134,6 +132,23 @@ function set(newState) {
 	this._root._lock = false;
 }
 
+function _set(newState) {
+	var oldState = this._state,
+		changed = {},
+		dirty = false;
+
+	for (var key in newState) {
+		if (differs(newState[key], oldState[key])) changed[key] = dirty = true;
+	}
+	if (!dirty) return;
+
+	this._state = assign({}, oldState, newState);
+	this._recompute(changed, this._state, oldState, false);
+	dispatchObservers(this, this._observers.pre, changed, this._state, oldState);
+	this._fragment.update(changed, this._state);
+	dispatchObservers(this, this._observers.post, changed, this._state, oldState);
+}
+
 function callAll(fns) {
 	while (fns && fns.length) fns.pop()();
 }
@@ -145,7 +160,9 @@ var proto = {
 	observe: observe,
 	on: on,
 	set: set,
-	teardown: destroy
+	teardown: destroy,
+	_recompute: noop,
+	_set: _set
 };
 
 function create_main_fragment ( state, component ) {
@@ -419,12 +436,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
diff --git a/test/js/samples/use-elements-as-anchors/expected.js b/test/js/samples/use-elements-as-anchors/expected.js
index 3ed3f1e7db..28f0a660d4 100644
--- a/test/js/samples/use-elements-as-anchors/expected.js
+++ b/test/js/samples/use-elements-as-anchors/expected.js
@@ -1,4 +1,4 @@
-import { appendNode, assign, createComment, createElement, createText, detachNode, dispatchObservers, insertNode, noop, proto } from "svelte/shared.js";
+import { appendNode, assign, createComment, createElement, createText, detachNode, insertNode, noop, proto } from "svelte/shared.js";
 
 function create_main_fragment ( state, component ) {
 	var div, text, p, text_1, text_2, text_3, text_4, p_1, text_5, text_6, text_8, if_block_4_anchor;
@@ -271,12 +271,4 @@ function SvelteComponent ( options ) {
 
 assign( SvelteComponent.prototype, proto );
 
-SvelteComponent.prototype._set = function _set ( newState ) {
-	var oldState = this._state;
-	this._state = assign( {}, oldState, newState );
-	dispatchObservers( this, this._observers.pre, newState, oldState );
-	this._fragment.update( newState, this._state );
-	dispatchObservers( this, this._observers.post, newState, oldState );
-};
-
 export default SvelteComponent;
\ No newline at end of file
diff --git a/test/runtime/samples/each-block-keyed/_config.js b/test/runtime/samples/each-block-keyed/_config.js
index 35b47f5be8..29cf422fbf 100644
--- a/test/runtime/samples/each-block-keyed/_config.js
+++ b/test/runtime/samples/each-block-keyed/_config.js
@@ -6,7 +6,10 @@ export default {
 		]
 	},
 
-	html: '<p>implement keyed each blocks</p><p>implement client-side hydration</p>',
+	html: `
+		<p>1: implement keyed each blocks</p>
+		<p>2: implement client-side hydration</p>
+	`,
 
 	test ( assert, component, target ) {
 		const [ p1, p2 ] = target.querySelectorAll( 'p' );
@@ -16,7 +19,7 @@ export default {
 				{ id: 234, description: 'implement client-side hydration' }
 			]
 		});
-		assert.htmlEqual( target.innerHTML, '<p>implement client-side hydration</p>' );
+		assert.htmlEqual( target.innerHTML, '<p>1: implement client-side hydration</p>' );
 
 		const [ p3 ] = target.querySelectorAll( 'p' );
 
diff --git a/test/runtime/samples/each-block-keyed/main.html b/test/runtime/samples/each-block-keyed/main.html
index 7d5b90a9f8..da74eaa701 100644
--- a/test/runtime/samples/each-block-keyed/main.html
+++ b/test/runtime/samples/each-block-keyed/main.html
@@ -1,3 +1,3 @@
-{{#each todos as todo @id}}
-	<p>{{todo.description}}</p>
+{{#each todos as todo, i @id}}
+	<p>{{i+1}}: {{todo.description}}</p>
 {{/each}}
diff --git a/test/runtime/samples/flush-before-bindings/Nested.html b/test/runtime/samples/flush-before-bindings/Nested.html
index 9b5a838cb4..d4071beee9 100644
--- a/test/runtime/samples/flush-before-bindings/Nested.html
+++ b/test/runtime/samples/flush-before-bindings/Nested.html
@@ -1,7 +1,7 @@
 {{#each things as thing}}
 	<Visibility bind:isVisible="visibilityMap[thing]">
 		<p>{{thing}} ({{visibilityMap[thing]}})</p>
-	</Visibility> 
+	</Visibility>
 {{/each}}
 
 <script>
diff --git a/test/runtime/samples/ignore-unchanged-attribute-compound/_config.js b/test/runtime/samples/ignore-unchanged-attribute-compound/_config.js
new file mode 100644
index 0000000000..5780a83e6c
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-attribute-compound/_config.js
@@ -0,0 +1,26 @@
+import counter from './counter.js';
+
+export default {
+	data: {
+		x: 1,
+		y: 2
+	},
+
+	html: `
+		<p>1</p>
+		<p class='-2-'></p>
+	`,
+
+	test(assert, component) {
+		counter.count = 0;
+
+		component.set({ x: 3 });
+		assert.equal(counter.count, 0);
+
+		component.set({ x: 4, y: 5 });
+		assert.equal(counter.count, 1);
+
+		component.set({ x: 5, y: 5 });
+		assert.equal(counter.count, 1);
+	}
+};
diff --git a/test/runtime/samples/ignore-unchanged-attribute-compound/counter.js b/test/runtime/samples/ignore-unchanged-attribute-compound/counter.js
new file mode 100644
index 0000000000..63872cd6a2
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-attribute-compound/counter.js
@@ -0,0 +1,3 @@
+export default {
+	count: 0
+};
\ No newline at end of file
diff --git a/test/runtime/samples/ignore-unchanged-attribute-compound/main.html b/test/runtime/samples/ignore-unchanged-attribute-compound/main.html
new file mode 100644
index 0000000000..1e742befd9
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-attribute-compound/main.html
@@ -0,0 +1,15 @@
+<p>{{x}}</p>
+<p class='-{{myHelper(y)}}-'></p>
+
+<script>
+	import counter from './counter.js';
+
+	export default {
+		helpers: {
+			myHelper(value) {
+				counter.count += 1;
+				return value;
+			}
+		}
+	};
+</script>
diff --git a/test/runtime/samples/ignore-unchanged-attribute/_config.js b/test/runtime/samples/ignore-unchanged-attribute/_config.js
new file mode 100644
index 0000000000..eb62f5dd8e
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-attribute/_config.js
@@ -0,0 +1,26 @@
+import counter from './counter.js';
+
+export default {
+	data: {
+		x: 1,
+		y: 2
+	},
+
+	html: `
+		<p>1</p>
+		<p class='2'></p>
+	`,
+
+	test(assert, component) {
+		counter.count = 0;
+
+		component.set({ x: 3 });
+		assert.equal(counter.count, 0);
+
+		component.set({ x: 4, y: 5 });
+		assert.equal(counter.count, 1);
+
+		component.set({ x: 5, y: 5 });
+		assert.equal(counter.count, 1);
+	}
+};
diff --git a/test/runtime/samples/ignore-unchanged-attribute/counter.js b/test/runtime/samples/ignore-unchanged-attribute/counter.js
new file mode 100644
index 0000000000..63872cd6a2
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-attribute/counter.js
@@ -0,0 +1,3 @@
+export default {
+	count: 0
+};
\ No newline at end of file
diff --git a/test/runtime/samples/ignore-unchanged-attribute/main.html b/test/runtime/samples/ignore-unchanged-attribute/main.html
new file mode 100644
index 0000000000..afaa83c85f
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-attribute/main.html
@@ -0,0 +1,15 @@
+<p>{{x}}</p>
+<p class='{{myHelper(y)}}'></p>
+
+<script>
+	import counter from './counter.js';
+
+	export default {
+		helpers: {
+			myHelper(value) {
+				counter.count += 1;
+				return value;
+			}
+		}
+	};
+</script>
diff --git a/test/runtime/samples/ignore-unchanged-raw/_config.js b/test/runtime/samples/ignore-unchanged-raw/_config.js
new file mode 100644
index 0000000000..b8c58367b3
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-raw/_config.js
@@ -0,0 +1,26 @@
+import counter from './counter.js';
+
+export default {
+	data: {
+		x: 1,
+		y: 2
+	},
+
+	html: `
+		<p>1</p>
+		<p>2</p>
+	`,
+
+	test(assert, component) {
+		counter.count = 0;
+
+		component.set({ x: 3 });
+		assert.equal(counter.count, 0);
+
+		component.set({ x: 4, y: 5 });
+		assert.equal(counter.count, 1);
+
+		component.set({ x: 5, y: 5 });
+		assert.equal(counter.count, 1);
+	}
+};
diff --git a/test/runtime/samples/ignore-unchanged-raw/counter.js b/test/runtime/samples/ignore-unchanged-raw/counter.js
new file mode 100644
index 0000000000..63872cd6a2
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-raw/counter.js
@@ -0,0 +1,3 @@
+export default {
+	count: 0
+};
\ No newline at end of file
diff --git a/test/runtime/samples/ignore-unchanged-raw/main.html b/test/runtime/samples/ignore-unchanged-raw/main.html
new file mode 100644
index 0000000000..375985c0d2
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-raw/main.html
@@ -0,0 +1,15 @@
+<p>{{x}}</p>
+<p>{{{myHelper(y)}}}</p>
+
+<script>
+	import counter from './counter.js';
+
+	export default {
+		helpers: {
+			myHelper(value) {
+				counter.count += 1;
+				return value;
+			}
+		}
+	};
+</script>
diff --git a/test/runtime/samples/ignore-unchanged-tag/_config.js b/test/runtime/samples/ignore-unchanged-tag/_config.js
new file mode 100644
index 0000000000..b8c58367b3
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-tag/_config.js
@@ -0,0 +1,26 @@
+import counter from './counter.js';
+
+export default {
+	data: {
+		x: 1,
+		y: 2
+	},
+
+	html: `
+		<p>1</p>
+		<p>2</p>
+	`,
+
+	test(assert, component) {
+		counter.count = 0;
+
+		component.set({ x: 3 });
+		assert.equal(counter.count, 0);
+
+		component.set({ x: 4, y: 5 });
+		assert.equal(counter.count, 1);
+
+		component.set({ x: 5, y: 5 });
+		assert.equal(counter.count, 1);
+	}
+};
diff --git a/test/runtime/samples/ignore-unchanged-tag/counter.js b/test/runtime/samples/ignore-unchanged-tag/counter.js
new file mode 100644
index 0000000000..63872cd6a2
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-tag/counter.js
@@ -0,0 +1,3 @@
+export default {
+	count: 0
+};
\ No newline at end of file
diff --git a/test/runtime/samples/ignore-unchanged-tag/main.html b/test/runtime/samples/ignore-unchanged-tag/main.html
new file mode 100644
index 0000000000..f50864df94
--- /dev/null
+++ b/test/runtime/samples/ignore-unchanged-tag/main.html
@@ -0,0 +1,15 @@
+<p>{{x}}</p>
+<p>{{myHelper(y)}}</p>
+
+<script>
+	import counter from './counter.js';
+
+	export default {
+		helpers: {
+			myHelper(value) {
+				counter.count += 1;
+				return value;
+			}
+		}
+	};
+</script>