diff --git a/src/compile/Component.ts b/src/compile/Component.ts
index 560cc98961..919cfb1171 100644
--- a/src/compile/Component.ts
+++ b/src/compile/Component.ts
@@ -15,6 +15,7 @@ import { Node, Ast, CompileOptions, CustomElementOptions } from '../interfaces';
 import error from '../utils/error';
 import getCodeFrame from '../utils/getCodeFrame';
 import flattenReference from '../utils/flattenReference';
+import addToSet from '../utils/addToSet';
 
 // We need to tell estree-walker that it should always
 // look for an `else` block, otherwise it might get
@@ -44,31 +45,32 @@ export default class Component {
 
 	properties: Map<string, Node>;
 
-	imports: Node[];
+	imports: Node[] = [];
 	namespace: string;
 	hasComponents: boolean;
 	javascript: string;
 
-	declarations: string[];
-	exports: Array<{ name: string, as: string }>;
-	event_handlers: Array<{ name: string, body: string }>;
-	props: string[];
+	declarations: string[] = [];
+	writable_declarations: Set<string> = new Set();
+	initialised_declarations: Set<string> = new Set();
+	exports: Array<{ name: string, as: string }> = [];
+	event_handlers: Array<{ name: string, body: string }> = [];
 
 	code: MagicString;
 
-	indirectDependencies: Map<string, Set<string>>;
-	expectedProperties: Set<string>;
-	refs: Set<string>;
+	indirectDependencies: Map<string, Set<string>> = new Map();
+	expectedProperties: Set<string> = new Set();
+	refs: Set<string> = new Set();
 
 	file: string;
 	locate: (c: number) => { line: number, column: number };
 
 	stylesheet: Stylesheet;
 
-	userVars: Set<string>;
-	templateVars: Map<string, string>;
-	aliases: Map<string, string>;
-	usedNames: Set<string>;
+	userVars: Set<string> = new Set();
+	templateVars: Map<string, string> = new Map();
+	aliases: Map<string, string> = new Map();
+	usedNames: Set<string> = new Set();
 	init_uses_self = false;
 
 	locator: (search: number, startIndex?: number) => {
@@ -89,38 +91,17 @@ export default class Component {
 		this.source = source;
 		this.options = options;
 
-		this.imports = [];
-
-		this.declarations = [];
-		this.exports = [];
-		this.event_handlers = [];
-
-		this.refs = new Set();
-
-		this.indirectDependencies = new Map();
-
 		this.file = options.filename && (
 			typeof process !== 'undefined' ? options.filename.replace(process.cwd(), '').replace(/^[\/\\]/, '') : options.filename
 		);
 		this.locate = getLocator(this.source);
 
-		// track which properties are needed, so we can provide useful info
-		// in dev mode
-		this.expectedProperties = new Set();
-
 		this.code = new MagicString(source);
 
 		// styles
 		this.stylesheet = new Stylesheet(source, ast, options.filename, options.dev);
 		this.stylesheet.validate(this);
 
-		// allow compiler to deconflict user's `import { flush } from 'whatever'` and
-		// Svelte's builtin `import { flush, ... } from 'svelte/internal.ts'`;
-		this.userVars = new Set();
-		this.templateVars = new Map();
-		this.aliases = new Map();
-		this.usedNames = new Set();
-
 		this.properties = new Map();
 
 		this.walkJs();
@@ -150,6 +131,7 @@ export default class Component {
 
 		if (!this.ast.js) {
 			this.declarations = Array.from(this.expectedProperties);
+			addToSet(this.writable_declarations, this.expectedProperties);
 
 			this.exports = this.declarations.map(name => ({
 				name,
@@ -390,6 +372,9 @@ export default class Component {
 			this.declarations.push(name);
 		});
 
+		this.writable_declarations = scope.writable_declarations;
+		this.initialised_declarations = scope.initialised_declarations;
+
 		globals.forEach(name => {
 			this.userVars.add(name);
 		});
@@ -401,7 +386,7 @@ export default class Component {
 				this.error(node, {
 					code: `default-export`,
 					message: `A component cannot have a default export`
-				})
+				});
 			}
 
 			if (node.type === 'ExportNamedDeclaration') {
diff --git a/src/compile/render-dom/index.ts b/src/compile/render-dom/index.ts
index 2011ca85f5..84f879cd8c 100644
--- a/src/compile/render-dom/index.ts
+++ b/src/compile/render-dom/index.ts
@@ -61,7 +61,8 @@ export default function dom(
 	const globals = expectedProperties.filter(prop => globalWhitelist.has(prop));
 
 	if (component.customElement) {
-		const props = component.props || Array.from(component.expectedProperties);
+		// TODO use `export` to determine this
+		const props = Array.from(component.expectedProperties);
 
 		builder.addBlock(deindent`
 			class ${name} extends HTMLElement {
@@ -118,8 +119,8 @@ export default function dom(
 			);
 		}
 
-		builder.addBlock(deindent`
-			class ${name} extends ${superclass} {
+		const body = [
+			deindent`
 				$$init($$make_dirty) {
 					${component.init_uses_self && `const $$self = this;`}
 					${component.javascript || component.exports.map(x => `let ${x.name};`)}
@@ -145,17 +146,59 @@ export default function dom(
 				$$create_fragment(${component.alias('component')}, ctx) {
 					${block.getContents()}
 				}
+			`
+		];
+
+		if (component.options.dev) {
+			// TODO check no uunexpected props were passed, as well as
+			// checking that expected ones were passed
+			const expected = component.exports
+				.map(x => x.name)
+				.filter(name => !component.initialised_declarations.has(name));
+
+			if (expected.length) {
+				const debug_name = `<${component.customElement ? component.tag : name}>`;
+
+				body.push(deindent`
+					$$checkProps() {
+						const state = this.$$.get_state();
+						${expected.map(name => deindent`
+
+						if (state.${name} === undefined) {
+							console.warn("${debug_name} was created without expected data property '${name}'");
+						}
+						`)}
+					}
+				`);
+			}
+		}
 
-				${component.exports.map(x => deindent`
+		component.exports.forEach(x => {
+			body.push(deindent`
 				get ${x.as}() {
 					return this.$$.get_state().${x.name};
 				}
+			`);
 
-				set ${x.as}(value) {
-					this.$set({ ${x.name}: value });
-					@flush();
-				}
-				`)}
+			if (component.writable_declarations.has(x.as)) {
+				body.push(deindent`
+					set ${x.as}(value) {
+						this.$set({ ${x.name}: value });
+						@flush();
+					}
+				`);
+			} else if (component.options.dev) {
+				body.push(deindent`
+					set ${x.as}(value) {
+						throw new Error("${x.as} is read-only");
+					}
+				`);
+			}
+		});
+
+		builder.addBlock(deindent`
+			class ${name} extends ${superclass} {
+				${body.join('\n\n')}
 			}
 		`);
 	}
diff --git a/src/internal/Component.js b/src/internal/Component.js
index bc3f9a424a..96c6fb41bf 100644
--- a/src/internal/Component.js
+++ b/src/internal/Component.js
@@ -116,6 +116,7 @@ export class $$ComponentDev extends $$Component {
 		}
 
 		super(options);
+		this.$$checkProps();
 	}
 
 	$destroy() {
@@ -124,4 +125,8 @@ export class $$ComponentDev extends $$Component {
 			console.warn(`Component was already destroyed`);
 		};
 	}
+
+	$$checkProps() {
+		// noop by default
+	}
 }
\ No newline at end of file
diff --git a/src/utils/annotateWithScopes.ts b/src/utils/annotateWithScopes.ts
index 5b0a28efb5..5d316d0797 100644
--- a/src/utils/annotateWithScopes.ts
+++ b/src/utils/annotateWithScopes.ts
@@ -54,21 +54,28 @@ export function createScopes(expression: Node) {
 export class Scope {
 	parent: Scope;
 	block: boolean;
-	declarations: Set<string>;
+
+	declarations: Set<string> = new Set();
+	writable_declarations: Set<string> = new Set();
+	initialised_declarations: Set<string> = new Set();
 
 	constructor(parent: Scope, block: boolean) {
 		this.parent = parent;
 		this.block = block;
-		this.declarations = new Set();
 	}
 
 	addDeclaration(node: Node) {
 		if (node.kind === 'var' && !this.block && this.parent) {
 			this.parent.addDeclaration(node);
 		} else if (node.type === 'VariableDeclaration') {
+			const writable = node.kind !== 'const';
+			const initialised = !!node.init;
+
 			node.declarations.forEach((declarator: Node) => {
 				extractNames(declarator.id).forEach(name => {
 					this.declarations.add(name);
+					if (writable) this.writable_declarations.add(name);
+					if (initialised) this.initialised_declarations.add(name);
 				});
 			});
 		} else {
diff --git a/test/runtime/samples/dev-warning-missing-data/_config.js b/test/runtime/samples/dev-warning-missing-data/_config.js
index aef14fd572..e45980bc1b 100644
--- a/test/runtime/samples/dev-warning-missing-data/_config.js
+++ b/test/runtime/samples/dev-warning-missing-data/_config.js
@@ -1,4 +1,6 @@
 export default {
+	solo: 1,
+
 	compileOptions: {
 		dev: true
 	},