throw if user sets read-only properties in dev mode

src/generators/dom/index.js

@@ -11,6 +11,8 @@ class DomGenerator extends Generator {
 		this.blocks = [];
 		this.uses = new Set();
 
+		this.readonly = new Set();
+
 		// initial values for e.g. window.innerWidth, if there's a <:Window> meta tag
 		this.builders = {
 			metaBindings: new CodeBuilder()
@@ -94,6 +96,13 @@ export default function dom ( parsed, source, options ) {
 		const differs = generator.helper( 'differs' );
 
 		computations.forEach( ({ key, deps }) => {
+			if ( generator.readonly.has( key ) ) {
+				// <:Window> bindings
+				throw new Error( `Cannot have a computed value '${key}' that clashes with a read-only property` );
+			}
+
+			generator.readonly.add( key );
+
 			builder.addBlock( deindent`
 				if ( isInitial || ${deps.map( dep => `( '${dep}' in newState && ${differs}( state.${dep}, oldState.${dep} ) )` ).join( ' || ' )} ) {
 					state.${key} = newState.${key} = ${generator.alias( 'template' )}.computed.${key}( ${deps.map( dep => `state.${dep}` ).join( ', ' )} );
@@ -106,7 +115,15 @@ export default function dom ( parsed, source, options ) {
 				${builder}
 			}
 		` );
+	}
 
+	if ( options.dev ) {
+		Array.from( generator.readonly ).forEach( prop => {
+			builders._set.addLine( `if ( '${prop}' in newState && !this._updatingReadonlyProperty ) throw new Error( "Cannot set read-only property '${prop}'" );` );
+		});
+	}
+
+	if ( computations.length ) {
 		builders._set.addLine( `${generator.alias( 'recompute' )}( this._state, newState, oldState, false )` );
 	}
 

src/generators/dom/visitors/Element/meta/Window.js

@@ -12,6 +12,14 @@ const associatedEvents = {
 	scrollY: 'scroll'
 };
 
+const readonly = new Set([
+	'innerWidth',
+	'innerHeight',
+	'outerWidth',
+	'outerHeight',
+	'online'
+]);
+
 export default function visitWindow ( generator, block, node ) {
 	const events = {};
 	const bindings = {};
@@ -47,6 +55,11 @@ export default function visitWindow ( generator, block, node ) {
 				throw new Error( `Bindings on <:Window/> must be to top-level properties, e.g. '${parts.pop()}' rather than '${keypath}'` );
 			}
 
+			// in dev mode, throw if read-only values are written to
+			if ( readonly.has( attribute.name ) ) {
+				generator.readonly.add( attribute.value.name );
+			}
+
 			bindings[ attribute.name ] = attribute.value.name;
 
 			// bind:online is a special case, we need to listen for two separate events
@@ -80,12 +93,16 @@ export default function visitWindow ( generator, block, node ) {
 			handlerBody.addLine( `${lock} = true;` );
 		}
 
+		if ( generator.options.dev ) handlerBody.addLine( `component._updatingReadonlyProperty = true;` );
+
 		handlerBody.addBlock( deindent`
 			component.set({
 				${props}
 			});
 		` );
 
+		if ( generator.options.dev ) handlerBody.addLine( `component._updatingReadonlyProperty = false;` );
+
 		if ( event === 'scroll' ) {
 			handlerBody.addLine( `${lock} = false;` );
 		}

test/runtime/samples/dev-warning-readonly-computed/_config.js

@@ -0,0 +1,12 @@
+export default {
+	dev: true,
+
+	test ( assert, component ) {
+		try {
+			component.set({ foo: 1 });
+			throw new Error( 'Expected an error' );
+		} catch ( err ) {
+			assert.equal( err.message, `Cannot set read-only property 'foo'` );
+		}
+	}
+};

test/runtime/samples/dev-warning-readonly-computed/main.html

@@ -0,0 +1,7 @@
+<script>
+	export default {
+		computed: {
+			foo: a => a + 1
+		}
+	};
+</script>

test/runtime/samples/dev-warning-readonly-window-binding/_config.js

@@ -0,0 +1,12 @@
+export default {
+	dev: true,
+
+	test ( assert, component ) {
+		try {
+			component.set({ width: 99 });
+			throw new Error( 'Expected an error' );
+		} catch ( err ) {
+			assert.equal( err.message, `Cannot set read-only property 'width'` );
+		}
+	}
+};

test/runtime/samples/dev-warning-readonly-window-binding/main.html

@@ -0,0 +1 @@
+<:Window bind:innerWidth='width'/>