add development warnings for missing data properties

src/generators/Generator.js

@@ -19,6 +19,10 @@ export default class Generator {
 		this.components = {};
 		this.events = {};
 
+		// track which properties are needed, so we can provide useful info
+		// in dev mode
+		this.expectedProperties = {};
+
 		this.elementDepth = 0;
 
 		this.code = new MagicString( source );
@@ -99,6 +103,10 @@ export default class Generator {
 			}
 		});
 
+		dependencies.forEach( name => {
+			this.expectedProperties[ name ] = true;
+		});
+
 		return {
 			dependencies,
 			contexts: usedContexts,

src/generators/dom/index.js

@@ -291,13 +291,32 @@ export default function dom ( parsed, source, options, names ) {
 		` );
 	}
 
-	const initialState = templateProperties.data ? `Object.assign( template.data(), options.data )` : `options.data || {}`;
+	const stateBlock = new CodeBuilder();
+
+	stateBlock.addLine(
+		`this._state = ${templateProperties.data ? `Object.assign( template.data(), options.data )` : `options.data || {}`};`
+	);
+
+	if ( templateProperties.computed ) {
+		stateBlock.addLine(
+			`applyComputations( this._state, this._state, {}, true );`
+		);
+	}
+
+	if ( options.dev ) {
+		Object.keys( generator.expectedProperties ).forEach( prop => {
+			stateBlock.addLine(
+				`if ( !( '${prop}' in this._state ) ) throw new Error( "Component was created without expected data property 'foo'" );`
+			);
+		});
+	}
 
 	builders.main.addBlock( deindent`
 		function ${name} ( options ) {
 			options = options || {};
 			${generator.usesRefs ? `\nthis.refs = {}` : ``}
-			this._state = ${initialState};${templateProperties.computed ? `\napplyComputations( this._state, this._state, {}, true );` : ``}
+
+			${stateBlock}
 
 			this._observers = {
 				pre: Object.create( null ),

test/generate.js

@@ -41,6 +41,7 @@ describe( 'generate', () => {
 			showCompiledCode = config.show;
 			compileOptions = config.compileOptions || {};
 			compileOptions.shared = shared;
+			compileOptions.dev = config.dev;
 
 			try {
 				const source = fs.readFileSync( `test/generator/${dir}/main.html`, 'utf-8' );
@@ -79,6 +80,8 @@ describe( 'generate', () => {
 				throw err;
 			}
 
+			let unintendedError = null;
+
 			return env()
 				.then( window => {
 					// Put the constructor on window for testing
@@ -91,6 +94,11 @@ describe( 'generate', () => {
 						data: config.data
 					});
 
+					if ( config.error ) {
+						unintendedError = true;
+						throw new Error( 'Expected a runtime error' );
+					}
+
 					if ( config.html ) {
 						assert.htmlEqual( target.innerHTML, config.html );
 					}
@@ -103,8 +111,14 @@ describe( 'generate', () => {
 					}
 				})
 				.catch( err => {
-					if ( !config.show ) console.log( addLineNumbers( code ) ); // eslint-disable-line no-console
-					throw err;
+					if ( config.error && !unintendedError ) {
+						config.error( assert, err );
+					}
+
+					else {
+						if ( !config.show ) console.log( addLineNumbers( code ) ); // eslint-disable-line no-console
+						throw err;
+					}
 				});
 		});
 	}

test/generator/dev-warning-missing-data/_config.js

@@ -0,0 +1,7 @@
+export default {
+	dev: true,
+
+	error ( assert, err ) {
+		assert.equal( err.message, `Component was created without expected data property 'foo'` );
+	}
+};

test/generator/dev-warning-missing-data/main.html

@@ -0,0 +1 @@
+<p>{{foo}}</p>