warn on missing format (fixes #1197)

src/Bundle.js

@@ -398,8 +398,6 @@ export default class Bundle {
 			options.format = 'es';
 		}
 
-		const format = options.format || 'es';
-
 		// Determine export mode - 'default', 'named', 'none'
 		const exportMode = getExportMode( this, options );
 
@@ -409,7 +407,7 @@ export default class Bundle {
 		timeStart( 'render modules' );
 
 		this.orderedModules.forEach( module => {
-			const source = module.render( format === 'es', this.legacy );
+			const source = module.render( options.format === 'es', this.legacy );
 
 			if ( source.toString().length ) {
 				magicString.addSource( source );
@@ -446,7 +444,7 @@ export default class Bundle {
 
 		const indentString = getIndentString( magicString, options );
 
-		const finalise = finalisers[ format ];
+		const finalise = finalisers[ options.format ];
 		if ( !finalise ) throw new Error( `You must specify an output type - valid options are ${keys( finalisers ).join( ', ' )}` );
 
 		timeStart( 'render format' );

src/rollup.js

@@ -67,7 +67,17 @@ export function rollup ( options ) {
 	return bundle.build().then( () => {
 		timeEnd( '--BUILD--' );
 
-		function generate ( options ) {
+		function generate ( options = {} ) {
+			if ( !options.format ) {
+				bundle.warn({
+					code: 'MISSING_FORMAT',
+					message: `No format option was supplied – defaulting to 'es'`,
+					url: `https://github.com/rollup/rollup/wiki/JavaScript-API#format`
+				});
+
+				options.format = 'es';
+			}
+
 			timeStart( '--GENERATE--' );
 
 			const rendered = bundle.render( options );

test/cli/sourcemap-newline/_config.js

@@ -2,7 +2,7 @@ const assert = require( 'assert' );
 
 module.exports = {
 	description: 'adds a newline after the sourceMappingURL comment (#756)',
-	command: 'rollup -i main.js -m inline',
+	command: 'rollup -i main.js -f es -m inline',
 	result: code => {
 		assert.equal( code.slice( -1 ), '\n' );
 	}

test/test.js

@@ -133,6 +133,25 @@ describe( 'rollup', function () {
 				assert.ok( code[ code.length - 1 ] === '\n' );
 			});
 		});
+
+		it( 'warns on missing format option', () => {
+			const warnings = [];
+
+			return rollup.rollup({
+				entry: 'x',
+				plugins: [ loader({ x: `console.log( 42 );` }) ],
+				onwarn: warning => warnings.push( warning )
+			}).then( bundle => {
+				bundle.generate();
+				compareWarnings( warnings, [
+					{
+						code: 'MISSING_FORMAT',
+						message: `No format option was supplied – defaulting to 'es'`,
+						url: `https://github.com/rollup/rollup/wiki/JavaScript-API#format`
+					}
+				]);
+			});
+		});
 	});
 
 	describe( 'bundle.write()', () => {