use an interopRequire function for external CJS deps with a default export

src/finalisers/cjs.js

@@ -3,17 +3,24 @@ import getExportBlock from './shared/getExportBlock.js';
 export default function cjs ( bundle, magicString, { exportMode }, options ) {
 	let intro = options.useStrict === false ? `` : `'use strict';\n\n`;
 
+	const hasDefaultImport = bundle.externalModules.some( mod => mod.declarations.default);
+
+	if (hasDefaultImport) {
+		intro += `function _interopRequire (id) { var ex = require(id); return 'default' in ex ? ex['default'] : ex; }\n\n`;
+	}
+
 	// TODO handle empty imports, once they're supported
 	const importBlock = bundle.externalModules
 		.map( module => {
-			let requireStatement = `var ${module.name} = require('${module.id}');`;
-
 			if ( module.declarations.default ) {
-				requireStatement += '\n' + ( module.exportsNames ? `var ${module.name}__default = ` : `${module.name} = ` ) +
-					`'default' in ${module.name} ? ${module.name}['default'] : ${module.name};`;
+				let importStatement = `var ${module.name} = _interopRequire('${module.id}');`;
+				if (module.exportsNames) {
+					importStatement += `\nvar ${module.name}__default = ${module.name};`;
+				}
+				return importStatement;
+			} else {
+				return `var ${module.name} = require('${module.id}');`;
 			}
-
-			return requireStatement;
 		})
 		.join( '\n' );
 

test/form/external-imports-custom-names/_expected/cjs.js

@@ -1,7 +1,8 @@
 'use strict';
 
-var $ = require('jquery');
-$ = 'default' in $ ? $['default'] : $;
+function _interopRequire (id) { var ex = require(id); return 'default' in ex ? ex['default'] : ex; }
+
+var $ = _interopRequire('jquery');
 
 $( function () {
 	$( 'body' ).html( '<h1>hello world!</h1>' );

test/form/external-imports/_expected/cjs.js

@@ -1,11 +1,12 @@
 'use strict';
 
-var factory = require('factory');
-factory = 'default' in factory ? factory['default'] : factory;
+function _interopRequire (id) { var ex = require(id); return 'default' in ex ? ex['default'] : ex; }
+
+var factory = _interopRequire('factory');
 var baz = require('baz');
 var containers = require('shipping-port');
-var alphabet = require('alphabet');
-var alphabet__default = 'default' in alphabet ? alphabet['default'] : alphabet;
+var alphabet = _interopRequire('alphabet');
+var alphabet__default = alphabet;
 
 factory( null );
 baz.foo( baz.bar, containers.port );