prevent `var undefined = sideEffectyDefault(foo)`

src/Module.js

@@ -636,6 +636,12 @@ export default class Module {
 						return;
 					}
 
+					// prevent `var undefined = sideEffectyDefault(foo)`
+					if ( canonicalName === undefined ) {
+						magicString.remove( statement.start, statement.node.declaration.start );
+						return;
+					}
+
 					// anonymous functions should be converted into declarations
 					if ( statement.node.declaration.type === 'FunctionExpression' ) {
 						magicString.overwrite( statement.node.start, statement.node.declaration.start + 8, `function ${canonicalName}` );

test/form/unused-default-exports/_config.js

@@ -0,0 +1,4 @@
+module.exports = {
+	description: 'does not name unused-but-included default export',
+	// solo: true
+};

test/form/unused-default-exports/_expected/amd.js

@@ -0,0 +1,14 @@
+define(function () { 'use strict';
+
+	var foo = { value: 1 };
+
+	function mutate ( obj ) {
+		obj.value += 1;
+		return obj;
+	}
+
+	mutate( foo );
+
+	assert.equal( foo.value, 2 );
+
+});

test/form/unused-default-exports/_expected/cjs.js

@@ -0,0 +1,12 @@
+'use strict';
+
+var foo = { value: 1 };
+
+function mutate ( obj ) {
+	obj.value += 1;
+	return obj;
+}
+
+mutate( foo );
+
+assert.equal( foo.value, 2 );

test/form/unused-default-exports/_expected/es6.js

@@ -0,0 +1,10 @@
+var foo = { value: 1 };
+
+function mutate ( obj ) {
+	obj.value += 1;
+	return obj;
+}
+
+mutate( foo );
+
+assert.equal( foo.value, 2 );

test/form/unused-default-exports/_expected/iife.js

@@ -0,0 +1,14 @@
+(function () { 'use strict';
+
+	var foo = { value: 1 };
+
+	function mutate ( obj ) {
+		obj.value += 1;
+		return obj;
+	}
+
+	mutate( foo );
+
+	assert.equal( foo.value, 2 );
+
+})();

test/form/unused-default-exports/_expected/umd.js

@@ -0,0 +1,18 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	factory();
+}(this, function () { 'use strict';
+
+	var foo = { value: 1 };
+
+	function mutate ( obj ) {
+		obj.value += 1;
+		return obj;
+	}
+
+	mutate( foo );
+
+	assert.equal( foo.value, 2 );
+
+}));

test/form/unused-default-exports/foo.js

@@ -0,0 +1,8 @@
+export var foo = { value: 1 };
+
+function mutate ( obj ) {
+	obj.value += 1;
+	return obj;
+}
+
+export default mutate( foo );

test/form/unused-default-exports/main.js

@@ -0,0 +1,2 @@
+import { foo } from './foo';
+assert.equal( foo.value, 2 );