prevent duplicate exports from single module (#679)

src/Module.js

@@ -77,7 +77,13 @@ export default class Module {
 
 			else {
 				node.specifiers.forEach( specifier => {
-					this.reexports[ specifier.exported.name ] = {
+					const name = specifier.exported.name;
+
+					if ( this.exports[ name ] || this.reexports[ name ] ) {
+						throw new Error( `A module cannot have multiple exports with the same name ('${name}')` );
+					}
+
+					this.reexports[ name ] = {
 						start: specifier.start,
 						source,
 						localName: specifier.local.name,
@@ -93,6 +99,11 @@ export default class Module {
 		else if ( node.type === 'ExportDefaultDeclaration' ) {
 			const identifier = ( node.declaration.id && node.declaration.id.name ) || node.declaration.name;
 
+			if ( this.exports.default ) {
+				// TODO indicate location
+				throw new Error( 'A module can only have one default export' );
+			}
+
 			this.exports.default = {
 				localName: 'default',
 				identifier
@@ -129,6 +140,10 @@ export default class Module {
 					const localName = specifier.local.name;
 					const exportedName = specifier.exported.name;
 
+					if ( this.exports[ exportedName ] || this.reexports[ exportedName ] ) {
+						throw new Error( `A module cannot have multiple exports with the same name ('${exportedName}')` );
+					}
+
 					this.exports[ exportedName ] = { localName };
 				});
 			} else {

test/function/double-default-export/_config.js

@@ -0,0 +1,8 @@
+const assert = require( 'assert' );
+
+module.exports = {
+	description: 'throws on double default exports',
+	error: err => {
+		assert.equal( err.message, 'A module can only have one default export' );
+	}
+};

test/function/double-default-export/foo.js

@@ -0,0 +1,2 @@
+export default 1;
+export default 2;

test/function/double-default-export/main.js

@@ -0,0 +1,2 @@
+import foo from './foo.js';
+console.log( foo );

test/function/double-named-export/_config.js

@@ -0,0 +1,8 @@
+const assert = require( 'assert' );
+
+module.exports = {
+	description: 'throws on duplicate named exports',
+	error: err => {
+		assert.equal( err.message, `A module cannot have multiple exports with the same name ('foo')` );
+	}
+};

test/function/double-named-export/foo.js

@@ -0,0 +1,3 @@
+var foo = 1;
+export { foo };
+export { foo };

test/function/double-named-export/main.js

@@ -0,0 +1,2 @@
+import { foo } from './foo.js';
+console.log( foo );

test/function/double-named-reexport/_config.js

@@ -0,0 +1,8 @@
+const assert = require( 'assert' );
+
+module.exports = {
+	description: 'throws on duplicate named exports',
+	error: err => {
+		assert.equal( err.message, `A module cannot have multiple exports with the same name ('foo')` );
+	}
+};

test/function/double-named-reexport/bar.js

@@ -0,0 +1 @@
+export var foo = 2;

test/function/double-named-reexport/foo.js

@@ -0,0 +1,3 @@
+var foo = 1;
+export { foo };
+export { foo } from './bar.js';

test/function/double-named-reexport/main.js

@@ -0,0 +1,2 @@
+import { foo } from './foo.js';
+console.log( foo );