Merge pull request ampproject#1 from developit/transform-safari-for-s…

…hadowing

Add transform to fix Safari for shadowing bug

src/plugins/transform-safari-for-shadowing/index.js

@@ -0,0 +1,32 @@
+/**
+ * Safari ~11 (unsure of version) had a bug where variable declarations in a For statement would throw if they shadowed parameter names.
+ * This is fixed simply by renaming any declarations in the left/init part of a For* statement so they don't shadow parameters.
+ * @see https://bugs.webkit.org/show_bug.cgi?id=171041
+ * @example
+ *   e => { for (let e of []) e }   // throws
+ *   e => { for (let _e of []) _e }   // works
+ */
+
+export default ({ types: t }) => ({
+	name: 'transform-safari-for-shadowing',
+	visitor: {
+		VariableDeclarator(path) {
+			if (!path.parentPath) return;
+			const name = path.node.id.name;
+			const parent = path.parentPath.parentPath;
+			const key = path.parentPath.parentKey;
+			if (
+				// for (let e of ..) & for (let e in ..)
+				(t.isForXStatement(parent) && key==='left') ||
+                // for (let e=..; ;)
+                (t.isForStatement(parent) && key==='init')
+			) {
+				// check if there is a shadowed binding coming from a parameter
+	            const fn = path.getFunctionParent();
+				if (fn && fn.scope.hasOwnBinding(name) && fn.scope.getOwnBinding(name).kind === 'param') {
+					path.scope.rename(name);
+				}
+			}
+		}
+	}
+});

src/plugins/transform-safari-for-shadowing/index.test.js

@@ -0,0 +1,26 @@
+import { babel } from '../../../test/_util';
+import plugin from '.';
+
+const CONFIG = {
+	plugins: [
+		plugin
+	]
+};
+
+describe('transform-safari-for-shadowing', () => {
+	it('should ignore unshadowed for declarations', () => {
+		expect(babel(`()=>{for(let x of{})x;};`, CONFIG)).toEqual(`()=>{for(let x of{})x;};`);
+		expect(babel(`()=>{for(const x in[])x;};`, CONFIG)).toEqual(`()=>{for(const x in[])x;};`);
+		expect(babel(`()=>{for(let x=0;;)x;};`, CONFIG)).toEqual(`()=>{for(let x=0;;)x;};`);
+		expect(babel(`x=>{for(let y of{})y;};`, CONFIG)).toEqual(`x=>{for(let y of{})y;};`);
+		expect(babel(`x=>{for(let y of{}){let x;x;}};`, CONFIG)).toEqual(`x=>{for(let y of{}){let x;x;}};`);
+	});
+
+	it('should rename parameter-shadowed declarations within for blocks', () => {
+		expect(babel(`x=>{for(let x of{})x;};`, CONFIG)).toEqual(`x=>{for(let _x of{})_x;};`);
+		expect(babel(`x=>{for(const x in[])x;};`, CONFIG)).toEqual(`x=>{for(const _x in[])_x;};`);
+		expect(babel(`x=>{for(let x=0;;)x;};`, CONFIG)).toEqual(`x=>{for(let _x=0;;)_x;};`);
+		expect(babel(`()=>{let x;for(let x of{})x;};`, CONFIG)).toEqual(`()=>{let x;for(let x of{})x;};`);
+		expect(babel(`x=>{for(let x of{}){let x;x;}};`, CONFIG)).toEqual(`x=>{for(let _x of{}){let x;x;}};`);
+	});
+});