rewrite top-level this as undefined

src/Statement.js

@@ -41,7 +41,7 @@ export default class Statement {
 		// find references
 		const statement = this;
 		let { module, references, scope, stringLiteralRanges } = this;
-		let readDepth = 0;
+		let contextDepth = 0;
 
 		walk( this.node, {
 			enter ( node, parent, prop ) {
@@ -59,8 +59,12 @@ export default class Statement {
 					stringLiteralRanges.push([ node.start + 1, node.end - 1 ]);
 				}
 
+				if ( node.type === 'ThisExpression' && contextDepth === 0 ) {
+					module.magicString.overwrite( node.start, node.end, 'undefined' );
+				}
+
 				if ( node._scope ) scope = node._scope;
-				if ( /Function/.test( node.type ) ) readDepth += 1;
+				if ( /^Function/.test( node.type ) ) contextDepth += 1;
 
 				let isReassignment;
 
@@ -121,7 +125,7 @@ export default class Statement {
 			},
 			leave ( node ) {
 				if ( node._scope ) scope = scope.parent;
-				if ( /Function/.test( node.type ) ) readDepth -= 1;
+				if ( /^Function/.test( node.type ) ) contextDepth -= 1;
 			}
 		});
 	}

test/form/this-is-undefined/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'top-level `this` expression is rewritten as `undefined`'
+};

test/form/this-is-undefined/_expected/amd.js

@@ -0,0 +1,21 @@
+define(function () { 'use strict';
+
+	const fooContext = {};
+
+	function foo () {
+		// inside a function, `this` should be untouched...
+		assert.strictEqual( this, fooContext );
+	}
+
+	const bar = () => {
+		// ...unless it's an arrow function
+		assert.strictEqual( undefined, undefined );
+	}
+
+	foo.call( fooContext );
+	bar.call( {} );
+
+	// outside a function, `this` is undefined
+	assert.strictEqual( undefined, undefined );
+
+});

test/form/this-is-undefined/_expected/cjs.js

@@ -0,0 +1,19 @@
+'use strict';
+
+const fooContext = {};
+
+function foo () {
+	// inside a function, `this` should be untouched...
+	assert.strictEqual( this, fooContext );
+}
+
+const bar = () => {
+	// ...unless it's an arrow function
+	assert.strictEqual( undefined, undefined );
+}
+
+foo.call( fooContext );
+bar.call( {} );
+
+// outside a function, `this` is undefined
+assert.strictEqual( undefined, undefined );

test/form/this-is-undefined/_expected/es6.js

@@ -0,0 +1,17 @@
+const fooContext = {};
+
+function foo () {
+	// inside a function, `this` should be untouched...
+	assert.strictEqual( this, fooContext );
+}
+
+const bar = () => {
+	// ...unless it's an arrow function
+	assert.strictEqual( undefined, undefined );
+}
+
+foo.call( fooContext );
+bar.call( {} );
+
+// outside a function, `this` is undefined
+assert.strictEqual( undefined, undefined );

test/form/this-is-undefined/_expected/iife.js

@@ -0,0 +1,22 @@
+(function () {
+	'use strict';
+
+	const fooContext = {};
+
+	function foo () {
+		// inside a function, `this` should be untouched...
+		assert.strictEqual( this, fooContext );
+	}
+
+	const bar = () => {
+		// ...unless it's an arrow function
+		assert.strictEqual( undefined, undefined );
+	}
+
+	foo.call( fooContext );
+	bar.call( {} );
+
+	// outside a function, `this` is undefined
+	assert.strictEqual( undefined, undefined );
+
+}());

test/form/this-is-undefined/_expected/umd.js

@@ -0,0 +1,25 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	(factory());
+}(this, function () { 'use strict';
+
+	const fooContext = {};
+
+	function foo () {
+		// inside a function, `this` should be untouched...
+		assert.strictEqual( this, fooContext );
+	}
+
+	const bar = () => {
+		// ...unless it's an arrow function
+		assert.strictEqual( undefined, undefined );
+	}
+
+	foo.call( fooContext );
+	bar.call( {} );
+
+	// outside a function, `this` is undefined
+	assert.strictEqual( undefined, undefined );
+
+}));

test/function/this-is-undefined/main.js → test/form/this-is-undefined/main.js

@@ -1,4 +1,4 @@
-const fooContext = {}
+const fooContext = {};
 
 function foo () {
 	// inside a function, `this` should be untouched...

test/function/this-is-undefined/_config.js

@@ -1,4 +0,0 @@
-module.exports = {
-	description: 'this at top level is undefined',
-	babel: true
-};