Do not create a temp for RHS if it's an identifier

src/compiler/emitter.ts

@@ -3523,17 +3523,26 @@ module ts {
                 
                 // Do not call create recordTempDeclaration because we are declaring the temps
                 // right here. Recording means they will be declared later.
+                // In the case where the user wrote an identifier as the RHS, like this:
+                //
+                //     for (var v of arr) { }
+                //
+                // we don't want to emit a temporary variable for the RHS, just use it directly.
+                var rhsIsIdentifier = node.expression.kind === SyntaxKind.Identifier;
                 var counter = createTempVariable(node, /*forLoopVariable*/ true);
-                var rhsReference = createTempVariable(node, /*forLoopVariable*/ false);
+                var rhsReference = rhsIsIdentifier ? <Identifier>node.expression : createTempVariable(node, /*forLoopVariable*/ false);
                 
-                // _i = 0, 
+                // _i = 0
                 emit(counter);
-                write(" = 0, ");
+                write(" = 0");
                 
-                // _a = expr;
-                emit(rhsReference);
-                write(" = ");
-                emit(node.expression);
+                if (!rhsIsIdentifier) {
+                    // , _a = expr
+                    write(", ");
+                    emit(rhsReference);
+                    write(" = ");
+                    emit(node.expression);
+                }
                 write("; ");
                 
                 // _i < _a.length;

tests/baselines/reference/ES5For-of24.js

@@ -0,0 +1,12 @@
+//// [ES5For-of24.ts]
+var a = [1, 2, 3];
+for (var v of a) {
+    let a = 0;
+}
+
+//// [ES5For-of24.js]
+var a = [1, 2, 3];
+for (var _i = 0; _i < a.length; _i++) {
+    var v = a[_i];
+    var _a = 0;
+}

tests/baselines/reference/ES5For-of25.js

@@ -0,0 +1,14 @@
+//// [ES5For-of25.ts]
+var a = [1, 2, 3];
+for (var v of a) {
+    v;
+    a;
+}
+
+//// [ES5For-of25.js]
+var a = [1, 2, 3];
+for (var _i = 0; _i < a.length; _i++) {
+    var v = a[_i];
+    v;
+    a;
+}

tests/baselines/reference/parserES5ForOfStatement10.js

@@ -3,6 +3,6 @@ for (const v of X) {
 }
 
 //// [parserES5ForOfStatement10.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var v = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var v = X[_i];
 }

tests/baselines/reference/parserES5ForOfStatement11.js

@@ -3,6 +3,6 @@ for (const [a, b] of X) {
 }
 
 //// [parserES5ForOfStatement11.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var _b = _a[_i], a = _b[0], b = _b[1];
+for (var _i = 0; _i < X.length; _i++) {
+    var _a = X[_i], a = _a[0], b = _a[1];
 }

tests/baselines/reference/parserES5ForOfStatement12.js

@@ -3,6 +3,6 @@ for (const {a, b} of X) {
 }
 
 //// [parserES5ForOfStatement12.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var _b = _a[_i], a = _b.a, b = _b.b;
+for (var _i = 0; _i < X.length; _i++) {
+    var _a = X[_i], a = _a.a, b = _a.b;
 }

tests/baselines/reference/parserES5ForOfStatement13.js

@@ -3,6 +3,6 @@ for (let {a, b} of X) {
 }
 
 //// [parserES5ForOfStatement13.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var _b = _a[_i], a = _b.a, b = _b.b;
+for (var _i = 0; _i < X.length; _i++) {
+    var _a = X[_i], a = _a.a, b = _a.b;
 }

tests/baselines/reference/parserES5ForOfStatement14.js

@@ -3,6 +3,6 @@ for (let [a, b] of X) {
 }
 
 //// [parserES5ForOfStatement14.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var _b = _a[_i], a = _b[0], b = _b[1];
+for (var _i = 0; _i < X.length; _i++) {
+    var _a = X[_i], a = _a[0], b = _a[1];
 }

tests/baselines/reference/parserES5ForOfStatement15.js

@@ -3,6 +3,6 @@ for (var [a, b] of X) {
 }
 
 //// [parserES5ForOfStatement15.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var _b = _a[_i], a = _b[0], b = _b[1];
+for (var _i = 0; _i < X.length; _i++) {
+    var _a = X[_i], a = _a[0], b = _a[1];
 }

tests/baselines/reference/parserES5ForOfStatement16.js

@@ -3,6 +3,6 @@ for (var {a, b} of X) {
 }
 
 //// [parserES5ForOfStatement16.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var _b = _a[_i], a = _b.a, b = _b.b;
+for (var _i = 0; _i < X.length; _i++) {
+    var _a = X[_i], a = _a.a, b = _a.b;
 }

tests/baselines/reference/parserES5ForOfStatement18.js

@@ -2,6 +2,6 @@
 for (var of of of) { }
 
 //// [parserES5ForOfStatement18.js]
-for (var _i = 0, _a = of; _i < _a.length; _i++) {
-    var of = _a[_i];
+for (var _i = 0; _i < of.length; _i++) {
+    var of = of[_i];
 }

tests/baselines/reference/parserES5ForOfStatement2.js

@@ -3,6 +3,6 @@ for (var of X) {
 }
 
 //// [parserES5ForOfStatement2.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var _b = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var _a = X[_i];
 }

tests/baselines/reference/parserES5ForOfStatement21.js

@@ -2,6 +2,6 @@
 for (var of of) { }
 
 //// [parserES5ForOfStatement21.js]
-for (var _i = 0, _a = of; _i < _a.length; _i++) {
-    var _b = _a[_i];
+for (var _i = 0; _i < of.length; _i++) {
+    var _a = of[_i];
 }

tests/baselines/reference/parserES5ForOfStatement3.js

@@ -3,6 +3,6 @@ for (var a, b of X) {
 }
 
 //// [parserES5ForOfStatement3.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var a = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var a = X[_i];
 }

tests/baselines/reference/parserES5ForOfStatement4.js

@@ -3,6 +3,6 @@ for (var a = 1 of X) {
 }
 
 //// [parserES5ForOfStatement4.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var a = 1 = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var a = 1 = X[_i];
 }

tests/baselines/reference/parserES5ForOfStatement5.js

@@ -3,6 +3,6 @@ for (var a: number of X) {
 }
 
 //// [parserES5ForOfStatement5.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var a = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var a = X[_i];
 }

tests/baselines/reference/parserES5ForOfStatement6.js

@@ -3,6 +3,6 @@ for (var a = 1, b = 2 of X) {
 }
 
 //// [parserES5ForOfStatement6.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var a = 1 = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var a = 1 = X[_i];
 }

tests/baselines/reference/parserES5ForOfStatement7.js

@@ -3,6 +3,6 @@ for (var a: number = 1, b: string = "" of X) {
 }
 
 //// [parserES5ForOfStatement7.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var a = 1 = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var a = 1 = X[_i];
 }

tests/baselines/reference/parserES5ForOfStatement8.js

@@ -3,6 +3,6 @@ for (var v of X) {
 }
 
 //// [parserES5ForOfStatement8.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var v = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var v = X[_i];
 }

tests/baselines/reference/parserES5ForOfStatement9.js

@@ -3,6 +3,6 @@ for (let v of X) {
 }
 
 //// [parserES5ForOfStatement9.js]
-for (var _i = 0, _a = X; _i < _a.length; _i++) {
-    var v = _a[_i];
+for (var _i = 0; _i < X.length; _i++) {
+    var v = X[_i];
 }

tests/cases/conformance/statements/for-ofStatements/ES5For-of24.ts

@@ -0,0 +1,4 @@
+var a = [1, 2, 3];
+for (var v of a) {
+    let a = 0;
+}

tests/cases/conformance/statements/for-ofStatements/ES5For-of25.ts

@@ -0,0 +1,5 @@
+var a = [1, 2, 3];
+for (var v of a) {
+    v;
+    a;
+}