Use more efficient loops for const bindings

docs/999-big-list-of-options.md

@@ -473,7 +473,36 @@ Whether to use arrow functions for auto-generated code snippets. Note that in ce
 
 **output.generatedCode.constBindings**<br> Type: `boolean`<br> CLI: `--generatedCode.constBindings`/`--no-generatedCode.constBindings`<br> Default: `false`
 
-This will use `const` instead of `var` in certain places and helper functions. Depending on the engine, this can provide [marginally better performance](https://benediktmeurer.de/2017/06/29/javascript-optimization-patterns-part2) in optimized machine code.
+This will use `const` instead of `var` in certain places and helper functions. Depending on the engine, this can provide [marginally better performance](https://benediktmeurer.de/2017/06/29/javascript-optimization-patterns-part2) in optimized machine code. It will also allow Rollup to generate more efficient helpers due to block scoping.
+
+```js
+// input
+export * from 'external';
+
+// cjs output with constBindings: false
+var external = require('external');
+
+Object.keys(external).forEach(function (k) {
+  if (k !== 'default' && !exports.hasOwnProperty(k))
+    Object.defineProperty(exports, k, {
+      enumerable: true,
+      get: function () {
+        return external[k];
+      }
+    });
+});
+
+// cjs output with constBindings: true
+const external = require('external');
+
+for (const k in external) {
+  if (k !== 'default' && !exports.hasOwnProperty(k))
+    Object.defineProperty(exports, k, {
+      enumerable: true,
+      get: () => external[k]
+    });
+}
+```
 
 **output.generatedCode.objectShorthand**<br> Type: `boolean`<br> CLI: `--generatedCode.objectShorthand`/`--no-generatedCode.objectShorthand`<br> Default: `false`
 
@@ -1346,8 +1375,6 @@ Example:
 export { x } from 'external';
 
 // CJS output with externalLiveBindings: true
-('use strict');
-
 Object.defineProperty(exports, '__esModule', { value: true });
 
 var external = require('external');
@@ -1360,8 +1387,6 @@ Object.defineProperty(exports, 'x', {
 });
 
 // CJS output with externalLiveBindings: false
-('use strict');
-
 Object.defineProperty(exports, '__esModule', { value: true });
 
 var external = require('external');

src/finalisers/shared/getExportBlock.ts

@@ -14,12 +14,13 @@ export function getExportBlock(
 	interop: GetInterop,
 	{
 		_,
-		s,
+		cnst,
 		directReturnFunctionRight,
 		getDirectReturnFunctionLeft,
 		getFunctionIntro,
 		getPropertyAccess,
-		n
+		n,
+		s
 	}: GenerateCodeSnippets,
 	t: string,
 	externalLiveBindings: boolean,
@@ -98,12 +99,14 @@ export function getExportBlock(
 								name: null
 						  })}${name}[k]${directReturnFunctionRight}${n}${t}})`
 						: `exports[k]${_}=${_}${name}[k]`;
+					const copyPropertyIfNecessary = `{${n}${t}if${_}(k${_}!==${_}'default'${_}&&${_}!exports.hasOwnProperty(k))${_}${defineProperty}${s}${n}}`;
 					exportBlock +=
-						`Object.keys(${name}).forEach(${getFunctionIntro(['k'], {
-							isAsync: false,
-							name: null
-						})}{${n}` +
-						`${t}if${_}(k${_}!==${_}'default'${_}&&${_}!exports.hasOwnProperty(k))${_}${defineProperty}${s}${n}});`;
+						cnst === 'var' && specifier.needsLiveBinding
+							? `Object.keys(${name}).forEach(${getFunctionIntro(['k'], {
+									isAsync: false,
+									name: null
+							  })}${copyPropertyIfNecessary});`
+							: `for${_}(${cnst} k in ${name})${_}${copyPropertyIfNecessary}`;
 				}
 			}
 		}

src/utils/interopHelpers.ts

@@ -153,6 +153,7 @@ const HELPER_GENERATORS: {
 	[MERGE_NAMESPACES_VARIABLE](t, snippets, liveBindings, freeze) {
 		const { _, directReturnFunctionRight, getDirectReturnFunctionLeft, getFunctionIntro, n } =
 			snippets;
+		// TODO Lukas use more efficient loops if we can use const
 		return (
 			`function ${MERGE_NAMESPACES_VARIABLE}(n, m)${_}{${n}` +
 			`${t}m.forEach(${getDirectReturnFunctionLeft(['e'], {
@@ -186,30 +187,41 @@ const createNamespaceObject = (
 	freeze: boolean,
 	namespaceToStringTag: boolean
 ) => {
-	const { _, cnst, getFunctionIntro, getPropertyAccess, n } = snippets;
+	const { _, cnst, getPropertyAccess, n, s } = snippets;
+	const copyProperty =
+		`{${n}` +
+		(liveBindings ? copyNonDefaultPropertyLiveBinding : copyPropertyStatic)(
+			t,
+			i + t + t,
+			snippets
+		) +
+		`${i}${t}}`;
 	return (
 		`${i}${cnst} n${_}=${_}${
 			namespaceToStringTag
 				? `{__proto__:${_}null,${_}[Symbol.toStringTag]:${_}'Module'}`
 				: 'Object.create(null)'
 		};${n}` +
 		`${i}if${_}(e)${_}{${n}` +
-		`${i}${t}Object.keys(e).forEach(${getFunctionIntro(['k'], {
-			isAsync: false,
-			name: null
-		})}{${n}` +
-		(liveBindings ? copyNonDefaultPropertyLiveBinding : copyPropertyStatic)(
-			t,
-			i + t + t,
-			snippets
-		) +
-		`${i}${t}});${n}` +
+		`${i}${t}${loopOverKeys(copyProperty, !liveBindings, snippets)}${n}` +
 		`${i}}${n}` +
 		`${i}n${getPropertyAccess('default')}${_}=${_}e;${n}` +
-		`${i}return ${getFrozen('n', freeze)};${n}`
+		`${i}return ${getFrozen('n', freeze)}${s}${n}`
 	);
 };
 
+const loopOverKeys = (
+	body: string,
+	allowVarLoopVariable: boolean,
+	{ _, cnst, getFunctionIntro, s }: GenerateCodeSnippets
+) =>
+	cnst !== 'var' || allowVarLoopVariable
+		? `for${_}(${cnst} k in e)${_}${body}`
+		: `Object.keys(e).forEach(${getFunctionIntro(['k'], {
+				isAsync: false,
+				name: null
+		  })}${body})${s}`;
+
 const copyNonDefaultPropertyLiveBinding = (
 	t: string,
 	i: string,

test/form/samples/deprecated/interop-per-dependency-no-live-binding-boolean/_expected.js

@@ -9,9 +9,9 @@ function _interopNamespace(e) {
 	if (e && e.__esModule) return e;
 	var n = Object.create(null);
 	if (e) {
-		Object.keys(e).forEach(function (k) {
+		for (var k in e) {
 			n[k] = e[k];
-		});
+		}
 	}
 	n["default"] = e;
 	return Object.freeze(n);