fix #2347: strip "use strict" when targeting ESM

CHANGELOG.md

@@ -13,6 +13,22 @@
     * `.woff` => `font/woff`
     * `.woff2` => `font/woff2`
 
+* Remove `"use strict";` when targeting ESM ([#2347](https://github.com/evanw/esbuild/issues/2347))
+
+    All ES module code is automatically in strict mode, so a `"use strict";` directive is unnecessary. With this release, esbuild will now remove the `"use strict";` directive if the output format is ESM. This change makes the generated output file a few bytes smaller:
+
+    ```js
+    // Original code
+    'use strict'
+    export let foo = 123
+
+    // Old output (with --format=esm --minify)
+    "use strict";let t=123;export{t as foo};
+
+    // New output (with --format=esm --minify)
+    let t=123;export{t as foo};
+    ```
+
 ## 0.14.47
 
 * Make global names more compact when `||=` is available ([#2331](https://github.com/evanw/esbuild/issues/2331))

internal/bundler/bundler_default_test.go

@@ -2788,7 +2788,41 @@ func TestUseStrictDirectiveBundleIssue1837(t *testing.T) {
 	})
 }
 
-func TestUseStrictDirectiveBundleIssue2264(t *testing.T) {
+func TestUseStrictDirectiveBundleIIFEIssue2264(t *testing.T) {
+	default_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				'use strict'
+				export let a = 1
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			AbsOutputFile: "/out.js",
+			OutputFormat:  config.FormatIIFE,
+		},
+	})
+}
+
+func TestUseStrictDirectiveBundleCJSIssue2264(t *testing.T) {
+	default_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				'use strict'
+				export let a = 1
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			AbsOutputFile: "/out.js",
+			OutputFormat:  config.FormatCommonJS,
+		},
+	})
+}
+
+func TestUseStrictDirectiveBundleESMIssue2264(t *testing.T) {
 	default_suite.expectBundled(t, bundled{
 		files: map[string]string{
 			"/entry.js": `

internal/bundler/bundler_tsconfig_test.go

@@ -1837,7 +1837,7 @@ func TestTsConfigAlwaysStrictTrueEmitDirectiveFormat(t *testing.T) {
 	})
 }
 
-func TestTsConfigAlwaysStrictTrueEmitDirectiveBundle(t *testing.T) {
+func TestTsConfigAlwaysStrictTrueEmitDirectiveBundleIIFE(t *testing.T) {
 	tsconfig_suite.expectBundled(t, bundled{
 		files: map[string]string{
 			"/Users/user/project/src/implicit.ts": `
@@ -1860,6 +1860,63 @@ func TestTsConfigAlwaysStrictTrueEmitDirectiveBundle(t *testing.T) {
 		options: config.Options{
 			Mode:         config.ModeBundle,
 			AbsOutputDir: "/Users/user/project/out",
+			OutputFormat: config.FormatIIFE,
+		},
+	})
+}
+
+func TestTsConfigAlwaysStrictTrueEmitDirectiveBundleCJS(t *testing.T) {
+	tsconfig_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/Users/user/project/src/implicit.ts": `
+				console.log('this file should start with "use strict"')
+			`,
+			"/Users/user/project/src/explicit.ts": `
+				'use strict'
+				console.log('this file should start with "use strict"')
+			`,
+			"/Users/user/project/tsconfig.json": `{
+				"compilerOptions": {
+					"alwaysStrict": true
+				}
+			}`,
+		},
+		entryPaths: []string{
+			"/Users/user/project/src/implicit.ts",
+			"/Users/user/project/src/explicit.ts",
+		},
+		options: config.Options{
+			Mode:         config.ModeBundle,
+			AbsOutputDir: "/Users/user/project/out",
+			OutputFormat: config.FormatCommonJS,
+		},
+	})
+}
+
+func TestTsConfigAlwaysStrictTrueEmitDirectiveBundleESM(t *testing.T) {
+	tsconfig_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/Users/user/project/src/implicit.ts": `
+				console.log('this file should not start with "use strict"')
+			`,
+			"/Users/user/project/src/explicit.ts": `
+				'use strict'
+				console.log('this file should not start with "use strict"')
+			`,
+			"/Users/user/project/tsconfig.json": `{
+				"compilerOptions": {
+					"alwaysStrict": true
+				}
+			}`,
+		},
+		entryPaths: []string{
+			"/Users/user/project/src/implicit.ts",
+			"/Users/user/project/src/explicit.ts",
+		},
+		options: config.Options{
+			Mode:         config.ModeBundle,
+			AbsOutputDir: "/Users/user/project/out",
+			OutputFormat: config.FormatESModule,
 		},
 	})
 }

internal/bundler/linker.go

@@ -4871,8 +4871,9 @@ func (c *linkerContext) generateChunkJS(chunks []chunkInfo, chunkIndex int, chun
 			isExecutable = true
 		}
 
-		// Add the top-level directive if present
-		if repr.AST.Directive != "" {
+		// Add the top-level directive if present (but omit "use strict" in ES
+		// modules because all ES modules are automatically in strict mode)
+		if repr.AST.Directive != "" && (repr.AST.Directive != "use strict" || c.options.OutputFormat != config.FormatESModule) {
 			quoted := string(js_printer.QuoteForJSON(repr.AST.Directive, c.options.ASCIIOnly)) + ";" + newline
 			prevOffset.AdvanceString(quoted)
 			j.AddString(quoted)

internal/bundler/snapshots/snapshots_default.txt

@@ -3927,6 +3927,37 @@ await foo;
 for await (foo of bar)
   ;
 
+================================================================================
+TestUseStrictDirectiveBundleCJSIssue2264
+---------- /out.js ----------
+"use strict";
+
+// entry.js
+var entry_exports = {};
+__export(entry_exports, {
+  a: () => a
+});
+module.exports = __toCommonJS(entry_exports);
+var a = 1;
+
+================================================================================
+TestUseStrictDirectiveBundleESMIssue2264
+---------- /out.js ----------
+// entry.js
+var a = 1;
+export {
+  a
+};
+
+================================================================================
+TestUseStrictDirectiveBundleIIFEIssue2264
+---------- /out.js ----------
+"use strict";
+(() => {
+  // entry.js
+  var a = 1;
+})();
+
 ================================================================================
 TestUseStrictDirectiveBundleIssue1837
 ---------- /out.js ----------
@@ -3953,17 +3984,6 @@ TestUseStrictDirectiveBundleIssue1837
   console.log(require_cjs());
 })();
 
-================================================================================
-TestUseStrictDirectiveBundleIssue2264
----------- /out.js ----------
-"use strict";
-
-// entry.js
-var a = 1;
-export {
-  a
-};
-
 ================================================================================
 TestUseStrictDirectiveMinifyNoBundle
 ---------- /out.js ----------

internal/bundler/snapshots/snapshots_tsconfig.txt

@@ -14,7 +14,7 @@ var import_util = __toESM(require_util());
 console.log((0, import_util.default)());
 
 ================================================================================
-TestTsConfigAlwaysStrictTrueEmitDirectiveBundle
+TestTsConfigAlwaysStrictTrueEmitDirectiveBundleCJS
 ---------- /Users/user/project/out/implicit.js ----------
 "use strict";
 
@@ -27,6 +27,32 @@ console.log('this file should start with "use strict"');
 // Users/user/project/src/explicit.ts
 console.log('this file should start with "use strict"');
 
+================================================================================
+TestTsConfigAlwaysStrictTrueEmitDirectiveBundleESM
+---------- /Users/user/project/out/implicit.js ----------
+// Users/user/project/src/implicit.ts
+console.log('this file should not start with "use strict"');
+
+---------- /Users/user/project/out/explicit.js ----------
+// Users/user/project/src/explicit.ts
+console.log('this file should not start with "use strict"');
+
+================================================================================
+TestTsConfigAlwaysStrictTrueEmitDirectiveBundleIIFE
+---------- /Users/user/project/out/implicit.js ----------
+"use strict";
+(() => {
+  // Users/user/project/src/implicit.ts
+  console.log('this file should start with "use strict"');
+})();
+
+---------- /Users/user/project/out/explicit.js ----------
+"use strict";
+(() => {
+  // Users/user/project/src/explicit.ts
+  console.log('this file should start with "use strict"');
+})();
+
 ================================================================================
 TestTsConfigAlwaysStrictTrueEmitDirectiveFormat
 ---------- /Users/user/project/out/implicit.js ----------