enable tree-shaking in transform mode when both --minify and --format flags are specified

CHANGELOG.md

@@ -1,5 +1,17 @@
 # Changelog
 
+## Unreleased
+
+* Enable tree-shaking in transform mode when target format is specified ([#1610](https://github.com/evanw/esbuild/issues/1610))
+
+    Previously esbuild does not perform tree-shaking when transforming single files because esbuild cannot assume whether the file will be concatenated with other files, and thus it would be unsafe to remove unused bindings.
+
+    However, when an output format is explicitly specified, it implies the file is a module so tree-shaking can be safely applied. In addition, the previous behavior when both `--minify` and `--format` are specified was already concatenation-unsafe, since esbuild renames top-level bindings in this case.
+
+    Before this release, there was no API that allows the user to force tree-shaking when transforming a single file. As of this release, esbuild will now enable tree-shaking when transforming a single file with both `--minify` and `--format` specified. This behavior can also be individually enabled with the `--minify-unused` flag.
+
+    This improvement is important for tools that intend to use esbuild as a non-bundling minifier to replace Terser - for example, a Vite or Rollup plugin that minifies ESM chunks produced by Rollup.
+
 ## 0.12.28
 
 * Fix U+30FB and U+FF65 in identifier names in ES5 vs. ES6+ ([#1599](https://github.com/evanw/esbuild/issues/1599))

internal/bundler/bundler_dce_test.go

@@ -1630,6 +1630,63 @@ func TestTreeShakingNoBundleIIFE(t *testing.T) {
 	})
 }
 
+func TestTreeShakingNoBundleMinifyESM(t *testing.T) {
+	dce_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				function keep() {}
+				function unused() {}
+				keep()
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeConvertFormat,
+			OutputFormat:  config.FormatESModule,
+			RemoveUnused:  true,
+			AbsOutputFile: "/out.js",
+		},
+	})
+}
+
+func TestTreeShakingNoBundleMinifyIIFE(t *testing.T) {
+	dce_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				function keep() {}
+				function unused() {}
+				keep()
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeConvertFormat,
+			OutputFormat:  config.FormatIIFE,
+			RemoveUnused:  true,
+			AbsOutputFile: "/out.js",
+		},
+	})
+}
+
+func TestTreeShakingNoBundleMinifyCJS(t *testing.T) {
+	dce_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				function keep() {}
+				function unused() {}
+				keep()
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeConvertFormat,
+			OutputFormat:  config.FormatCommonJS,
+			RemoveUnused:  true,
+			AbsOutputFile: "/out.js",
+		},
+	})
+}
+
 func TestTreeShakingInESMWrapper(t *testing.T) {
 	dce_suite.expectBundled(t, bundled{
 		files: map[string]string{

internal/bundler/linker.go

@@ -2571,7 +2571,7 @@ func (c *linkerContext) markFileLiveForTreeShaking(sourceIndex uint32) {
 
 	switch repr := file.InputFile.Repr.(type) {
 	case *graph.JSRepr:
-		isTreeShakingEnabled := config.IsTreeShakingEnabled(c.options.Mode, c.options.OutputFormat)
+		isTreeShakingEnabled := config.IsTreeShakingEnabled(c.options.Mode, c.options.OutputFormat, c.options.RemoveUnused)
 
 		// If the JavaScript stub for a CSS file is included, also include the CSS file
 		if repr.CSSSourceIndex.IsValid() {

internal/bundler/snapshots/snapshots_dce.txt

@@ -720,6 +720,29 @@ TestTreeShakingNoBundleIIFE
   keep();
 })();
 
+================================================================================
+TestTreeShakingNoBundleMinifyCJS
+---------- /out.js ----------
+function keep() {
+}
+keep();
+
+================================================================================
+TestTreeShakingNoBundleMinifyESM
+---------- /out.js ----------
+function keep() {
+}
+keep();
+
+================================================================================
+TestTreeShakingNoBundleMinifyIIFE
+---------- /out.js ----------
+(() => {
+  function keep() {
+  }
+  keep();
+})();
+
 ================================================================================
 TestTreeShakingReactElements
 ---------- /out.js ----------

internal/config/config.go

@@ -206,6 +206,7 @@ type Options struct {
 	RemoveWhitespace  bool
 	MinifyIdentifiers bool
 	MangleSyntax      bool
+	RemoveUnused      bool
 	ProfilerNames     bool
 	CodeSplitting     bool
 	WatchMode         bool
@@ -384,8 +385,8 @@ func SubstituteTemplate(template []PathTemplate, placeholders PathPlaceholders)
 	return result
 }
 
-func IsTreeShakingEnabled(mode Mode, outputFormat Format) bool {
-	return mode == ModeBundle || (mode == ModeConvertFormat && outputFormat == FormatIIFE)
+func IsTreeShakingEnabled(mode Mode, outputFormat Format, minifyUnused bool) bool {
+	return mode == ModeBundle || (mode == ModeConvertFormat && (outputFormat == FormatIIFE || minifyUnused))
 }
 
 func ShouldCallRuntimeRequire(mode Mode, outputFormat Format) bool {

internal/js_parser/js_parser.go

@@ -334,6 +334,7 @@ type optionsThatSupportStructuralEquality struct {
 	keepNames               bool
 	mangleSyntax            bool
 	minifyIdentifiers       bool
+	removeUnused            bool
 	omitRuntimeForTests     bool
 	ignoreDCEAnnotations    bool
 	preserveUnusedImportsTS bool
@@ -359,6 +360,7 @@ func OptionsFromConfig(options *config.Options) Options {
 			keepNames:               options.KeepNames,
 			mangleSyntax:            options.MangleSyntax,
 			minifyIdentifiers:       options.MinifyIdentifiers,
+			removeUnused:            options.RemoveUnused,
 			omitRuntimeForTests:     options.OmitRuntimeForTests,
 			ignoreDCEAnnotations:    options.IgnoreDCEAnnotations,
 			preserveUnusedImportsTS: options.PreserveUnusedImportsTS,
@@ -13830,7 +13832,7 @@ func Parse(log logger.Log, source logger.Source, options Options) (result js_ast
 	// single pass, but it turns out it's pretty much impossible to do this
 	// correctly while handling arrow functions because of the grammar
 	// ambiguities.
-	if !config.IsTreeShakingEnabled(p.options.mode, p.options.outputFormat) {
+	if !config.IsTreeShakingEnabled(p.options.mode, p.options.outputFormat, p.options.removeUnused) {
 		// When not bundling, everything comes in a single part
 		parts = p.appendPart(parts, stmts)
 	} else {

lib/shared/common.ts

@@ -107,6 +107,7 @@ function pushCommonFlags(flags: string[], options: CommonOptions, keys: OptionKe
   let minifySyntax = getFlag(options, keys, 'minifySyntax', mustBeBoolean);
   let minifyWhitespace = getFlag(options, keys, 'minifyWhitespace', mustBeBoolean);
   let minifyIdentifiers = getFlag(options, keys, 'minifyIdentifiers', mustBeBoolean);
+  let minifyUnused = getFlag(options, keys, 'minifyUnused', mustBeBoolean);
   let charset = getFlag(options, keys, 'charset', mustBeString);
   let treeShaking = getFlag(options, keys, 'treeShaking', mustBeStringOrBoolean);
   let jsx = getFlag(options, keys, 'jsx', mustBeString);
@@ -130,6 +131,7 @@ function pushCommonFlags(flags: string[], options: CommonOptions, keys: OptionKe
   if (minifySyntax) flags.push('--minify-syntax');
   if (minifyWhitespace) flags.push('--minify-whitespace');
   if (minifyIdentifiers) flags.push('--minify-identifiers');
+  if (minifyUnused) flags.push('--minify-unused');
   if (charset) flags.push(`--charset=${charset}`);
   if (treeShaking !== void 0 && treeShaking !== true) flags.push(`--tree-shaking=${treeShaking}`);
 

lib/shared/types.ts

@@ -19,6 +19,7 @@ interface CommonOptions {
   minifyWhitespace?: boolean;
   minifyIdentifiers?: boolean;
   minifySyntax?: boolean;
+  minifyUnused?: boolean;
   charset?: Charset;
   treeShaking?: TreeShaking;
 

pkg/api/api.go

@@ -256,6 +256,7 @@ type BuildOptions struct {
 	MinifyWhitespace  bool
 	MinifyIdentifiers bool
 	MinifySyntax      bool
+	MinifyUnused      bool
 	Charset           Charset
 	TreeShaking       TreeShaking
 	LegalComments     LegalComments
@@ -364,6 +365,7 @@ type TransformOptions struct {
 	MinifyWhitespace  bool
 	MinifyIdentifiers bool
 	MinifySyntax      bool
+	MinifyUnused      bool
 	Charset           Charset
 	TreeShaking       TreeShaking
 	LegalComments     LegalComments

pkg/api/api_impl.go

@@ -804,7 +804,7 @@ func rebuildImpl(
 	outJS, outCSS := validateOutputExtensions(log, buildOpts.OutExtensions)
 	bannerJS, bannerCSS := validateBannerOrFooter(log, "banner", buildOpts.Banner)
 	footerJS, footerCSS := validateBannerOrFooter(log, "footer", buildOpts.Footer)
-	minify := buildOpts.MinifyWhitespace && buildOpts.MinifyIdentifiers && buildOpts.MinifySyntax
+	minify := buildOpts.MinifyWhitespace && buildOpts.MinifyIdentifiers && buildOpts.MinifySyntax && buildOpts.MinifyUnused
 	defines, injectedDefines := validateDefines(log, buildOpts.Define, buildOpts.Pure, buildOpts.Platform, minify)
 	options := config.Options{
 		IsTargetUnconfigured:   isTargetUnconfigured,
@@ -826,6 +826,7 @@ func rebuildImpl(
 		MangleSyntax:          buildOpts.MinifySyntax,
 		RemoveWhitespace:      buildOpts.MinifyWhitespace,
 		MinifyIdentifiers:     buildOpts.MinifyIdentifiers,
+		RemoveUnused:          buildOpts.MinifyUnused,
 		AllowOverwrite:        buildOpts.AllowOverwrite,
 		ASCIIOnly:             validateASCIIOnly(buildOpts.Charset),
 		IgnoreDCEAnnotations:  validateIgnoreDCEAnnotations(buildOpts.TreeShaking),
@@ -1310,6 +1311,7 @@ func transformImpl(input string, transformOpts TransformOptions) TransformResult
 		MangleSyntax:            transformOpts.MinifySyntax,
 		RemoveWhitespace:        transformOpts.MinifyWhitespace,
 		MinifyIdentifiers:       transformOpts.MinifyIdentifiers,
+		RemoveUnused:            transformOpts.MinifyUnused,
 		ASCIIOnly:               validateASCIIOnly(transformOpts.Charset),
 		IgnoreDCEAnnotations:    validateIgnoreDCEAnnotations(transformOpts.TreeShaking),
 		AbsOutputFile:           transformOpts.Sourcefile + "-out",

pkg/cli/cli_impl.go

@@ -68,10 +68,12 @@ func parseOptionsImpl(
 				buildOpts.MinifySyntax = true
 				buildOpts.MinifyWhitespace = true
 				buildOpts.MinifyIdentifiers = true
+				buildOpts.MinifyUnused = true
 			} else {
 				transformOpts.MinifySyntax = true
 				transformOpts.MinifyWhitespace = true
 				transformOpts.MinifyIdentifiers = true
+				transformOpts.MinifyUnused = true
 			}
 
 		case arg == "--minify-syntax":
@@ -95,6 +97,13 @@ func parseOptionsImpl(
 				transformOpts.MinifyIdentifiers = true
 			}
 
+		case arg == "--minify-unused":
+			if buildOpts != nil {
+				buildOpts.MinifyUnused = true
+			} else {
+				transformOpts.MinifyUnused = true
+			}
+
 		case strings.HasPrefix(arg, "--legal-comments="):
 			value := arg[len("--legal-comments="):]
 			var legalComments api.LegalComments

scripts/js-api-tests.js

@@ -3231,6 +3231,21 @@ let transformTests = {
     assert.strictEqual(code, `fn(), React.createElement("div", null);\n`)
   },
 
+  async treeShakingMinifyWithFormat({ esbuild }) {
+    const { code } = await esbuild.transform(`let unused = 1`, {
+      minifyUnused: true,
+      format: 'esm'
+    })
+    assert.strictEqual(code, ``)
+  },
+
+  async treeShakingMinifyWithoutFormat({ esbuild }) {
+    const { code } = await esbuild.transform(`let unused = 1`, {
+      minifyUnused: true
+    })
+    assert.strictEqual(code, `let unused = 1;\n`)
+  },
+
   async jsCharsetDefault({ esbuild }) {
     const { code } = await esbuild.transform(`let π = 'π'`, {})
     assert.strictEqual(code, `let \\u03C0 = "\\u03C0";\n`)