Add JSX side effects option

CHANGELOG.md

@@ -99,6 +99,12 @@
 
     This release changes esbuild's parsing of `@keyframes` to now consider this case to be an unrecognized CSS rule. That means it will be passed through unmodified (so you can now use esbuild to bundle this Firefox-specific CSS) but the CSS will not be pretty-printed or minified. I don't think it makes sense for esbuild to have special code to handle this Firefox-specific syntax at this time. This decision can be revisited in the future if other browsers add support for this feature.
 
+* Add the `--jsx-side-effects` API option ([#2539](https://github.com/evanw/esbuild/issues/2539), [#2546](https://github.com/evanw/esbuild/pull/2546))
+
+    By default esbuild assumes that JSX expressions are side-effect free, which means they are annoated with `/* @__PURE__ */` comments and are removed during bundling when they are unused. This follows the common use of JSX for virtual DOM and applies to the vast majority of JSX libraries. However, some people have written JSX libraries that don't have this property. JSX expressions can have arbitrary side effects and can't be removed. If you are using such a library, you can now pass `--jsx-side-effects` to tell esbuild that JSX expressions have side effects so it won't remove them when they are unused.
+
+    This feature was contributed by [@rtsao](https://github.com/rtsao).
+
 ## 0.15.7
 
 * Add `--watch=forever` to allow esbuild to never terminate ([#1511](https://github.com/evanw/esbuild/issues/1511), [#1885](https://github.com/evanw/esbuild/issues/1885))

cmd/esbuild/main.go

@@ -82,6 +82,7 @@ var helpText = func(colors logger.Colors) string {
   --jsx-fragment=...        What to use for JSX instead of React.Fragment
   --jsx-import-source=...   Override the package name for the automatic runtime
                             (default "react")
+  --jsx-side-effects        Do not remove unused JSX expressions
   --jsx=...                 Set to "automatic" to use React's automatic runtime
                             or to "preserve" to disable transforming JSX to JS
   --keep-names              Preserve "name" on functions and classes

internal/config/config.go

@@ -20,6 +20,7 @@ type JSXOptions struct {
 	AutomaticRuntime bool
 	ImportSource     string
 	Development      bool
+	SideEffects      bool
 }
 
 type TSJSX uint8

internal/js_parser/js_parser.go

@@ -12401,7 +12401,7 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
 					CloseParenLoc: e.CloseLoc,
 
 					// Enable tree shaking
-					CanBeUnwrappedIfUnused: !p.options.ignoreDCEAnnotations,
+					CanBeUnwrappedIfUnused: !p.options.ignoreDCEAnnotations && !p.options.jsx.SideEffects,
 				}}, exprOut{}
 			} else {
 				// Arguments to jsx()
@@ -12529,7 +12529,7 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
 					CloseParenLoc: e.CloseLoc,
 
 					// Enable tree shaking
-					CanBeUnwrappedIfUnused: !p.options.ignoreDCEAnnotations,
+					CanBeUnwrappedIfUnused: !p.options.ignoreDCEAnnotations && !p.options.jsx.SideEffects,
 				}}, exprOut{}
 			}
 		}

internal/js_parser/js_parser_test.go

@@ -156,6 +156,16 @@ func expectPrintedJSX(t *testing.T, contents string, expected string) {
 	})
 }
 
+func expectPrintedJSXSideEffects(t *testing.T, contents string, expected string) {
+	t.Helper()
+	expectPrintedCommon(t, contents, expected, config.Options{
+		JSX: config.JSXOptions{
+			Parse:       true,
+			SideEffects: true,
+		},
+	})
+}
+
 func expectPrintedMangleJSX(t *testing.T, contents string, expected string) {
 	t.Helper()
 	expectPrintedCommon(t, contents, expected, config.Options{
@@ -170,6 +180,7 @@ type JSXAutomaticTestOptions struct {
 	Development            bool
 	ImportSource           string
 	OmitJSXRuntimeForTests bool
+	SideEffects            bool
 }
 
 func expectParseErrorJSXAutomatic(t *testing.T, options JSXAutomaticTestOptions, contents string, expected string) {
@@ -181,6 +192,7 @@ func expectParseErrorJSXAutomatic(t *testing.T, options JSXAutomaticTestOptions,
 			Parse:            true,
 			Development:      options.Development,
 			ImportSource:     options.ImportSource,
+			SideEffects:      options.SideEffects,
 		},
 	})
 }
@@ -194,6 +206,7 @@ func expectPrintedJSXAutomatic(t *testing.T, options JSXAutomaticTestOptions, co
 			Parse:            true,
 			Development:      options.Development,
 			ImportSource:     options.ImportSource,
+			SideEffects:      options.SideEffects,
 		},
 	})
 }
@@ -4813,6 +4826,11 @@ NOTE: Both "__source" and "__self" are set automatically by esbuild when using R
 	expectPrintedJSXAutomatic(t, pri, "<div/>", "import { jsx } from \"my-jsx-lib/jsx-runtime\";\n/* @__PURE__ */ jsx(\"div\", {});\n")
 	expectPrintedJSXAutomatic(t, pri, "<div {...props} key=\"key\" />", "import { createElement } from \"my-jsx-lib\";\n/* @__PURE__ */ createElement(\"div\", {\n  ...props,\n  key: \"key\"\n});\n")
 
+	// Impure JSX call expressions
+	pi := JSXAutomaticTestOptions{SideEffects: true, ImportSource: "my-jsx-lib"}
+	expectPrintedJSXAutomatic(t, pi, "<a/>", "import { jsx } from \"my-jsx-lib/jsx-runtime\";\njsx(\"a\", {});\n")
+	expectPrintedJSXAutomatic(t, pi, "<></>", "import { Fragment, jsx } from \"my-jsx-lib/jsx-runtime\";\njsx(Fragment, {});\n")
+
 	// Dev, without runtime imports
 	d := JSXAutomaticTestOptions{Development: true, OmitJSXRuntimeForTests: true}
 	expectPrintedJSXAutomatic(t, d, "<div>></div>", "/* @__PURE__ */ jsxDEV(\"div\", {\n  children: \">\"\n}, void 0, false, {\n  fileName: \"<stdin>\",\n  lineNumber: 1,\n  columnNumber: 1\n}, this);\n")
@@ -4930,6 +4948,14 @@ NOTE: You can enable React's "automatic" JSX transform for this file by using a
 	expectParseErrorJSX(t, "// @jsxRuntime automatic @jsxFrag f\n<></>", "<stdin>: WARNING: The JSX fragment cannot be set when using React's \"automatic\" JSX transform\n")
 }
 
+func TestJSXSideEffects(t *testing.T) {
+	expectPrintedJSX(t, "<a/>", "/* @__PURE__ */ React.createElement(\"a\", null);\n")
+	expectPrintedJSX(t, "<></>", "/* @__PURE__ */ React.createElement(React.Fragment, null);\n")
+
+	expectPrintedJSXSideEffects(t, "<a/>", "React.createElement(\"a\", null);\n")
+	expectPrintedJSXSideEffects(t, "<></>", "React.createElement(React.Fragment, null);\n")
+}
+
 func TestPreserveOptionalChainParentheses(t *testing.T) {
 	expectPrinted(t, "a?.b.c", "a?.b.c;\n")
 	expectPrinted(t, "(a?.b).c", "(a?.b).c;\n")

lib/shared/common.ts

@@ -145,6 +145,7 @@ function pushCommonFlags(flags: string[], options: CommonOptions, keys: OptionKe
   let jsxFragment = getFlag(options, keys, 'jsxFragment', mustBeString);
   let jsxImportSource = getFlag(options, keys, 'jsxImportSource', mustBeString);
   let jsxDev = getFlag(options, keys, 'jsxDev', mustBeBoolean);
+  let jsxSideEffects = getFlag(options, keys, 'jsxSideEffects', mustBeBoolean);
   let define = getFlag(options, keys, 'define', mustBeObject);
   let logOverride = getFlag(options, keys, 'logOverride', mustBeObject);
   let supported = getFlag(options, keys, 'supported', mustBeObject);
@@ -180,6 +181,7 @@ function pushCommonFlags(flags: string[], options: CommonOptions, keys: OptionKe
   if (jsxFragment) flags.push(`--jsx-fragment=${jsxFragment}`);
   if (jsxImportSource) flags.push(`--jsx-import-source=${jsxImportSource}`);
   if (jsxDev) flags.push(`--jsx-dev`);
+  if (jsxSideEffects) flags.push(`--jsx-side-effects`);
 
   if (define) {
     for (let key in define) {

lib/shared/types.ts

@@ -61,6 +61,8 @@ interface CommonOptions {
   jsxImportSource?: string;
   /** Documentation: https://esbuild.github.io/api/#jsx-development */
   jsxDev?: boolean;
+  /** Documentation: https://esbuild.github.io/api/#jsx-side-effects */
+  jsxSideEffects?: boolean;
 
   /** Documentation: https://esbuild.github.io/api/#define */
   define?: { [key: string]: string };

pkg/api/api.go

@@ -281,6 +281,7 @@ type BuildOptions struct {
 	JSXFragment     string  // Documentation: https://esbuild.github.io/api/#jsx-fragment
 	JSXImportSource string  // Documentation: https://esbuild.github.io/api/#jsx-import-source
 	JSXDev          bool    // Documentation: https://esbuild.github.io/api/#jsx-dev
+	JSXSideEffects  bool    // Documentation: https://esbuild.github.io/api/#jsx-side-effects
 
 	Define    map[string]string // Documentation: https://esbuild.github.io/api/#define
 	Pure      []string          // Documentation: https://esbuild.github.io/api/#pure
@@ -403,6 +404,7 @@ type TransformOptions struct {
 	JSXFragment     string  // Documentation: https://esbuild.github.io/api/#jsx-fragment
 	JSXImportSource string  // Documentation: https://esbuild.github.io/api/#jsx-import-source
 	JSXDev          bool    // Documentation: https://esbuild.github.io/api/#jsx-dev
+	JSXSideEffects  bool    // Documentation: https://esbuild.github.io/api/#jsx-side-effects
 
 	TsconfigRaw string // Documentation: https://esbuild.github.io/api/#tsconfig-raw
 	Banner      string // Documentation: https://esbuild.github.io/api/#banner

pkg/api/api_impl.go

@@ -903,6 +903,7 @@ func rebuildImpl(
 			Fragment:         validateJSXExpr(log, buildOpts.JSXFragment, "fragment"),
 			Development:      buildOpts.JSXDev,
 			ImportSource:     buildOpts.JSXImportSource,
+			SideEffects:      buildOpts.JSXSideEffects,
 		},
 		Defines:               defines,
 		InjectedDefines:       injectedDefines,
@@ -1365,6 +1366,7 @@ func transformImpl(input string, transformOpts TransformOptions) TransformResult
 		Fragment:         validateJSXExpr(log, transformOpts.JSXFragment, "fragment"),
 		Development:      transformOpts.JSXDev,
 		ImportSource:     transformOpts.JSXImportSource,
+		SideEffects:      transformOpts.JSXSideEffects,
 	}
 
 	// Settings from "tsconfig.json" override those

pkg/cli/cli_impl.go

@@ -657,6 +657,15 @@ func parseOptionsImpl(
 				transformOpts.JSXDev = value
 			}
 
+		case isBoolFlag(arg, "--jsx-side-effects"):
+			if value, err := parseBoolFlag(arg, true); err != nil {
+				return parseOptionsExtras{}, err
+			} else if buildOpts != nil {
+				buildOpts.JSXSideEffects = value
+			} else {
+				transformOpts.JSXSideEffects = value
+			}
+
 		case strings.HasPrefix(arg, "--banner=") && transformOpts != nil:
 			transformOpts.Banner = arg[len("--banner="):]
 
@@ -754,6 +763,7 @@ func parseOptionsImpl(
 				"bundle":             true,
 				"ignore-annotations": true,
 				"jsx-dev":            true,
+				"jsx-side-effects":   true,
 				"keep-names":         true,
 				"minify-identifiers": true,
 				"minify-syntax":      true,

scripts/js-api-tests.js

@@ -4587,6 +4587,11 @@ let transformTests = {
     assert.strictEqual(code, `import { jsx } from "notreact/jsx-runtime";\nconsole.log(/* @__PURE__ */ jsx("div", {}));\n`)
   },
 
+  async jsxSideEffects({ esbuild }) {
+    const { code } = await esbuild.transform(`<b/>`, { loader: 'jsx', jsxSideEffects: true })
+    assert.strictEqual(code, `React.createElement("b", null);\n`)
+  },
+
   async ts({ esbuild }) {
     const { code } = await esbuild.transform(`enum Foo { FOO }`, { loader: 'ts' })
     assert.strictEqual(code, `var Foo = /* @__PURE__ */ ((Foo2) => {\n  Foo2[Foo2["FOO"] = 0] = "FOO";\n  return Foo2;\n})(Foo || {});\n`)