fix #1407: add "undefined" literals in "--define:"

CHANGELOG.md

@@ -2,6 +2,21 @@
 
 ## Unreleased
 
+* Treat `--define:foo=undefined` as an undefined literal instead of an identifier ([#1407](https://github.com/evanw/esbuild/issues/1407))
+
+    References to the global variable `undefined` are automatically replaced with the literal value for undefined, which appears as `void 0` when printed. This allows for additional optimizations such as collapsing `undefined ?? bar` into just `bar`. However, this substitution was not done for values specified via `--define:`. As a result, esbuild could potentially miss out on certain optimizations in these cases. With this release, it's now possible to use `--define:` to substitute something with an undefined literal:
+
+    ```js
+    // Original code
+    let win = typeof window !== 'undefined' ? window : {}
+
+    // Old output (with --define:window=undefined --minify)
+    let win=typeof undefined!="undefined"?undefined:{};
+
+    // New output (with --define:window=undefined --minify)
+    let win={};
+    ```
+
 * Add the `--drop:debugger` flag ([#1809](https://github.com/evanw/esbuild/issues/1809))
 
     Passing this flag causes all [`debugger;` statements](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/debugger) to be removed from the output. This is similar to the `drop_debugger: true` flag available in the popular UglifyJS and Terser JavaScript minifiers.

pkg/api/api_impl.go

@@ -471,11 +471,26 @@ func validateDefines(
 		// Allow substituting for an identifier
 		if js_lexer.IsIdentifier(value) {
 			if _, ok := js_lexer.Keywords[value]; !ok {
-				name := value // The closure must close over a variable inside the loop
-				rawDefines[key] = config.DefineData{
-					DefineFunc: func(args config.DefineArgs) js_ast.E {
-						return &js_ast.EIdentifier{Ref: args.FindSymbol(args.Loc, name)}
-					},
+				switch value {
+				case "undefined":
+					rawDefines[key] = config.DefineData{
+						DefineFunc: func(config.DefineArgs) js_ast.E { return js_ast.EUndefinedShared },
+					}
+				case "NaN":
+					rawDefines[key] = config.DefineData{
+						DefineFunc: func(config.DefineArgs) js_ast.E { return &js_ast.ENumber{Value: math.NaN()} },
+					}
+				case "Infinity":
+					rawDefines[key] = config.DefineData{
+						DefineFunc: func(config.DefineArgs) js_ast.E { return &js_ast.ENumber{Value: math.Inf(1)} },
+					}
+				default:
+					name := value // The closure must close over a variable inside the loop
+					rawDefines[key] = config.DefineData{
+						DefineFunc: func(args config.DefineArgs) js_ast.E {
+							return &js_ast.EIdentifier{Ref: args.FindSymbol(args.Loc, name)}
+						},
+					}
 				}
 				continue
 			}

scripts/js-api-tests.js

@@ -3727,6 +3727,12 @@ let transformTests = {
     assert.strictEqual(code, `console.log("production");\n`)
   },
 
+  async defineBuiltInConstants({ esbuild }) {
+    const define = { a: 'NaN', b: 'Infinity', c: 'undefined', d: 'something' }
+    const { code } = await esbuild.transform(`console.log([typeof a, typeof b, typeof c, typeof d])`, { define })
+    assert.strictEqual(code, `console.log(["number", "number", "undefined", typeof something]);\n`)
+  },
+
   async defineArray({ esbuild }) {
     const define = { 'process.env.NODE_ENV': '[1,2,3]', 'something.else': '[2,3,4]' }
     const { code } = await esbuild.transform(`console.log(process.env.NODE_ENV)`, { define })