minify undefined checks to typeof x < u

CHANGELOG.md

@@ -79,6 +79,23 @@
 
     You can now target [Opera](https://www.opera.com/) and/or [Internet Explorer](https://www.microsoft.com/en-us/download/internet-explorer.aspx) using the `--target=` setting. For example, `--target=opera45,ie9` targets Opera 45 and Internet Explorer 9. This change does not add any additional features to esbuild's code transformation pipeline to transform newer syntax so that it works in Internet Explorer. It just adds information about what features are supported in these browsers to esbuild's internal feature compatibility table.
 
+* Minify `typeof x !== 'undefined'` to `typeof x < 'u'`
+
+    This release introduces a small improvement for code that does a lot of `typeof` checks against `undefined`:
+
+    ```js
+    // Original code
+    y = typeof x !== 'undefined';
+
+    // Old output (with --minify)
+    y=typeof x!="undefined";
+
+    // New output (with --minify)
+    y=typeof x<"u";
+    ```
+
+    This transformation is only active when minification is enabled, and is disabled if the language target is set lower than ES2020 or if Internet Explorer is set as a target environment. Before ES2020, implementations were allowed to return non-standard values from the `typeof` operator for a few objects. Internet Explorer took advantage of this to sometimes return the string `'unknown'` instead of `'undefined'`. But this has been removed from the specification and Internet Explorer was the only engine to do this, so this minification is valid for code that does not need to target Internet Explorer.
+
 ## 0.14.17
 
 * Attempt to fix an install script issue on Ubuntu Linux ([#1711](https://github.com/evanw/esbuild/issues/1711))

internal/bundler/bundler_dce_test.go

@@ -1829,6 +1829,74 @@ func TestDCETypeOfEqualsStringGuardCondition(t *testing.T) {
 	})
 }
 
+func TestDCETypeOfCompareStringGuardCondition(t *testing.T) {
+	dce_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				// Everything here should be removed as dead code due to tree shaking
+				var REMOVE_1 = typeof x <= 'u' ? x : null
+				var REMOVE_1 = typeof x < 'u' ? x : null
+				var REMOVE_1 = typeof x >= 'u' ? null : x
+				var REMOVE_1 = typeof x > 'u' ? null : x
+				var REMOVE_1 = typeof x <= 'u' && x
+				var REMOVE_1 = typeof x < 'u' && x
+				var REMOVE_1 = typeof x >= 'u' || x
+				var REMOVE_1 = typeof x > 'u' || x
+				var REMOVE_1 = 'u' >= typeof x ? x : null
+				var REMOVE_1 = 'u' > typeof x ? x : null
+				var REMOVE_1 = 'u' <= typeof x ? null : x
+				var REMOVE_1 = 'u' < typeof x ? null : x
+				var REMOVE_1 = 'u' >= typeof x && x
+				var REMOVE_1 = 'u' > typeof x && x
+				var REMOVE_1 = 'u' <= typeof x || x
+				var REMOVE_1 = 'u' < typeof x || x
+
+				// Everything here should be kept as live code because it has side effects
+				var keep_1 = typeof x <= 'u' ? y : null
+				var keep_1 = typeof x < 'u' ? y : null
+				var keep_1 = typeof x >= 'u' ? null : y
+				var keep_1 = typeof x > 'u' ? null : y
+				var keep_1 = typeof x <= 'u' && y
+				var keep_1 = typeof x < 'u' && y
+				var keep_1 = typeof x >= 'u' || y
+				var keep_1 = typeof x > 'u' || y
+				var keep_1 = 'u' >= typeof x ? y : null
+				var keep_1 = 'u' > typeof x ? y : null
+				var keep_1 = 'u' <= typeof x ? null : y
+				var keep_1 = 'u' < typeof x ? null : y
+				var keep_1 = 'u' >= typeof x && y
+				var keep_1 = 'u' > typeof x && y
+				var keep_1 = 'u' <= typeof x || y
+				var keep_1 = 'u' < typeof x || y
+
+				// Everything here should be kept as live code because it has side effects
+				var keep_2 = typeof x <= 'u' ? null : x
+				var keep_2 = typeof x < 'u' ? null : x
+				var keep_2 = typeof x >= 'u' ? x : null
+				var keep_2 = typeof x > 'u' ? x : null
+				var keep_2 = typeof x <= 'u' || x
+				var keep_2 = typeof x < 'u' || x
+				var keep_2 = typeof x >= 'u' && x
+				var keep_2 = typeof x > 'u' && x
+				var keep_2 = 'u' >= typeof x ? null : x
+				var keep_2 = 'u' > typeof x ? null : x
+				var keep_2 = 'u' <= typeof x ? x : null
+				var keep_2 = 'u' < typeof x ? x : null
+				var keep_2 = 'u' >= typeof x || x
+				var keep_2 = 'u' > typeof x || x
+				var keep_2 = 'u' <= typeof x && x
+				var keep_2 = 'u' < typeof x && x
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			OutputFormat:  config.FormatIIFE,
+			AbsOutputFile: "/out.js",
+		},
+	})
+}
+
 // These unused imports should be removed since they aren't used, and removing
 // them makes the code shorter.
 func TestRemoveUnusedImports(t *testing.T) {

internal/bundler/snapshots/snapshots_dce.txt

@@ -47,6 +47,45 @@ var e = `${alsoKeep}`;
 TestDCETypeOf
 ---------- /out.js ----------
 
+================================================================================
+TestDCETypeOfCompareStringGuardCondition
+---------- /out.js ----------
+(() => {
+  // entry.js
+  var keep_1 = typeof x <= "u" ? y : null;
+  var keep_1 = typeof x < "u" ? y : null;
+  var keep_1 = typeof x >= "u" ? null : y;
+  var keep_1 = typeof x > "u" ? null : y;
+  var keep_1 = typeof x <= "u" && y;
+  var keep_1 = typeof x < "u" && y;
+  var keep_1 = typeof x >= "u" || y;
+  var keep_1 = typeof x > "u" || y;
+  var keep_1 = "u" >= typeof x ? y : null;
+  var keep_1 = "u" > typeof x ? y : null;
+  var keep_1 = "u" <= typeof x ? null : y;
+  var keep_1 = "u" < typeof x ? null : y;
+  var keep_1 = "u" >= typeof x && y;
+  var keep_1 = "u" > typeof x && y;
+  var keep_1 = "u" <= typeof x || y;
+  var keep_1 = "u" < typeof x || y;
+  var keep_2 = typeof x <= "u" ? null : x;
+  var keep_2 = typeof x < "u" ? null : x;
+  var keep_2 = typeof x >= "u" ? x : null;
+  var keep_2 = typeof x > "u" ? x : null;
+  var keep_2 = typeof x <= "u" || x;
+  var keep_2 = typeof x < "u" || x;
+  var keep_2 = typeof x >= "u" && x;
+  var keep_2 = typeof x > "u" && x;
+  var keep_2 = "u" >= typeof x ? null : x;
+  var keep_2 = "u" > typeof x ? null : x;
+  var keep_2 = "u" <= typeof x ? x : null;
+  var keep_2 = "u" < typeof x ? x : null;
+  var keep_2 = "u" >= typeof x || x;
+  var keep_2 = "u" > typeof x || x;
+  var keep_2 = "u" <= typeof x && x;
+  var keep_2 = "u" < typeof x && x;
+})();
+
 ================================================================================
 TestDCETypeOfEqualsString
 ---------- /out.js ----------

internal/compat/js_table.go

@@ -87,6 +87,7 @@ const (
 	RestArgument
 	TemplateLiteral
 	TopLevelAwait
+	TypeofExoticObjectIsObject
 	UnicodeEscapes
 )
 
@@ -485,6 +486,16 @@ var jsTable = map[JSFeature]map[Engine][]versionRange{
 		Node:    {{start: v{14, 8, 0}}},
 		Safari:  {{start: v{15, 0, 0}}},
 	},
+	TypeofExoticObjectIsObject: {
+		Chrome:  {{start: v{0, 0, 0}}},
+		Edge:    {{start: v{0, 0, 0}}},
+		ES:      {{start: v{2020, 0, 0}}},
+		Firefox: {{start: v{0, 0, 0}}},
+		IOS:     {{start: v{0, 0, 0}}},
+		Node:    {{start: v{0, 0, 0}}},
+		Opera:   {{start: v{0, 0, 0}}},
+		Safari:  {{start: v{0, 0, 0}}},
+	},
 	UnicodeEscapes: {
 		Chrome:  {{start: v{44, 0, 0}}},
 		Edge:    {{start: v{12, 0, 0}}},

internal/js_parser/js_parser.go

@@ -10587,19 +10587,41 @@ func canChangeStrictToLoose(a js_ast.Expr, b js_ast.Expr) bool {
 	return x == y && x != js_ast.PrimitiveUnknown && x != js_ast.PrimitiveMixed
 }
 
-func maybeSimplifyEqualityComparison(e *js_ast.EBinary, isNotEqual bool) (js_ast.Expr, bool) {
+func (p *parser) maybeSimplifyEqualityComparison(loc logger.Loc, e *js_ast.EBinary) (js_ast.Expr, bool) {
 	// "!x === true" => "!x"
 	// "!x === false" => "!!x"
 	// "!x !== true" => "!!x"
 	// "!x !== false" => "!x"
 	if boolean, ok := e.Right.Data.(*js_ast.EBoolean); ok && js_ast.KnownPrimitiveType(e.Left) == js_ast.PrimitiveBoolean {
-		if boolean.Value == isNotEqual {
+		if boolean.Value == (e.Op == js_ast.BinOpLooseNe || e.Op == js_ast.BinOpStrictNe) {
 			return js_ast.Not(e.Left), true
 		} else {
 			return e.Left, true
 		}
 	}
 
+	// "typeof x != 'undefined'" => "typeof x < 'u'"
+	// "typeof x == 'undefined'" => "typeof x > 'u'"
+	if !p.options.unsupportedJSFeatures.Has(compat.TypeofExoticObjectIsObject) {
+		// Only do this optimization if we know that the "typeof" operator won't
+		// return something random. The only case of this happening was Internet
+		// Explorer returning "unknown" for some objects, which messes with this
+		// optimization. So we don't do this when targeting Internet Explorer.
+		if typeof, ok := e.Left.Data.(*js_ast.EUnary); ok && typeof.Op == js_ast.UnOpTypeof {
+			if str, ok := e.Right.Data.(*js_ast.EString); ok && js_lexer.UTF16EqualsString(str.Value, "undefined") {
+				op := js_ast.BinOpLt
+				if e.Op == js_ast.BinOpLooseEq || e.Op == js_ast.BinOpStrictEq {
+					op = js_ast.BinOpGt
+				}
+				return js_ast.Expr{Loc: loc, Data: &js_ast.EBinary{
+					Op:    op,
+					Left:  e.Left,
+					Right: js_ast.Expr{Loc: e.Right.Loc, Data: &js_ast.EString{Value: []uint16{'u'}}},
+				}}, true
+			}
+		}
+	}
+
 	return js_ast.Expr{}, false
 }
 
@@ -11733,7 +11755,7 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
 					e.Right.Data = js_ast.ENullShared
 				}
 
-				if result, ok := maybeSimplifyEqualityComparison(e, false /* isNotEqual */); ok {
+				if result, ok := p.maybeSimplifyEqualityComparison(expr.Loc, e); ok {
 					return result, exprOut{}
 				}
 			}
@@ -11754,7 +11776,7 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
 					e.Op = js_ast.BinOpLooseEq
 				}
 
-				if result, ok := maybeSimplifyEqualityComparison(e, false /* isNotEqual */); ok {
+				if result, ok := p.maybeSimplifyEqualityComparison(expr.Loc, e); ok {
 					return result, exprOut{}
 				}
 			}
@@ -11775,7 +11797,7 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
 					e.Right.Data = js_ast.ENullShared
 				}
 
-				if result, ok := maybeSimplifyEqualityComparison(e, true /* isNotEqual */); ok {
+				if result, ok := p.maybeSimplifyEqualityComparison(expr.Loc, e); ok {
 					return result, exprOut{}
 				}
 			}
@@ -11796,7 +11818,7 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
 					e.Op = js_ast.BinOpLooseNe
 				}
 
-				if result, ok := maybeSimplifyEqualityComparison(e, true /* isNotEqual */); ok {
+				if result, ok := p.maybeSimplifyEqualityComparison(expr.Loc, e); ok {
 					return result, exprOut{}
 				}
 			}
@@ -14557,6 +14579,14 @@ func (p *parser) exprCanBeRemovedIfUnused(expr js_ast.Expr) bool {
 		// we must also consider "typeof x == 'object'" to be side-effect free.
 		case js_ast.BinOpLooseEq, js_ast.BinOpLooseNe:
 			return canChangeStrictToLoose(e.Left, e.Right) && p.exprCanBeRemovedIfUnused(e.Left) && p.exprCanBeRemovedIfUnused(e.Right)
+
+		// Special-case "<" and ">" with string, number, or bigint arguments
+		case js_ast.BinOpLt, js_ast.BinOpGt, js_ast.BinOpLe, js_ast.BinOpGe:
+			left := js_ast.KnownPrimitiveType(e.Left)
+			switch left {
+			case js_ast.PrimitiveString, js_ast.PrimitiveNumber, js_ast.PrimitiveBigInt:
+				return js_ast.KnownPrimitiveType(e.Right) == left && p.exprCanBeRemovedIfUnused(e.Left) && p.exprCanBeRemovedIfUnused(e.Right)
+			}
 		}
 
 	case *js_ast.ETemplate:
@@ -14599,6 +14629,25 @@ func (p *parser) isSideEffectFreeUnboundIdentifierRef(value js_ast.Expr, guardCo
 						}
 					}
 				}
+
+			case js_ast.BinOpLt, js_ast.BinOpGt, js_ast.BinOpLe, js_ast.BinOpGe:
+				// Pattern match for "typeof x < <string>"
+				typeof, string := binary.Left, binary.Right
+				if _, ok := typeof.Data.(*js_ast.EString); ok {
+					typeof, string = string, typeof
+					isYesBranch = !isYesBranch
+				}
+				if typeof, ok := typeof.Data.(*js_ast.EUnary); ok && typeof.Op == js_ast.UnOpTypeof {
+					if text, ok := string.Data.(*js_ast.EString); ok && js_lexer.UTF16EqualsString(text.Value, "u") {
+						// In "typeof x < 'u' ? x : null", the reference to "x" is side-effect free
+						// In "typeof x > 'u' ? x : null", the reference to "x" is side-effect free
+						if isYesBranch == (binary.Op == js_ast.BinOpLt || binary.Op == js_ast.BinOpLe) {
+							if id2, ok := typeof.Value.Data.(*js_ast.EIdentifier); ok && id2.Ref == id.Ref {
+								return true
+							}
+						}
+					}
+				}
 			}
 		}
 	}

internal/js_parser/js_parser_test.go

@@ -3554,6 +3554,18 @@ func TestMangleTypeofIdentifier(t *testing.T) {
 	expectPrintedMangle(t, "return typeof (false || x); var x", "return typeof x;\nvar x;\n")
 }
 
+func TestMangleTypeofEqualsUndefined(t *testing.T) {
+	expectPrintedMangle(t, "return typeof x !== 'undefined'", "return typeof x < \"u\";\n")
+	expectPrintedMangle(t, "return typeof x != 'undefined'", "return typeof x < \"u\";\n")
+	expectPrintedMangle(t, "return 'undefined' !== typeof x", "return typeof x < \"u\";\n")
+	expectPrintedMangle(t, "return 'undefined' != typeof x", "return typeof x < \"u\";\n")
+
+	expectPrintedMangle(t, "return typeof x === 'undefined'", "return typeof x > \"u\";\n")
+	expectPrintedMangle(t, "return typeof x == 'undefined'", "return typeof x > \"u\";\n")
+	expectPrintedMangle(t, "return 'undefined' === typeof x", "return typeof x > \"u\";\n")
+	expectPrintedMangle(t, "return 'undefined' == typeof x", "return typeof x > \"u\";\n")
+}
+
 func TestMangleEquals(t *testing.T) {
 	expectPrintedMangle(t, "return typeof x === y", "return typeof x === y;\n")
 	expectPrintedMangle(t, "return typeof x !== y", "return typeof x !== y;\n")

scripts/compat-table.js

@@ -155,6 +155,7 @@ mergeVersions('BigInt', { es2020: true })
 mergeVersions('ImportMeta', { es2020: true })
 mergeVersions('NullishCoalescing', { es2020: true })
 mergeVersions('OptionalChain', { es2020: true })
+mergeVersions('TypeofExoticObjectIsObject', { es2020: true }) // https://github.com/tc39/ecma262/pull/1441
 mergeVersions('LogicalAssignment', { es2021: true })
 mergeVersions('TopLevelAwait', {})
 mergeVersions('ArbitraryModuleNamespaceNames', {})
@@ -201,6 +202,18 @@ mergeVersions('DynamicImport', {
   safari11_1: true,
 })
 
+// This is a problem specific to Internet explorer. See https://github.com/tc39/ecma262/issues/1440
+mergeVersions('TypeofExoticObjectIsObject', {
+  chrome0: true,
+  edge0: true,
+  es0: true,
+  firefox0: true,
+  ios0: true,
+  node0: true,
+  opera0: true,
+  safari0: true,
+})
+
 // This is a special case. Node added support for it to both v12.20+ and v13.2+
 // so the range is inconveniently discontiguous. Sources:
 //

scripts/js-api-tests.js

@@ -4249,6 +4249,15 @@ let transformTests = {
     assert.strictEqual(fromPromiseResolve(code4), `Promise.resolve().then(function(){return __toESM(require(foo))});\n`)
   },
 
+  async typeofEqualsUndefinedTarget({ esbuild }) {
+    assert.strictEqual((await esbuild.transform(`a = typeof b !== 'undefined'`, { minify: true })).code, `a=typeof b<"u";\n`)
+    assert.strictEqual((await esbuild.transform(`a = typeof b !== 'undefined'`, { minify: true, target: 'es2020' })).code, `a=typeof b<"u";\n`)
+    assert.strictEqual((await esbuild.transform(`a = typeof b !== 'undefined'`, { minify: true, target: 'chrome11' })).code, `a=typeof b<"u";\n`)
+
+    assert.strictEqual((await esbuild.transform(`a = typeof b !== 'undefined'`, { minify: true, target: 'es2019' })).code, `a=typeof b!="undefined";\n`)
+    assert.strictEqual((await esbuild.transform(`a = typeof b !== 'undefined'`, { minify: true, target: 'ie11' })).code, `a=typeof b!="undefined";\n`)
+  },
+
   async caseInsensitiveTarget({ esbuild }) {
     assert.strictEqual((await esbuild.transform(`a ||= b`, { target: 'eS5' })).code, `a || (a = b);\n`)
     assert.strictEqual((await esbuild.transform(`a ||= b`, { target: 'eSnExT' })).code, `a ||= b;\n`)