minify some switch statements to if-else statement

Author: evanw

CHANGELOG.md

@@ -21,7 +21,7 @@
     switch(x){case 0:foo();break;case 1:default:bar()}
 
     // New output (with --minify)
-    switch(x){case 0:foo();break;default:bar()}
+    x===0?foo():bar();
     ```
 
 * Forbid `using` declarations inside `switch` clauses ([#4323](https://github.com/evanw/esbuild/issues/4323))

internal/js_parser/js_parser.go

@@ -11625,26 +11625,59 @@ func (p *parser) minifySwitchStmt(loc logger.Loc, s *js_ast.SSwitch, stmts []js_
 		}
 	}
 
-	// Handle switch statements with only one case remaining
+	// Handle a switch statement containing only a "default" clause
 	if len(s.Cases) == 1 {
-		c := s.Cases[0]
-		var isTaken bool
-		var ok bool
-		if c.ValueOrNil.Data != nil {
-			// Non-default case
-			isTaken, ok = js_ast.CheckEqualityIfNoSideEffects(s.Test.Data, c.ValueOrNil.Data, js_ast.StrictEquality)
-		} else {
-			// Default case
-			isTaken, ok = true, p.astHelpers.ExprCanBeRemovedIfUnused(s.Test)
-		}
-		if ok && isTaken {
+		if c := s.Cases[0]; c.ValueOrNil.Data == nil && p.astHelpers.ExprCanBeRemovedIfUnused(s.Test) {
 			if body, ok := tryToInlineCaseBody(s.BodyLoc, c.Body, s.CloseBraceLoc); ok {
-				// Inline the case body
 				return append(stmts, body...)
 			}
 		}
 	}
 
+	// Try to turn this into an if-else statement
+	var yesCase js_ast.Case
+	var noCase js_ast.Case
+	if len(s.Cases) == 1 {
+		if yes := s.Cases[0]; yes.ValueOrNil.Data != nil {
+			yesCase = yes
+		}
+	} else if len(s.Cases) == 2 {
+		// "switch (x) { case y: a(); break; default: b() }"
+		if yes := s.Cases[0]; yes.ValueOrNil.Data != nil && !caseBodyCouldHaveFallThrough(yes.Body) {
+			if no := s.Cases[1]; no.ValueOrNil.Data == nil {
+				yesCase = yes
+				noCase = no
+			}
+		}
+
+		// "switch (x) { default: a(); break; case y: b() }"
+		if no := s.Cases[0]; no.ValueOrNil.Data == nil && !caseBodyCouldHaveFallThrough(no.Body) {
+			if yes := s.Cases[1]; yes.ValueOrNil.Data != nil {
+				yesCase = yes
+				noCase = no
+			}
+		}
+	}
+	if yesCase.ValueOrNil.Data != nil {
+		if yesBody, ok := tryToInlineCaseBody(s.BodyLoc, yesCase.Body, s.CloseBraceLoc); ok {
+			if noBody, ok := tryToInlineCaseBody(s.BodyLoc, noCase.Body, s.CloseBraceLoc); ok {
+				ifElse := js_ast.SIf{
+					Test: js_ast.Expr{Loc: s.Test.Loc},
+					Yes:  stmtsToSingleStmt(yesCase.Loc, yesBody, logger.Loc{}),
+				}
+				if isEqualToTest, ok := js_ast.CheckEqualityIfNoSideEffects(s.Test.Data, yesCase.ValueOrNil.Data, js_ast.StrictEquality); ok {
+					ifElse.Test.Data = &js_ast.EBoolean{Value: isEqualToTest}
+				} else {
+					ifElse.Test.Data = &js_ast.EBinary{Op: js_ast.BinOpStrictEq, Left: s.Test, Right: yesCase.ValueOrNil}
+				}
+				if len(noBody) > 0 {
+					ifElse.NoOrNil = stmtsToSingleStmt(noCase.Loc, noBody, logger.Loc{})
+				}
+				return p.mangleIf(stmts, loc, &ifElse)
+			}
+		}
+	}
+
 	return append(stmts, js_ast.Stmt{Loc: loc, Data: s})
 }
 

internal/js_parser/js_parser_test.go

@@ -1865,7 +1865,7 @@ func TestSuperCall(t *testing.T) {
 	expectPrintedMangleTarget(t, 2015, "class A extends B { x = 1; constructor() { super(); if (c) throw c } }",
 		"class A extends B {\n  constructor() {\n    super();\n    __publicField(this, \"x\", 1);\n    if (c) throw c;\n  }\n}\n")
 	expectPrintedMangleTarget(t, 2015, "class A extends B { x = 1; constructor() { super(); switch (c) { case 0: throw c } } }",
-		"class A extends B {\n  constructor() {\n    super();\n    __publicField(this, \"x\", 1);\n    switch (c) {\n      case 0:\n        throw c;\n    }\n  }\n}\n")
+		"class A extends B {\n  constructor() {\n    super();\n    __publicField(this, \"x\", 1);\n    if (c === 0)\n      throw c;\n  }\n}\n")
 	expectPrintedMangleTarget(t, 2015, "class A extends B { x = 1; constructor() { super(); while (!c) throw c } }",
 		"class A extends B {\n  constructor() {\n    super();\n    __publicField(this, \"x\", 1);\n    for (; !c; ) throw c;\n  }\n}\n")
 	expectPrintedMangleTarget(t, 2015, "class A extends B { x = 1; constructor() { super(); return c } }",
@@ -3664,21 +3664,23 @@ func TestMangleBlock(t *testing.T) {
 }
 
 func TestMangleSwitch(t *testing.T) {
-	expectPrintedMangle(t, "x(); switch (y) { case z: return w; }", "switch (x(), y) {\n  case z:\n    return w;\n}\n")
-	expectPrintedMangle(t, "if (t) { x(); switch (y) { case z: return w; } }", "if (t)\n  switch (x(), y) {\n    case z:\n      return w;\n  }\n")
+	expectPrintedMangle(t, "x(); switch (y) { case z: return w; }", "if (x(), y === z)\n  return w;\n")
+	expectPrintedMangle(t, "if (t) { x(); switch (y) { case z: return w; } }", "if (t && (x(), y === z))\n  return w;\n")
 
 	// We potentially need to keep let/const declarations in dead cases
 	expectPrintedMangle(t, "switch (1) { case 0: x; case 1: return x }", "return x;\n")
 	expectPrintedMangle(t, "switch (1) { case 0: var x; case 1: return x }", "switch (1) {\n  case 0:\n    var x;\n  case 1:\n    return x;\n}\n")
 	expectPrintedMangle(t, "switch (1) { case 0: let x; case 1: return x }", "switch (1) {\n  case 0:\n    let x;\n  case 1:\n    return x;\n}\n")
 	expectPrintedMangle(t, "switch (1) { case 0: const x = 0; case 1: return x }", "switch (1) {\n  case 0:\n    const x = 0;\n  case 1:\n    return x;\n}\n")
-	expectPrintedMangle(t, "switch (2) { case 0: var x; case 1: return x }", "switch (2) {\n  case 0:\n    var x;\n}\n")
+	expectPrintedMangle(t, "switch (2) { case 0: var x; case 1: return x }", "if (0)\n  var x;\n")
 	expectPrintedMangle(t, "switch (2) { case 0: let x; case 1: return x }", "")
 	expectPrintedMangle(t, "switch (2) { case 0: const x = 0; case 1: return x }", "")
 
 	// https://github.com/evanw/esbuild/issues/4359
+	expectPrintedMangle(t, "switch (x) { case 0: a(); break; default: b() }", "x === 0 ? a() : b();\n")
+	expectPrintedMangle(t, "switch (x) { default: a(); break; case 0: b() }", "x === 0 ? b() : a();\n")
 	expectPrintedMangle(t, "switch (x) { case p: a(); break; case q: default: b() }", "switch (x) {\n  case p:\n    a();\n    break;\n  case q:\n  default:\n    b();\n}\n")
-	expectPrintedMangle(t, "switch (x) { case 0: a(); break; case 1: case 2: default: b() }", "switch (x) {\n  case 0:\n    a();\n    break;\n  default:\n    b();\n}\n")
+	expectPrintedMangle(t, "switch (x) { case 0: a(); break; case 1: case 2: default: b() }", "x === 0 ? a() : b();\n")
 	expectPrintedMangle(t, "switch (x) { case 0: default: a(); break; case 0: b() }", "switch (x) {\n  case 0:\n  default:\n    a();\n    break;\n  case 0:\n    b();\n}\n")
 
 	// https://github.com/evanw/esbuild/issues/4176
@@ -5389,7 +5391,7 @@ func TestMangleInlineLocals(t *testing.T) {
 	check("let x = arg0; throw x;", "throw arg0;")
 	check("let x = arg0; return x;", "return arg0;")
 	check("let x = arg0; if (x) return 1;", "if (arg0) return 1;")
-	check("let x = arg0; switch (x) { case 0: return 1; }", "switch (arg0) {\n  case 0:\n    return 1;\n}")
+	check("let x = arg0; switch (x) { case 0: return 1; }", "if (arg0 === 0)\n  return 1;")
 	check("let x = arg0; let y = x; return y + y;", "let y = arg0;\nreturn y + y;")
 
 	// Loops must not be substituted into because they evaluate multiple times