fix #3659: trim code in dead switch statements

CHANGELOG.md

@@ -2,6 +2,21 @@
 
 ## Unreleased
 
+* Improve dead code removal of `switch` statements ([#3659](https://github.com/evanw/esbuild/issues/3659))
+
+    With this release, esbuild will now remove `switch` statements in branches when minifying if they are known to never be evaluated:
+
+    ```js
+    // Original code
+    if (true) foo(); else switch (bar) { case 1: baz(); break }
+
+    // Old output (with --minify)
+    if(1)foo();else switch(bar){case 1:}
+
+    // New output (with --minify)
+    foo();
+    ```
+
 * Empty enums should behave like an object literal ([#3657](https://github.com/evanw/esbuild/issues/3657))
 
     TypeScript allows you to create an empty enum and add properties to it at run time. While people usually use an empty object literal for this instead of a TypeScript enum, esbuild's enum transform didn't anticipate this use case and generated `undefined` instead of `{}` for an empty enum. With this release, you can now use an empty enum to generate an empty object literal.

internal/js_parser/js_parser.go

@@ -10704,6 +10704,14 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
 			}
 		}
 
+		// Unwrap switch statements in dead code
+		if p.options.minifySyntax && p.isControlFlowDead {
+			for _, c := range s.Cases {
+				stmts = append(stmts, c.Body...)
+			}
+			return stmts
+		}
+
 		// "using" declarations inside switch statements must be special-cased
 		if lowered := p.maybeLowerUsingDeclarationsInSwitch(stmt.Loc, s); lowered != nil {
 			return append(stmts, lowered...)

internal/js_parser/js_parser_test.go

@@ -5232,6 +5232,7 @@ func TestTrimCodeInDeadControlFlow(t *testing.T) {
 	expectPrintedMangle(t, "b: while (1) if (0) a(); else { continue b }", "b:\n  for (; ; )\n    continue b;\n")
 	expectPrintedMangle(t, "if (1) a(); else { class b {} }", "a();\n")
 	expectPrintedMangle(t, "if (1) a(); else { debugger }", "a();\n")
+	expectPrintedMangle(t, "if (1) a(); else { switch (1) { case 1: b() } }", "a();\n")
 
 	expectPrintedMangle(t, "if (0) {let a = 1} else a()", "a();\n")
 	expectPrintedMangle(t, "if (1) {let a = 1} else a()", "{\n  let a = 1;\n}\n")
@@ -5246,6 +5247,7 @@ func TestTrimCodeInDeadControlFlow(t *testing.T) {
 	expectPrintedMangle(t, "if (1) a(); else { function a() {} }", "if (1)\n  a();\nelse\n  var a;\n")
 	expectPrintedMangle(t, "if (1) a(); else { for(;;){var a} }", "if (1)\n  a();\nelse\n  for (; ; )\n    var a;\n")
 	expectPrintedMangle(t, "if (1) { a(); b() } else { var a; var b; }", "if (1)\n  a(), b();\nelse\n  var a, b;\n")
+	expectPrintedMangle(t, "if (1) a(); else { switch (1) { case 1: case 2: var a } }", "if (1)\n  a();\nelse\n  var a;\n")
 }
 
 func TestPreservedComments(t *testing.T) {