fix #4359: prune empty case before a default

Author: evanw

CHANGELOG.md

@@ -2,6 +2,28 @@
 
 ## Unreleased
 
+* More minifications for `switch` statements ([#4359](https://github.com/evanw/esbuild/issues/4359))
+
+    This release contains additional minification patterns for reducing `switch` statements. Here is an example:
+
+    ```js
+    // Original code
+    switch (x) {
+      case 0:
+        foo()
+        break
+      case 1:
+      default:
+        bar()
+    }
+
+    // Old output (with --minify)
+    switch(x){case 0:foo();break;case 1:default:bar()}
+
+    // New output (with --minify)
+    switch(x){case 0:foo();break;default:bar()}
+    ```
+
 * Forbid `using` declarations inside `switch` clauses ([#4323](https://github.com/evanw/esbuild/issues/4323))
 
     This is a rare change to remove something that was previously possible. The [Explicit Resource Management](https://github.com/tc39/proposal-explicit-resource-management) proposal introduced `using` declarations. These were previously allowed inside `case` and `default` clauses in `switch` statements. This had well-defined semantics and was already widely implemented (by V8, SpiderMonkey, TypeScript, esbuild, and others). However, it was considered to be too confusing because of how scope works in switch statements, so it has been removed from the specification. This edge case will now be a syntax error. See [tc39/proposal-explicit-resource-management#215](https://github.com/tc39/proposal-explicit-resource-management/issues/215) and [rbuckton/ecma262#14](https://github.com/rbuckton/ecma262/pull/14) for details.

internal/js_parser/js_parser.go

@@ -11232,8 +11232,19 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
 			return stmts
 		}
 
-		// Attempt to remove statically-determined switch statements
 		if p.options.minifySyntax {
+			// Trim empty cases before a trailing default clause
+			if len(s.Cases) > 0 {
+				if i := len(s.Cases) - 1; s.Cases[i].ValueOrNil.Data == nil {
+					// "switch (x) { case 0: default: y() }" => "switch (x) { default: y() }"
+					for i > 0 && len(s.Cases[i-1].Body) == 0 && js_ast.IsPrimitiveLiteral(s.Cases[i-1].ValueOrNil.Data) {
+						i--
+					}
+					s.Cases = append(s.Cases[:i], s.Cases[len(s.Cases)-1])
+				}
+			}
+
+			// Attempt to remove statically-determined switch statements
 			if len(s.Cases) == 0 {
 				if p.astHelpers.ExprCanBeRemovedIfUnused(s.Test) {
 					// Remove everything

internal/js_parser/js_parser_test.go

@@ -3666,6 +3666,10 @@ 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, "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: default: a(); break; case 0: b() }", "switch (x) {\n  case 0:\n  default:\n    a();\n    break;\n  case 0:\n    b();\n}\n")
 }
 
 func TestMangleAddEmptyString(t *testing.T) {

scripts/end-to-end-tests.js

@@ -3406,6 +3406,95 @@ for (const minify of [[], ['--minify-syntax']]) {
         var x = class { static [-0xFFFF_FFFF_FFFF] = 1 }; if (x['-281474976710655'] !== 1) throw 'fail: -0xFFFF_FFFF_FFFF'
       `,
     }),
+    test(['in.js', '--outfile=node.js'].concat(minify), {
+      'in.js': `
+        function test(x) {
+          let log = ''
+          switch (x) {
+            case 0:
+              log += '0'
+            case 1:
+            default:
+              log += '1'
+          }
+          return log
+        }
+        if (test(0) !== '01') throw 'fail: 0'
+        if (test(1) !== '1') throw 'fail: 1'
+        if (test(2) !== '1') throw 'fail: 2'
+      `,
+    }),
+    test(['in.js', '--outfile=node.js'].concat(minify), {
+      'in.js': `
+        function test(x) {
+          let log = ''
+          switch (x) {
+            case 0:
+              log += '0'
+              break
+            case 1:
+            default:
+              log += '1'
+          }
+          return log
+        }
+        if (test(0) !== '0') throw 'fail: 0'
+        if (test(1) !== '1') throw 'fail: 1'
+        if (test(2) !== '1') throw 'fail: 2'
+      `,
+    }),
+    test(['in.js', '--outfile=node.js'].concat(minify), {
+      'in.js': `
+        function test(x) {
+          let log = ''
+          switch (x) {
+            case 0:
+              log += '0'
+            case 1:
+            default:
+              log += '1'
+            case 2:
+              log += '2'
+          }
+          return log
+        }
+        if (test(0) !== '012') throw 'fail: 0'
+        if (test(1) !== '12') throw 'fail: 1'
+        if (test(2) !== '2') throw 'fail: 2'
+        if (test(3) !== '12') throw 'fail: 3'
+      `,
+    }),
+    test(['in.js', '--outfile=node.js'].concat(minify), {
+      'in.js': `
+        function test(x) {
+          let log = ''
+          switch (x) {
+            case 0: // Note: This case must not be removed
+            default:
+              log += '1'
+            case 0:
+              log += '2'
+          }
+          return log
+        }
+        if (test(0) !== '12') throw 'fail: 0'
+        if (test(1) !== '12') throw 'fail: 1'
+      `,
+    }, {
+      expectedStderr: `▲ [WARNING] This case clause will never be evaluated because it duplicates an earlier case clause [duplicate-case]
+
+    in.js:8:12:
+      8 │             case 0:
+        ╵             ~~~~
+
+  The earlier case clause is here:
+
+    in.js:5:12:
+      5 │             case 0: // Note: This case must not be removed
+        ╵             ~~~~
+
+`,
+    }),
 
     // See: https://github.com/evanw/esbuild/issues/3195
     test(['in.js', '--outfile=node.js', '--keep-names'].concat(minify), {