fix #2496: minify removes useless break statements

CHANGELOG.md

@@ -35,6 +35,10 @@
         * `class-static-blocks=false`
         * `class-static-field=false`
 
+* Implement a small minification improvement ([#2496](https://github.com/evanw/esbuild/issues/2496))
+
+    Some people write code that contains a label with an immediate break such as `x: break x`. Previously this code was not removed during minification but it will now be removed during minification starting with this release.
+
 ## 0.15.5
 
 * Fix issues with Yarn PnP and Yarn's workspaces feature ([#2476](https://github.com/evanw/esbuild/issues/2476))

internal/js_parser/js_parser.go

@@ -9454,6 +9454,13 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
 		s.Stmt = p.visitSingleStmt(s.Stmt, stmtsNormal)
 		p.popScope()
 
+		// Optimize "x: break x" which some people apparently write by hand
+		if p.options.minifySyntax {
+			if child, ok := s.Stmt.Data.(*js_ast.SBreak); ok && child.Label != nil && child.Label.Ref == s.Name.Ref {
+				return stmts
+			}
+		}
+
 	case *js_ast.SLocal:
 		// Local statements do not end the const local prefix
 		p.currentScope.IsAfterConstLocalPrefix = wasAfterAfterConstLocalPrefix

internal/js_parser/js_parser_test.go

@@ -2031,6 +2031,13 @@ func TestLabels(t *testing.T) {
 	expectPrinted(t, "x: ({ f() { x: 1; } }).f()", "x:\n  ({ f() {\n    x:\n      1;\n  } }).f();\n")
 	expectPrinted(t, "x: (function() { x: 1; })()", "x:\n  (function() {\n    x:\n      1;\n  })();\n")
 	expectParseError(t, "x: y: x: 1", "<stdin>: ERROR: Duplicate label \"x\"\n<stdin>: NOTE: The original label \"x\" is here:\n")
+
+	expectPrinted(t, "x: break x", "x:\n  break x;\n")
+	expectPrinted(t, "x: { break x; foo() }", "x: {\n  break x;\n  foo();\n}\n")
+	expectPrintedMangle(t, "x: break x", "")
+	expectPrintedMangle(t, "x: { break x; foo() }", "")
+	expectPrintedMangle(t, "y: while (foo()) x: { break x; foo() }", "y:\n  for (; foo(); )\n    ;\n")
+	expectPrintedMangle(t, "y: while (foo()) x: { break y; foo() }", "y:\n  for (; foo(); )\n    x:\n      break y;\n")
 }
 
 func TestArrow(t *testing.T) {