fix #2439: webpack magic comments in new Worker

CHANGELOG.md

@@ -19,6 +19,10 @@
 
     TypeScript code that does this should now be parsed correctly.
 
+* Preserve Webpack comments inside constructor calls ([#2439](https://github.com/evanw/esbuild/issues/2439))
+
+    This improves the use of esbuild as a faster TypeScript-to-JavaScript frontend for Webpack, which has special [magic comments](https://webpack.js.org/api/module-methods/#magic-comments) inside `new Worker()` expressions that affect Webpack's behavior.
+
 ## 0.15.16
 
 * Add a package alias feature ([#2191](https://github.com/evanw/esbuild/issues/2191))

internal/js_ast/js_ast.go

@@ -492,8 +492,12 @@ var SEmptyShared = &SEmpty{}
 var SDebuggerShared = &SDebugger{}
 
 type ENew struct {
-	Target        Expr
-	Args          []Expr
+	Target Expr
+	Args   []Expr
+
+	// See this for more context: https://github.com/evanw/esbuild/issues/2439
+	WebpackComments []Comment
+
 	CloseParenLoc logger.Loc
 	IsMultiLine   bool
 

internal/js_parser/js_parser.go

@@ -3341,19 +3341,21 @@ func (p *parser) parsePrefix(level js_ast.L, errors *deferredErrors, flags exprF
 		}
 
 		target := p.parseExprWithFlags(js_ast.LMember, flags)
+		var webpackComments []js_ast.Comment
 		args := []js_ast.Expr{}
 		var closeParenLoc logger.Loc
 		var isMultiLine bool
 
 		if p.lexer.Token == js_lexer.TOpenParen {
-			args, closeParenLoc, isMultiLine = p.parseCallArgs()
+			webpackComments, args, closeParenLoc, isMultiLine = p.parseCallArgs()
 		}
 
 		return js_ast.Expr{Loc: loc, Data: &js_ast.ENew{
-			Target:        target,
-			Args:          args,
-			CloseParenLoc: closeParenLoc,
-			IsMultiLine:   isMultiLine,
+			Target:          target,
+			Args:            args,
+			WebpackComments: webpackComments,
+			CloseParenLoc:   closeParenLoc,
+			IsMultiLine:     isMultiLine,
 		}}
 
 	case js_lexer.TOpenBracket:
@@ -3839,7 +3841,7 @@ func (p *parser) parseSuffix(left js_ast.Expr, level js_ast.L, errors *deferredE
 				if level >= js_ast.LCall {
 					return left
 				}
-				args, closeParenLoc, isMultiLine := p.parseCallArgs()
+				_, args, closeParenLoc, isMultiLine := p.parseCallArgs()
 				left = js_ast.Expr{Loc: left.Loc, Data: &js_ast.ECall{
 					Target:        left,
 					Args:          args,
@@ -3861,7 +3863,7 @@ func (p *parser) parseSuffix(left js_ast.Expr, level js_ast.L, errors *deferredE
 				if level >= js_ast.LCall {
 					return left
 				}
-				args, closeParenLoc, isMultiLine := p.parseCallArgs()
+				_, args, closeParenLoc, isMultiLine := p.parseCallArgs()
 				left = js_ast.Expr{Loc: left.Loc, Data: &js_ast.ECall{
 					Target:        left,
 					Args:          args,
@@ -3963,7 +3965,7 @@ func (p *parser) parseSuffix(left js_ast.Expr, level js_ast.L, errors *deferredE
 			if level >= js_ast.LCall {
 				return left
 			}
-			args, closeParenLoc, isMultiLine := p.parseCallArgs()
+			_, args, closeParenLoc, isMultiLine := p.parseCallArgs()
 			left = js_ast.Expr{Loc: left.Loc, Data: &js_ast.ECall{
 				Target:        left,
 				Args:          args,
@@ -4474,11 +4476,13 @@ func (p *parser) parseExprOrLetStmt(opts parseStmtOpts) (js_ast.Expr, js_ast.Stm
 	return p.parseSuffix(expr, js_ast.LLowest, nil, 0), js_ast.Stmt{}, nil
 }
 
-func (p *parser) parseCallArgs() (args []js_ast.Expr, closeParenLoc logger.Loc, isMultiLine bool) {
+func (p *parser) parseCallArgs() (webpackComments []js_ast.Comment, args []js_ast.Expr, closeParenLoc logger.Loc, isMultiLine bool) {
 	// Allow "in" inside call arguments
 	oldAllowIn := p.allowIn
 	p.allowIn = true
 
+	oldWebpackComments := p.lexer.WebpackComments
+	p.lexer.WebpackComments = &webpackComments
 	p.lexer.Expect(js_lexer.TOpenParen)
 
 	for p.lexer.Token != js_lexer.TCloseParen {
@@ -4509,6 +4513,7 @@ func (p *parser) parseCallArgs() (args []js_ast.Expr, closeParenLoc logger.Loc,
 		isMultiLine = true
 	}
 	closeParenLoc = p.lexer.Loc()
+	p.lexer.WebpackComments = oldWebpackComments
 	p.lexer.Expect(js_lexer.TCloseParen)
 	p.allowIn = oldAllowIn
 	return

internal/js_printer/js_printer.go

@@ -1743,27 +1743,40 @@ func (p *printer) printExpr(expr js_ast.Expr, level js_ast.L, flags printExprFla
 
 		// Omit the "()" when minifying, but only when safe to do so
 		if !p.options.MinifyWhitespace || len(e.Args) > 0 || level >= js_ast.LPostfix {
-			isMultiLine := e.IsMultiLine && len(e.Args) > 0 && !p.options.MinifyWhitespace
+			isMultiLine := !p.options.MinifyWhitespace && ((e.IsMultiLine && len(e.Args) > 0 && !p.options.MinifyWhitespace) || len(e.WebpackComments) > 0)
+			needsNewline := true
 			p.print("(")
 			if isMultiLine {
 				p.options.Indent++
+				if len(e.WebpackComments) > 0 {
+					p.printNewline()
+					needsNewline = false
+					for _, comment := range e.WebpackComments {
+						p.printIndentedComment(comment.Text)
+					}
+				}
 			}
 			for i, arg := range e.Args {
 				if isMultiLine {
 					if i != 0 {
 						p.print(",")
 					}
-					p.printNewline()
+					if needsNewline {
+						p.printNewline()
+					}
 					p.printIndent()
 				} else if i != 0 {
 					p.print(",")
 					p.printSpace()
 				}
 				p.printExpr(arg, js_ast.LComma, 0)
+				needsNewline = true
 			}
 			if isMultiLine {
 				p.options.Indent--
-				p.printNewline()
+				if needsNewline {
+					p.printNewline()
+				}
 				p.printIndent()
 			}
 			if e.CloseParenLoc.Start > expr.Loc.Start {

internal/js_printer/js_printer_test.go

@@ -319,6 +319,20 @@ func TestNew(t *testing.T) {
 	expectPrintedMinify(t, "new x().y", "new x().y;")
 	expectPrintedMinify(t, "new x() + y", "new x+y;")
 	expectPrintedMinify(t, "new x() ** 2", "new x**2;")
+
+	// Test preservation of Webpack-specific comments
+	expectPrinted(t, "new Worker(// webpackFoo: 1\n // webpackBar: 2\n 'path');", "new Worker(\n  // webpackFoo: 1\n  // webpackBar: 2\n  \"path\"\n);\n")
+	expectPrinted(t, "new Worker(/* webpackFoo: 1 */ /* webpackBar: 2 */ 'path');", "new Worker(\n  /* webpackFoo: 1 */\n  /* webpackBar: 2 */\n  \"path\"\n);\n")
+	expectPrinted(t, "new Worker(\n    /* multi\n     * line\n     * webpackBar: */ 'path');", "new Worker(\n  /* multi\n   * line\n   * webpackBar: */\n  \"path\"\n);\n")
+	expectPrinted(t, "new Worker(/* webpackFoo: 1 */ 'path' /* webpackBar:2 */);", "new Worker(\n  /* webpackFoo: 1 */\n  /* webpackBar:2 */\n  \"path\"\n);\n")
+	expectPrinted(t, "new Worker(/* webpackFoo: 1 */ 'path' /* webpackBar:2 */ ,);", "new Worker(\n  /* webpackFoo: 1 */\n  /* webpackBar:2 */\n  \"path\"\n);\n")
+	expectPrinted(t, "new Worker(/* webpackFoo: 1 */ 'path', /* webpackBar:2 */ );", "new Worker(\n  /* webpackFoo: 1 */\n  /* webpackBar:2 */\n  \"path\"\n);\n")
+	expectPrinted(t, "new Worker(new URL('path', /* webpackFoo: these can go anywhere */ import.meta.url))",
+		"new Worker(new URL(\n  /* webpackFoo: these can go anywhere */\n  \"path\",\n  import.meta.url\n));\n")
+
+	// Other comments should not be preserved
+	expectPrinted(t, "new Worker(// comment 1\n // comment 2\n 'path');", "new Worker(\n  \"path\"\n);\n")
+	expectPrinted(t, "new Worker(/* comment 1 */ /* comment 2 */ 'path');", "new Worker(\"path\");\n")
 }
 
 func TestCall(t *testing.T) {
@@ -677,7 +691,8 @@ func TestImport(t *testing.T) {
 	expectPrinted(t, "import(/* webpackFoo: 1 */ 'path' /* webpackBar:2 */ ,);", "import(\n  /* webpackFoo: 1 */\n  /* webpackBar:2 */\n  \"path\"\n);\n")
 	expectPrinted(t, "import(/* webpackFoo: 1 */ 'path', /* webpackBar:2 */ );", "import(\n  /* webpackFoo: 1 */\n  /* webpackBar:2 */\n  \"path\"\n);\n")
 	expectPrinted(t, "import(/* webpackFoo: 1 */ 'path', { type: 'module' } /* webpackBar:2 */ );", "import(\n  /* webpackFoo: 1 */\n  /* webpackBar:2 */\n  \"path\",\n  { type: \"module\" }\n);\n")
-	expectPrinted(t, "import(new URL('path', /* webpackFoo: these can go anywhere */ import.meta.url))", "import(\n  /* webpackFoo: these can go anywhere */\n  new URL(\"path\", import.meta.url)\n);\n")
+	expectPrinted(t, "import(new URL('path', /* webpackFoo: these can go anywhere */ import.meta.url))",
+		"import(new URL(\n  /* webpackFoo: these can go anywhere */\n  \"path\",\n  import.meta.url\n));\n")
 
 	// Other comments should not be preserved
 	expectPrinted(t, "import(// comment 1\n // comment 2\n 'path');", "import(\"path\");\n")