fix #734: ts decorators on class ctor args

CHANGELOG.md

@@ -2,6 +2,18 @@
 
 ## Unreleased
 
+* Fix TypeScript parameter decorators on class constructors ([#734](https://github.com/evanw/esbuild/issues/734))
+
+    This release fixes a TypeScript translation bug where parameter decorators on class constructors were translated incorrectly. Affected code looks like this:
+
+    ```js
+    class Example {
+      constructor(@decorator param: any) {}
+    }
+    ```
+
+    This bug has been fixed. In addition, decorators are no longer allowed on class constructors themselves because they are not allowed in TypeScript.
+
 * Resolve `browser` entries in `package.json` with no file extension ([#740](https://github.com/evanw/esbuild/issues/740))
 
     This fix changes how esbuild interprets the `browser` field in `package.json`. It will now remap imports without a file extension to `browser` map entries without a file extension, which improves compatibility with Webpack. Specifically, a `package.json` file with `"browser": {"./file": "./something.js"}` will now match an import of `./file`. Previously the `package.json` file had to contain something like `"browser": {"./file.js": "./something.js"}` instead. Note that for compatibility with the rest of the ecosystem, a remapping of `./file` will counter-intuitively _not_ match an import of `./file.js` even though it works fine in the other direction.

internal/bundler/bundler_ts_test.go

@@ -529,6 +529,7 @@ func TestTypeScriptDecorators(t *testing.T) {
 				export default class Foo {
 					@x @y mUndef
 					@x @y mDef = 1
+					constructor(@x0 @y0 arg0, @x1 @y1 arg1) {}
 					@x @y method(@x0 @y0 arg0, @x1 @y1 arg1) { return new Foo }
 					@x @y static sUndef
 					@x @y static sDef = new Foo

internal/bundler/snapshots/snapshots_ts.txt

@@ -311,7 +311,7 @@ TestTypeScriptDecorators
 ---------- /out.js ----------
 // all.ts
 var Foo = class {
-  constructor() {
+  constructor(arg0, arg1) {
     this.mDef = 1;
   }
   method(arg0, arg1) {
@@ -356,7 +356,11 @@ __decorate([
 ], Foo, "sMethod", 1);
 Foo = __decorate([
   x.y(),
-  new y.x()
+  new y.x(),
+  __param(0, x0),
+  __param(0, y0),
+  __param(1, x1),
+  __param(1, y1)
 ], Foo);
 var all_default = Foo;
 

internal/js_parser/js_parser.go

@@ -4501,13 +4501,21 @@ func (p *parser) parseClass(name *js_ast.LocRef, classOpts parseClassOpts) js_as
 		}
 
 		// Parse decorators for this property
+		firstDecoratorLoc := p.lexer.Loc()
 		if opts.allowTSDecorators {
 			opts.tsDecorators = p.parseTypeScriptDecorators()
 		}
 
 		// This property may turn out to be a type in TypeScript, which should be ignored
 		if property, ok := p.parseProperty(js_ast.PropertyNormal, opts, nil); ok {
 			properties = append(properties, property)
+
+			// Forbid decorators on class constructors
+			if len(opts.tsDecorators) > 0 {
+				if key, ok := property.Key.Data.(*js_ast.EString); ok && js_lexer.UTF16EqualsString(key.Value, "constructor") {
+					p.log.AddError(&p.source, firstDecoratorLoc, "TypeScript does not allow decorators on class constructors")
+				}
+			}
 		}
 	}
 

internal/js_parser/js_parser_lower.go

@@ -1585,10 +1585,18 @@ func (p *parser) lowerClass(stmt js_ast.Stmt, expr js_ast.Expr, shadowRef js_ast
 		// Merge parameter decorators with method decorators
 		if p.options.ts.Parse && prop.IsMethod {
 			if fn, ok := prop.Value.Data.(*js_ast.EFunction); ok {
+				isConstructor := false
+				if key, ok := prop.Key.Data.(*js_ast.EString); ok {
+					isConstructor = js_lexer.UTF16EqualsString(key.Value, "constructor")
+				}
 				for i, arg := range fn.Fn.Args {
 					for _, decorator := range arg.TSDecorators {
 						// Generate a call to "__param()" for this parameter decorator
-						prop.TSDecorators = append(prop.TSDecorators,
+						var decorators *[]js_ast.Expr = &prop.TSDecorators
+						if isConstructor {
+							decorators = &class.TSDecorators
+						}
+						*decorators = append(*decorators,
 							p.callRuntime(decorator.Loc, "__param", []js_ast.Expr{
 								{Loc: decorator.Loc, Data: &js_ast.ENumber{Value: float64(i)}},
 								decorator,

internal/js_parser/ts_parser_test.go

@@ -1183,6 +1183,10 @@ func TestTSDecorator(t *testing.T) {
 	expectParseErrorTS(t, "class Foo { @dec static *#foo() {} }", "<stdin>: error: Expected identifier but found \"#foo\"\n")
 	expectParseErrorTS(t, "class Foo { @dec static async #foo() {} }", "<stdin>: error: Expected identifier but found \"#foo\"\n")
 	expectParseErrorTS(t, "class Foo { @dec static async* #foo() {} }", "<stdin>: error: Expected identifier but found \"#foo\"\n")
+
+	// Decorators aren't allowed on class constructors
+	expectParseErrorTS(t, "class Foo { @dec constructor() {} }", "<stdin>: error: TypeScript does not allow decorators on class constructors\n")
+	expectParseErrorTS(t, "class Foo { @dec public constructor() {} }", "<stdin>: error: TypeScript does not allow decorators on class constructors\n")
 }
 
 func TestTSTry(t *testing.T) {