Runtime parameter info (name, optional, default) · Issue #8126 · microsoft/TypeScript

yinso · 2016-04-17T02:33:11.000Z

**TypeScript Version:** 1.8.9 **Code** I'm trying to leverage decorators to create runtime contracts to check against incoming parameters. Below is the ts code ``` ts class Foo { @contract test (a : integer, b : string) : boolean { return true } @contract testOptional (a : integer, b? : string) : boolean { return true } @contract testDefault (a : integer, b : string = 'hello') : boolean { return true } } ``` The above generated the following Javascript. ``` js var Foo = (function () { function Foo() { } Foo.prototype.test = function (a, b) { return true; }; Foo.prototype.testOptional = function (a, b) { return true; }; Foo.prototype.testDefault = function (a, b) { if (b === void 0) { b = 'hello'; } return true; }; __decorate([ contract, __metadata('design:type', Function), __metadata('design:paramtypes', [integer, String]), __metadata('design:returntype', Boolean) ], Foo.prototype, "test", null); __decorate([ contract, __metadata('design:type', Function), __metadata('design:paramtypes', [integer, String]), // same as above __metadata('design:returntype', Boolean) ], Foo.prototype, "testOptional", null); __decorate([ contract, __metadata('design:type', Function), __metadata('design:paramtypes', [integer, String]), // same as above __metadata('design:returntype', Boolean) ], Foo.prototype, "testDefault", null); return Foo; }()); ``` Note that the three different ts functions generated the exact sets of `__metadata` calls. The `design:paramtypes` alone cannot distinguish whether a particular parameter is optional or has default values, and thus insufficient for building a runtime contract. It would be great for TypeScript to support a full parameter info so decorators can be made more capable.