Runtime parameter info (name, optional, default)

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

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.

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.

Start with the decorator metadata emission shown in the generated JavaScript, especially the design:paramtypes and __metadata calls. Compare the metadata produced for test, testOptional, and testDefault. Done means runtime decorators can distinguish parameter names, optional parameters, and default values, with the relevant behavior covered by tests.