Add explicit error to __extends when base class is undefined

[Arnavion]

We occasionally get people in the IRC channel asking about their transpiled JS failing with TypeError: undefined is not an object. It happens when accessing b.prototype inside __extends because the child class is before the base class, but they often think it's a bug in __extends. Would it make sense to detect if b is undefined at the start of the function and throw an appropriately worded error? Eg:

var __extends = this.__extends || function (d, b) {
    if (b === undefined) { throw new Error("The base class is not defined. Did you put the child class before the base class?"); }
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    __.prototype = b.prototype;
    d.prototype = new __();
};

[mhegazy]

would #4343 be sufficient to address the underlying problem?

[Arnavion]

It seems to me that PR only detects child before base in the same file? Does it handle, say, base class imported from another module that also has circular reference so base class is still undefined?

[frankwallis]

Coincidentally the circular referenced superclass issue came up on stack overflow just the other day: http://stackoverflow.com/questions/33897273/typescript-and-systemjs-circle-dependency/33905499#33905499

[mhegazy]

It seems to me that PR only detects child before base in the same file? Does it handle, say, base class imported from another module that also has circular reference so base class is still undefined?

you are correct, it does not handle circular references in modules, just in the same file, or if you are using --out.

[RyanCavanaugh]

The other common case I see is that someone refactors into two script files and forgets to add a script tag for the file containing the base class.

[DavidKDeutsch]

Out of curiosity, has the implementation of __extends changed recently? Ever since upgrading to 1.8, I seem to get these run time errors a lot more. Not sure if it is just a coincidence or what.

[Arnavion]

@DavidKDeutsch __extends is not the source of the problem (base class being undefined). It's the messenger. That's what this issue is about - of clarifying that the problem is not with __extends but with whether the classes are defined / loaded in the right order or not.

If you're seeing it more with 1.8 then it might be 1.8 changed the ordering of your classes such that the base class is undefined (although it sounds unlikely).

[DavidKDeutsch]

BTW, this can be especially confusing to developers when including other libraries that were also compiled with TypeScript, as the first file that defines __extends is where the error is thrown from.

[yssource]

I am having the same issue even if the class is used after the other:

BaseClass.ts:

module MyModule {
  export class Base {
    constructor() { }
  }
}

ChildClass.ts:

module MyModule {
  export class Child extends Base {
    constructor() { super(); }
  }
}

Generates:

/****************************************************************************
  Generated by T4TS.tt - don't make any changes in this file
****************************************************************************/

var MyModule;
(function (MyModule) {
  /** Generated from MyModule.Base **/
  var Base = (function () {
    function Base () {
    }
    return Base;
  })();
  MyModule.Base = Base;
})(MyModule || (MyModule = {}));

var __extends = (this && this.__extends) || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
};

var MyModule;
(function (MyModule) {
  /** Generated from MyModule.Child **/
  var Child = (function (_super) {
      __extends(Child, _super);
      function Child() {
          _super.apply(this, arguments);
      }
      return Child;
  })(MyModule.Base);
  MyModule.Child = Child;
})(MyModule|| (MyModule = {}));

It seems that the order is correct, although it doesn't look like it is working as expected.

But if I am merging two source files into one, then it is working well.

BaseClassAndChileClass.ts:

module MyModule {
  export class Base {
    constructor() { }
  }
  export class Child extends Base {
    constructor() { super(); }
  }
}