tslib.__extends called on undefined prototype

[jsayol]

TypeScript Version: 2.2.1

Code
To make things simple, here's a repo to easily reproduce this: https://github.com/jsayol/typescript-issue-14734

$ git clone https://github.com/jsayol/typescript-issue-14734.git
$ cd typescript-issue-14734
$ npm install
$ npm run build

And load index.html in the browser.

a.ts:

import { B } from './b';

export class A {
  constructor(public prop: string) {
  }

  b(): B {
    return new B(this.prop, 123);
  }

  log() {
    console.log(`Prop: ${this.prop}`);
  }
}

b.ts

import { A } from './a';

export class B extends A {
  constructor(prop: string, public otherStuff: number) {
    super(`B ${prop}`);
  }
}

index.ts

import { A } from './a';
import { B } from './b';

const a = new A('hello');
const b: B = a.b();
b.log();

Expected behavior:
Seeing "B hello" logged in the console.

For reference, putting all the code in a single file generates the correct output:

class A {
  constructor(public prop: string) {
  }

  b(): B {
    return new B(this.prop, 123);
  }

  log() {
    console.log(`Prop: ${this.prop}`);
  }
}

class B extends A {
  constructor(prop: string, public otherStuff: number) {
    super(`B ${prop}`);
  }
}

const a = new A('hello');
const b: B = a.b();
b.log();

Actual behavior:

tslib.es6.js:22 Uncaught TypeError: Object prototype may only be an Object or null: undefined
    at setPrototypeOf (<anonymous>)
    at Object.__extends [as a] (tslib.es6.js:22)
    at b.js:4
    at Object.__webpack_exports__.a (b.js:11)
    at __webpack_require__ (bootstrap 4672cf4…:19)
    at Object.defineProperty.value (bootstrap 4672cf4…:65)
    at __webpack_require__ (bootstrap 4672cf4…:19)
    at Object.<anonymous> (bundle.umd.js:108)
    at __webpack_require__ (bootstrap 4672cf4…:19)
    at Object.<anonymous> (bundle.umd.js:321)

This is the generated b.js, where the problem is:

import * as tslib_1 from "tslib";
import { A } from './a';
var B = (function (_super) {
    tslib_1.__extends(B, _super); // <--- Exception thrown here. "B" is undefined.
    function B(prop, otherStuff) {
        var _this = _super.call(this, "B " + prop) || this;
        _this.otherStuff = otherStuff;
        return _this;
    }
    return B;
}(A));
export { B };
//# sourceMappingURL=b.js.map

[jsayol]

Possibly related: #10837 and #5207

[kitsonk]

You are dealing with a circular dependency. TypeScript can statically identify the shape of everything, therefore it is fine, but it seems WebPack (you module loader of choice) is not resolving the circular dependency effectively at runtime. I suspect that WebPack is attempting to instantiate module B before module A is ready, therefore module B can be loaded properly.

When they are in the same module, B isn't technically required by A until a certain method is called, therefore everything is resolved fine.

[jsayol]

Thanks @kitsonk, makes sense. I wasn't sure at first if this was a TypeScript or Webpack issue. I'll move this over there and close it here for now.

[cvsguimaraes]

I'd like to add to @kitsonk's excellent explanation that you might be able to workaround this by changing the export order of some of your barrels/modules.

I can't explain it well in generic terms since it really depends on how you organized your barrels and where your webpack entry points are, but a good rule of thumb would be to always export higher level classes at the bottom and its dependencies at the top.

[jsayol]

Thanks for your imput @cvsguimaraes. In the end it all came down to the fact that TypeScript assumes that module imports do not have side effects.

See here for more info: webpack/webpack#4520 (comment)

[304NotModified]

Hi,

We have the same error. Currently it is difficult to find the cycle and I was hoping this could be fixed or the error could be improved.

But it's unclear to me if this is now a Webpack or typescript issue? (it's closed on both repos)

[cvsguimaraes]

But it's unclear to me if this is now a Webpack or typescript issue? (it's closed on both repos)

@304NotModified See this comment from webpack author on the issue.

[kodeine]

i am facing the same issue and cant find exact root cause of this error.