var __extends as --extends CLI option

[streamich]

Every time you extend a class in TypeScript this code is appended to the transpiled .js file:

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 __());
};

Here is a possible quick fix to allow developers remove this code and specify a global __extends function for all files. In TypeScript command line tool add an extra parameter --extends that allows to provide the code for the __extends function.

For example:

tsc --extends 'window.myExtends;` hello-world.ts

This would output the __extends function as:

var __extends = window.myExtends;

[DanielRosenwasser]

We already have a --noEmitHelpers compiler option so that you can define your own __extends function yourself in code. Is there any reason that scenario doesn't suffice?

[streamich]

@DanielRosenwasser thanks, looks like it does the job. But I'm not sure what exactly that option does, I found in documentation:

Do not generate custom helper functions like __extends in compiled output.

So, I'm not sure what else besides __extends it stops to generate, otherwise it's good.

[DanielRosenwasser]

That's a good point - it might not be quite as granular as you'd want. But here are all of the helpers TypeScript will currently emit.

They're used for:

• Class inheritance
• Decorators & decorator metadata
• Async functions (i.e. async/await)
• JSX Spread attributes

In any case, for now I think we'll close this (since the extra emit isn't so bad anyhow), but if it becomes a real issue, we can consider making things more granular.

[streamich]

1. Yes, more granular would indeed be helpful.
2. What do you exactly mean by JSX Spread attributes? For me in .tsx files it just generates Reacts's React.__spread.
3. BTW, React.__spread is deprecated now, will it be replaced in the future?

[DanielRosenwasser]

Check out #7270.