global-constructors-fix.md

fix Generator and AsyncGenerator

This feature makes breaking change, not included in breaking-free version

There are some faulty designs with the prototype chains of Generator and AsyncGenerator.

For example, here are a generator function and returns a generator, and the generator has a constructor.

const generatorFunction= function*(){};
const generator= generatorFunction();
const constructor= generator.constructor;

The generator is an object, so it certainly should be

generator instanceof constructor // false

But as we've tried out, it unexpectedly false.

The generatorFunction, in some sense, construct the generator, so it looks should be

generatorFunction === constructor // false

but it's also false. At the least fortune, there is one thing true.

generator instanceof generatorFunction // true

And all generators have the same constructor, that's the global Generator provided by BetterJS.

constructor === Generator // true

Let's see the prototype graph

generator
    |
    |
    |     --constructor-→ undefined
    ↓    /
__proto__ ←--prototype--- generatorFunction --name--→  'generatorFunction'
    |
    |
    |     --constructor--
    ↓    /               ↘
__proto__ ←--prototype--- Generator --name--→ ''

There are three mistakes:

1. the constructor of the first depth point to undefined, but should be generatorFunction.
2. the Generator is in the prototype chain, but instanceof returrns false, that should be true.
3. the Generator has a empty name, it should be 'Generator'.

We fix 2 and 3, but unable to fix the mistake 1.

With BetterJS, we have

generator instanceof constructor       // true
generator instanceof generatorFunction // true

generatorFunction === constructor      // false
constructor === Generator              // true
Generator.name === 'Generator'