Re: Subclassing ES6 objects with ES5 syntax.

Alex Kocharin Fri, 24 Apr 2015 07:26:47 -0700

True, but it should be your responsibility (as a person who implements a subclass) to resolve those issues.

Thankfully, it's easily done:

```

function MyArray(...args) {

var self = Array.apply(null, args)

Object.setPrototypeOf(self, MyArray.prototype)

return self

}

Object.setPrototypeOf(MyArray.prototype, Array.prototype)

;[ 'slice', 'map', 'concat' /* whatever else */ ].forEach(function (method) {

MyArray.prototype[method] = function (...args) {

var result = Array.prototype[method].apply(this, args)

Object.setPrototypeOf(result, MyArray.prototype)

return result

}

})

console.log(MyArray(1).slice(1) instanceof MyArray)

// true

```

24.04.2015, 14:13, "Andrea Giammarchi" <[email protected]>:

Not exactly ... if it's an Array, as example, the moment you slice/map/splice/concat, etc will return an instanceof Array, not an instance of whatever you have sublcassed.

Regards

On Fri, Apr 24, 2015 at 10:24 AM, Alex Kocharin <[email protected]> wrote:

I believe you can subclass anything using code like this:

function MyPromise(executor) {
var self = new Promise(executor)
self.setPrototypeOf(self, MyPromise.prototype)
return self
}
Object.setPrototypeOf(MyPromise, Promise)

... and it can be easily subclassed itself in the same way.

24.04.2015, 04:02, "C. Scott Ananian" <[email protected]>:
Is there any way to access `new.target` using ES5 syntax?
It appears that the "correct" way to create a subclass using ES5 syntax is:
```
function MyPromise(executor) {
var self = Reflect.construct(Promise, [executor], new.target);
return self;
}
Object.setPrototypeOf(MyPromise, Promise);
```
But since `new.target` isn't accessible, we have to do something like:
```
function MyPromise(executor) {
var self = Reflect.construct(Promise, [executor], MyPromise); // <-- THIS
return self;
}
Object.setPrototypeOf(MyPromise, Promise);
```
which works for only a single level of subclassing. That is, it allows us to create and instantiate MyPromise, but now nobody can subclass MyPromise. That's too bad.
Is there any way around this?
--scott
ps. Use case: My `prfun` package on npm subclasses `Promise` in order to add all the useful utility helpers without stomping on the global `Promise` object. I'd like to do so in a way which is compatible with both native ES6 promises (if they are available) and properly-written ES5 shims.
,
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Re: Subclassing ES6 objects with ES5 syntax.

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