On Apr 2, 2012, at 9:47 PM, Brendan Eich wrote:
> Douglas Crockford wrote:
>>
>>
>> It seems I misunderstood what we were agreeing to. I think the (this...)
>> form is critically important, and the immutability thing as well.
>
> (...)
>
> I agree that leading |this| could be important for dynamic non-method
> use-cases, but those are relatively rare (let's not discount JQuery, but
> again, it could use long functions and survive). We could put
> leading-this-parameterization on the agenda for May, but we'll have to be
> careful not to lose consensus on arrows.
I was chatting this evening with a friend about this (ruby) thing:
class Numeric
(Math.methods - Module.methods - ["hypot", "ldexp"]).each do |method|
define_method method do
Math.send method, self
end
end
end
And I showed him how one could do that easily in JavaScript:
Object.getOwnPropertyNames(Math).forEach(function (key) {
if (typeof Math[key] === 'function') {
Number.prototype[key]= function () {
return Math[key](this);
};
}
});
Then I thought, let's see how would that look like with arrows ?
Object.getOwnPropertyNames(Math).forEach((key) => {
if (typeof Math[key] === 'function') Number.prototype[key]= () =>
Math[key](this);
});
And it turns out that here, the outer/enclosing context `this` isn't the one
you want/need. You'd need a dynamically bound `this` instead:
Object.getOwnPropertyNames(Math).forEach((key) => {
if (typeof Math[key] === 'function') Number.prototype[key]= (this) =>
Math[key](this);
});
and thus the this-in-the-parameters-list trick.
I don't think it's such a rare case: Inside a constructor function called with
`new`, yes, `this` is most likely going to be the right `this` always, but when
you are building objects with a factory (e.g. with a .create() method), the
enclosing `this` usually isn't going to be the right one.
--
Jorge.
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