Function.prototype.bind

Question

I\'ve got pretty interesting question about EcmaScript-5 Function.prototype.bind implementation. Usually when you use bind, you do it this way:

var myFunctio         


        

Answer 1

The precedent for Function.prototype.bind was the implementation of the idea in various JS frameworks. To the best of my knowledge, none of them allowed this-binding to be changed by subsequent binding. You might as well ask why none of them allowed this-binding changing, as to ask why ES5 doesn't allow it.

You're not the only person I've heard who thought this odd. Chris Leary, who works on Mozilla's JS engine (as I do), thought it a bit odd, raising the issue on Twitter a couple months ago. And in a somewhat different form, I remember one of the Mozilla Labs hackers questioning if there were some way to "unbind" a function, to extract the target function from it. (If you could do that, you could of course bind it to a different this, at least if you could also extract the bound arguments list to also pass it along.)

I don't remember the issue being discussed when bind was being specified. However, I wasn't paying particularly close attention to the es-discuss mailing list at the time this stuff was hashed out. That said, I don't believe ES5 was looking to innovate in the area much, just "pave a cowpath", to borrow a phrase.

You might possibly be able to propose some introspective methods to address these concerns to es-discuss, if you wrote a sufficiently detailed proposal. On the other hand, binding is a form of information-hiding mechanism, which would cut against its adoption. It might be worth a try to propose something, if you have time. My guess is the information-hiding concern would block a proposal from being adopted. But that's just a guess that could well be wrong. Only one way to find out...