Javascript function bind override (how to bind it to another object)

Question

Is there a way to rebind a function that is already bound to another object via Function.prototype.bind?

var a={};
var b={};
var c=function(){ alert(this===a); };
c(); // alerts false
c=c.bind(a);
c(); // alerts true
c=c.bind(b);
c(); // still alerts true

I know that I can use a different approach and keep a "clean" function for binding, but I just wonder how to reuse an already bound function.

Answer 1

Is there a way to rebind a function that is already bound to another object via Function.prototype.bind?

No. From the ES2015 spec about Function.prototype.bind:

19.2.3.2 Function.prototype.bind ( thisArg , ...args)

[...]

Note 2: If Target is an arrow function or a bound function then the thisArg passed to this method will not be used by subsequent calls to F.

This was already true for earlier versions as well.

Answer 2

What .bind() does is almost the same as this:

function likeBind(fun, thisValue) {
  return function() {
    var args = [].slice.call(arguments, 0);
    return fun.apply(thisValue, args);
  };
}

So:

c = likeBind(c, a);

gives you a bound function. Now, even if you attempt to re-bind, the original bound function still exists in that closure with the value you originally requested to be used as this. Values of variables inside closures can only be changed from inside the closure, so there's nothing you can do to un-bind a bound function like that. You have to start over from the original function.

So, no.