functions extracted from objects unbound by default

[tvcutsem]

A common mistake in JS is to extract a method from an object (as a first-class function value), then call the function without providing a binding for this. Example:

var obj = {
  offset: 0,
  arr: [1,2,3,4,5],
  nth: function(i) { return this.arr[this.offset + i]; }
};

// incorrect:
var nth = obj.nth;
nth(1); // oops, crashes as `this.offset` is undefined, or in strict-mode because `this` is undefined

// must explicitly bind the function on extraction (easy since ES5):
var nth = obj.nth.bind(obj);
nth(1);

I think the ability to be able to extract methods as unbound functions is useful and powerful (it allows for easy sharing of methods among different objects), but it seems this mistake is so common that extracting methods as bound functions could have been the default.

[michaelficarra]

@tvcutsem: But how would you unbind a function? Once bound, that function's context becomes useless.

[tvcutsem]

I wouldn't argue to ever unbind a bound function. Instead, if binding-on-extraction using the dot-operator would have been the default, I would imagine one could still get at the unbound function using reflection, e.g. Object.getOwnPropertyDescriptor(obj, "nth").value. This would make sharing methods between objects more painful though. It all depends on what the more frequent use case of extracting methods is: is the extracted method used as a stand-alone function, or is it installed as a method in another object?

[Ayms]

Finally maybe the default behavior is the one that indicates fat arrow :

var obj = {
  offset: 0,
  arr: [1,2,3,4,5],
  nth: (i) => { return this.arr[this.offset + i]; }
};

var nth = obj.nth;

obj.nth(); //|this| is the global or undefined if strict mode
nth(); //idem

Of course, one could have expected the contrary (ie |this| is obj for both)

I did not deeply think about it but doing this project https://github.com/Ayms/node-Tor where the use of "var self=this" is replaced by "bind" (for future replacement by =>) at a certain point of time it came to me the question : "why don't we have an unbind ?", but apparently you are saying that's it's not arguable