function def() {
console.log(this.x)
}
var f = def.bind({ x:777 })
f() // prints 777
The bind creates a function f which is identical to def, except that within f, this is set to { x:777 }.
Is it possible to access the object f was bound to outside of f? E.g., console.log(f.this.x) (but that doesn't work). Or is it impossible for code that comes after to see what object f was bound to?
I found some useful information on bind here:
http://dmitrysoshnikov.com/notes/note-1-ecmascript-bound-functions/
bind as specified in ECMAScript 5 produces a sort of lightweight function (which differs in some ways from usual functions, as described in the link above. Basically it provides a wrapper for calling the target function, and maintains internal properties which include the target function, the bound this, and the bound arguments. As these are internal properties, they aren't accessible in the way the OP is asking about (you can't take an arbitrary bound function f and do something like f.getBoundThis()).
It's worth noting that bind is not unique in capturing some state. Closures also capture state. However, bind (as specified in ECMAScript 5) is not a closure, because closures capture variables whereas bind captures values.
Here's an example:
(function () {
var x = 2;
function thisSquared() { return this * this; }
f = thisSquared.bind(x);
g = function() { return x * x; } // g is a closure
console.log(f()); // Squares the captured value (2), prints 4
console.log(g()); // Squares x, prints 4
x = 3;
})();
console.log(f()); // Squares the captured value (still 2), prints 4
console.log(g()); // Squares x, prints 9
Some previous implementations of bind (written in JavaScript before ECMAScript 5) didn't have this distinction from closures.
No, you cannot access it because the object is only bound temporarily to the function for the lifetime of the call, it does not change the functions prototype.
777, but the question is can you access the bound object if you only have a reference to a function instance returned by bind().
this or 2) assign this to a variable bound to an outer function (not really ideal). bind, call, and apply all return the value the function they are executing returns.apply() or call()?f which was created using bind as in exebook's example, how do I extract the object that was passed to the bind? Obviously if I stored the object in some variable, or if f returns the object, then I can see it. But in the general case, is there some way to read the object given f. E.g., I could imagine an f.this, but it doesn't exist.
bindchanges the context ofthisinside a function but it doesn't execute it likecallandapply. I don't see what's the point of your code.xwon't be accessible I don't think so.xfrom thethisargument?bind, is it possible to extract the bound object from the function