Where is a function's this value stored?

Question

I know that you can bind the this value of a function using

function.bind(new_this_value, arguments)

But is there a way to access the bound value? ie something like this:

console.log(my_function.boundValue)

In other words, suppose a module provides the following function:

function getACoolFunction () {
  var someFarAwayFunction = function() {console.log(this.name)}
  var bound_this_value = {name: "bound this value"}
  someFarAwayFunction.bind(bound_this_value)
  return someFarAwayFunction;
}

and I have this in my code:

import {getACoolFunction} from coolModule

var coolFunction = getACoolFunction();
// coolFunction.bound_value

How do I get the bound value of coolFunction from my code without changing the module?

You have access to both `new_this_value` and `arguments` before you pass them to `.bind()` — guest271314, Feb 13 '17 at 21:22
I guess I'm more interested in how the JS engine knows what the this value is and how to access it when debugging — CamJohnson26, Feb 13 '17 at 21:24
Have you read the specification? What do you mean by "access it"? — guest271314, Feb 13 '17 at 21:24
So given a function, can you find the bound this value without calling the function — CamJohnson26, Feb 13 '17 at 21:32
`this` is `new_this_value`. Do you mean where you cannot view `new_this_value`at the bound function itself? — guest271314, Feb 13 '17 at 21:34
Given the edited Question, at first glance may not be possible without calling function. — guest271314, Feb 13 '17 at 21:43
No, this is not possible. Your best bet would be to overwrite `console.log` and just call the `coolfunction` to get the string. But really, if the object should be accessible, you should rewrite the module to make it accessible. — Bergi, Feb 13 '17 at 21:46

score 1 · Accepted Answer · edited May 23 '17 at 12:17

1

Nothing prevents you from doing this:

let my_function = function.bind(new_this_value, arguments);
my_function.boundValue = new_this_value;

As from the standard, a bound function is:

an exotic object that wraps another function object.

It has internal properties containing original function, provided this and arguments.
As they are explicitly mentioned as internal, I would say that they are not directly exposed and accessible.

Probably this Q/A contains details that can help you to work around this limitation.

edited May 23 '17 at 12:17

Community

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answered Feb 13 '17 at 21:23

skypjack

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Sure but I'm wondering if there's a way to get new_this_value without defining your own property, particularly if you're debugging objects you didn't write for example – CamJohnson26 Feb 13 '17 at 21:31

Where is a function's this value stored?

1 Answers1