Javascript - Access variable in function closure from outside

Question

I was wondering if there was some way to obtain the value of a variable that has been captured by a nested function, from outside of that said function. A little tricky to explain in words so here's what I'm trying to achieve:

function counter(){
    let count = 0;
    return function counterIncrementer(){
      ++count;
    }
}

function someReceiever(counterIncrementer){
    // From here, somehow access the value of count captured by 
    // counterIncrementer.
    // -> Without modifying the counter/returned counterIncrementer function 
    //    before runtime
}
someReceiever(counter())

Thanks !

Because I didn't write the 'counter' function and the variable was meant to be hidden. This was just for easy illustrative purposes. — Eladian
– Eladian, Commented Nov 5, 2018 at 23:29
there is no way to do that, the count in that case is available only in its context. and it is private to that as it is not exported in any way shape of form. — Paul G Mihai
– Paul G Mihai, Commented Nov 5, 2018 at 23:31

Szab · Accepted Answer · 2018-11-05 23:31:06Z

1

By my best knowledge - it's not possible to do that. The count value is a part of the counter function and is accessible only in this scope or in the scope of functions created inside that function (so basically closures).

I think you might find this thread useful: Accessing variables trapped by closure

answered Nov 5, 2018 at 23:31

Szab

1,2638 silver badges18 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Eladian Over a year ago

Do you know if it'd be possible to somehow create a new counterIncrementer function with a clone of its original scope or is that beyond the realm of possibility? If that was possible you could return a function with the captured variable.

Szab Over a year ago

If you know the scope, then you can use eval to do that: jsbin.com/wekoxokibe/edit?js,console . If you don't know the scope, then you would probably have to manually parse the scope function, which in more complex scenarios might be virtually impossible to do.

Sam Clarke · Accepted Answer · 2018-11-05 23:40:37Z

The only way to do this would be to declare count as a global, or create another function just for accessing count, nested within counter; but given the structure of your code, it doesn't seem like that's a great answer.

function counter(){
    let count = 0;
    return [
        function counterIncrementer(){
            ++count;
        }, 
        function counterGetter() {
            return count;
        }
    ];
}

function someReceiever(counterIncrementerPack){
    let counterIncrementer = counterIncrementerPack[0];
    let counterGetter = counterIncrementerPack[1];

    console.log(counterIncrementer(), counterGetter(), counterGetter(), counterIncrementer(), counterGetter());
}
someReceiever(counter())

Outputs: undefined 1 1 undefined 2

Note: you may also want to make counterIncrementer return ++count, but that wasn't the question shrug.

Collectives™ on Stack Overflow

Javascript - Access variable in function closure from outside

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related