CODEWITHSUNDEEP
javascript
1252748
1252748

Reputation: 15369

understanding iterator in js closure

I do not understand how calling this function works

function setup(x) {
    var i = 0;
    return function () {
        console.log(i);
        return x[i++];

    };
}

var next = setup(['a', 'b', 'c']);

console.log(next());//a
console.log(next());//b
console.log(next());//c

how is i not reset to 0 with each call?

Upvotes: 0

Views: 431

Answers (2)

RobG
RobG

Reputation: 147423

In your code:

> function setup(x) {
>     var i = 0;

When setup is called, a new execution context is created with local variables x and i. When execution begins, x is assigned a value of the passed array and i is set to zero.

>     return function () {
>         console.log(i);
>         return x[i++];
>     };

This anonymous function is returned. It has a closure to the outer execution object that is placed on its scope chain (so is the global execution object).

So it has a closure to both i and x, whose values can now only be changed by the function.

> }
>
> var next = setup(['a', 'b', 'c']);

The returned function is assigned to next, so it's no longer anonymous.

> 
> console.log(next());//a

When this line executes, next returns x[0], which is "a", and increments i to 1.

> console.log(next());//b

When this line executes, next returns x[1], which is "b", and increments i to 2.

> console.log(next());//c

When this line executes, next returns x[2], which is "c", and increments i to 3.

Upvotes: 1

nfechner
nfechner

Reputation: 17525

i is not reset, because it was set outside of the function that is assigned to next. In fact in the line directly above it.

Upvotes: 0

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