For loop initializer capture

Question

I am quite confused about the closing behaviour of functions declared inside of for loops, in particular about variables defined in initializers:

function createFunctions(){
    const functions = []
    for(let i = 0; i < 5; i++)
        functions.push(() => i);
    return functions;
}
const results = createFunctions().map(m => m())
// results: [0, 1, 2, 3, 4]

vs

function createFunctions(){
    const functions = []
    let i;
    for(i = 0; i < 5; i++)
        functions.push(() => i);
    return functions;
}
const results = createFunctions().map(m => m())
// results: [5, 5, 5, 5, 5]

Since the anonymous arrow function declared in the for loop captures its scope, I would expect both cases to yield [5, 5, 5, 5, 5], since at call time, i's value is 5. The first result however seems to suggest that on each iteration through the loop, i is a different variable. However, if you repeat the test but the initialized variable is an object instead of a number:

function createFunctions(){
    const functions = []
    for(let obj = {}, i = 0; i < 5; i++)
        functions.push(() => obj);
    return functions;
}
const results = createFunctions().map(m => m())
// results: [{}, {}, {}, {}, {}]; results[0] === results[1]: true

we can see that all elements in the returned array are referentially equal, and so are not different variables. So it seems that the way that functions close over variables declared in the for loop initializer changes depending on whether the variable is a primitive or not, which sounds preposterous to me.

What am I missing?

score 0 · Answer 1 · answered Apr 21 '20 at 17:21

So, I went through all the trouble of writing this question, and while researching I found the answer somewhat by accident, so I thought I'd switch this to a self-answer question so that if others encounter the same behaviour, it may be easier to find.

I found the answer here

In loops, you get a fresh binding for each iteration if you let-declare a variable. The loops that allow you to do so are: for, for-in and for-of.

Which explains why the objects are equal: we get a new variable which gets assigned to the value of the previous one at the end of the loop. In the case of the object, the new variable is pointing to the same object, which explains the referential equality. It also explains why the first case yields [0, 1, 2, 3, 4]: numbers are values, not references, unlike objects.

For loop initializer capture

1 Answers1