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Javascript – Does returning early prevent map() from iterating over the entire array?

Does the slice method iterate over the entire array? For example here I check what index and stop map when its on 4-th element. Do I have to do like that or is it better to just slice the array?

{data?.map((direction, i) => {
  if (i === 4) {
    return;
  }
  return (
    <Link
        className="hover:underline"
        key={direction.id}
        to={{
            pathname: '/courses',
            search: `?searchFilter%5B0%5D=${direction.id}`,
        }}
        onClick={scrollToTop}
    >
        {direction?.name}
    </Link>
  );
})}

My attempt is shown above

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2

Answers


  1. 0

    Does the slice method iterate over the entire array?

    No, it iterates as many times as the size of your slice. And when you apply a .map call on that slice, it will only visit the elements in that slice.

    For example here I check what index and stop map when its on 4-th element.

    That is a misunderstanding. That return is not stopping the map-callbacks to continue until the end of the array. If there is an index 5, your map callback will be called with that index too.

    Do I have to do like that or is it better to just slice the array?

    As your alternative does not achieve what you want, there is no question about "better". Just slice:

    data?.slice(0, 4)?.map((direction, i) => 
    ...
)
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  2. 0

    What if i have an array than includes over 10000 elements? Will slice iterate through the entire array to return new array with first four elements or not?

    No, it will only iterate over the elements you’ve asked for. This is covered by the loop starting at Step 14 in the specification.

    Do I have to do like that or is it better to just slice the array?

    You certainly can use slice and then map, making a temporary copy of a portion of the array and then mapping it.

    Someday, you’ll be able to use iterator methods to skip to the index you want, take only as many elements as you want, and map them, all without making a copy. More in the TC39 proposal here. Applying that to your example would be data?.take(4).map(/*...*/) I believe.

    Until then, you could write your own version of take and map, or use one of the "userland implementations" the TC39 proposal links to, or write your own simple utility function that maps a subset of the array. For example (off-the-cuff, untested):

    function mapPartial(array, callback, start = 0, end = array.length) {
    const result = [];
    // ...add adjusting `start` and `end` to be within bounds here...
    for (let index = start; index < end; ++index) {
        const value = array[index];
        const mappedValue = callback(value, index, array);
        result.push(mappedValue);
    }
    return result;
}

    Then you’d use data && mapPartial(data, /*...callback...*/, 0, 4).

    You might want to tweak the order of parameters.

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