How to select elements from an array based on the indices of another array in JavaScript?

Question

This works, but I'm wondering if there is a better way to filter by index:

a = [10,20,30,40]
b = [1,3]
a.filter((x,i) => b.includes(i))
// [20, 40]

Answer 1

Another way would be b.map(aIndex => a[aIndex]) . If b is shorter than a this could also be faster. However if there are indexes in b that do not belong in a , you would end up with undefined "holes" in the array.

EDIT

Looking a bit into Array.includes , looks like it will run in O(n) for unsorted arrays. If we say A = a.length and B = b.length , your solution from the question should run in O(A * B) . The second solution (with map) will run in O(B) . To fix the undefined holes, you could add a .filter(element => typeof element !== 'undefined') . The final solution would then be b.map(i => a[i]).filter(e => typeof e !== 'undefined') . This now runs in O(2 * B) , which should still be better than O(A * B) .

Answer 2

I think your solution is just great. map is a great solution too.

For the record, you could use a for...of loop, but it becomes much more complex for nothing...

 let a = [10, 20, 30, 40], b = [1, 3]; let res = []; for (const entry of a.entries()) { if (b.includes(entry[0])) { res.push(entry[1]); } } console.log(res); 

Answer 3

This (and more) can be done with Lodash's at function:

_.at([10, 20, 30, 40], [1, 3]) // [20, 40]
_.at(['a', 'b'], [0, 1, 1, 0]) // ['a', 'b', 'b', 'a']