Find multiple indexes in an array

Question

a = [-1, 150, 190, 170, -1, -1, 160, 180]

Given the above, how can I get another array of the indexes where -1 is found?

I was able to do it with a for loop and checking against the desired number, then getting the number of the iteration and pushing into an array. Wondering if there's a one liner or a way that won't involve a for loop.

Does this answer your question? [How to find index of all occurrences of element in array?](https://stackoverflow.com/questions/20798477/how-to-find-index-of-all-occurrences-of-element-in-array) — Rickard Elimää, May 03 '21 at 14:20
Ave you tried any of the obvious approaches like `[...a.keys()].filter(i => a[i] === -1)` — Wyck, May 03 '21 at 14:20

score 1 · Answer 1 · answered May 03 '21 at 14:18

1

You can use Array.prototype.reduce() to generate a new array of indices:

const a = [-1, 150, 190, 170, -1, -1, 160, 180];
console.log(a.reduce((acc, cv, idx) => {
    if (cv === -1) acc.push(idx);
    return acc;
}, []);

answered May 03 '21 at 14:18

esqew

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Find multiple indexes in an array

1 Answers1