val
in an array.
If val
never occurs, returns an empty array.Array.prototype.reduce()
to loop over elements and store indexes for matching elements.const indexOfAll = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);
indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0, 3] indexOfAll([1, 2, 3], 4); // []
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