indexOfAll

Returns all indices of val in an array. If val never occurs, returns [].

Use Array.prototype.reduce() to loop over elements and store indices for matching elements. Return the array of indices.

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); // []