Array.prototype.reduce()

Index of all matches

Finds all indexes of val in an array. If val never occurs, returns an empty array.

• Use 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); // []

Example