30-seconds-of-code/snippets/index-of-all.md

title, tags, cover, firstSeen, lastUpdated

title	tags	cover	firstSeen	lastUpdated
Index of all matches	array	jars-on-shelf-2	2018-01-06T12:07:56+02:00	2020-10-22T20:23:47+03:00

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); // []