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.

代码片段

const indexOfAll = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);

使用样例

indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0, 3]
indexOfAll([1, 2, 3], 4); // []