30-seconds-of-code/snippets/indexesOf.md

indexesOf

Returns an array of indexes at which the val occurs in arr. If it occurs only once return the index and if it never occurs returns -1

const indexesOf = (arr, val) => {
    let indexes = [], i;
    arr.forEach((el,i) => {if(el === val) indexes.push(i)})
    return indexes.length === 0 ? [-1] : indexes
}

indexesOf([1,2,3],1); // [0]
indexesOf([1,2,3,1,2,3],1); // [0,3]
indexesOf([1,2,3],4); // [-1]
indexesOf([[1,2,3]],[1,2,3]); // [-1] (Array.prototype.indexOf()) has the same behaviour