SOURCE

function towSum(arr, target){
    let obj = {};
    for (let i in arr){
        let diff = target - arr[i];
        if(diff in obj){
            return [i,obj[diff]];
        } else {
            obj[arr[i]] = i;
        }
    }
}
console.log(towSum([1,2,4,3,5],8))