Returning an array from a higher order function

Question

function map(arr, fn){
for(var i = 0; i <arr.length; i++){
    fn(arr[i]);
    //some code here
    }
}

map([1,2,3,4], function(val){
    return console.log(val * 2);
});

I am having trouble trying to get the result into an array form. For example, [2,4,6,8]. I feel like I know how to do it but I'm blanking out.

Answer 1

The callback (fn) should mutate the array value at hand and map should return the array. See the snippet. Furthermore: just use the native Array.map method I'd say (see mdn for that)

function map(arr, fn){
  for(var i = 0; i <arr.length; i++){
    arr[i] = fn(arr[i]);
  }
  return arr;
}

console.log(
  map([1,2,3,4], function(val){  return val*2; })
);

// but hey, who needs it
console.log([1,2,3,4].map(v => v*2));