deleteNth.js

/* 
Task
Given a list and a number, create a new list that contains each number of list at most N times, without reordering.
For example if the input number is 2, and the input list is [1,2,3,1,2,1,2,3], you take [1,2,3,1,2], drop the next [1,2] since this would lead to 1 and 2 being in the result 3 times, and then take 3, which leads to [1,2,3,1,2,3].
With list [20,37,20,21] and number 1, the result would be [20,37,21]. 
*/

function deleteNth(arr,n){
  let numCount = {}
  let result = []

  for(num of arr){
    numCount[num] = (numCount[num] | 0) + 1;

    if(numCount[num] <= n){
      result.push(num)
    }
  }
  return result
}