array-diff.js

// https://www.codewars.com/kata/523f5d21c841566fde000009
// Solution 1
function arrayDiff(a, b) {
  var result = [];
  for(var n of a) {
    if(!b.includes(n)) {
      result.push(n);
    }
  }
  return result;
}

// Solution 2
function arrayDiff(a, b) {
  var result = [];
  var obj = getObject(b);
  for(var n of a) {
    if(!obj[n]) {
      result.push(n);
    }
  }
  return result;
}

function getObject(array) {
  var obj = {};
  for(var n of array) {
    obj[n] = obj[n] ? obj[n] + 1 : 1;
  }
  return obj;
}

// Solution 3
function arrayDiff(a, b) {
  var obj = getObject(b);
  
  return a.filter(e => !obj[e]);
}

function getObject(array) {
  var obj = {};
  for(var n of array) {
    obj[n] = obj[n] ? obj[n] + 1 : 1;
  }
  return obj;
}