function diffArray(arr1, arr2) {
 var newArr = [];

  arr1.map(function(val){
   arr2.indexOf(val) < 0 ? newArr.push(val) : '';
  });

  arr2.map(function(val){
   arr1.indexOf(val) < 0 ? newArr.push(val) : '';
  });

  return newArr;
}

diffArray([1, 2, 3, 5, 6, 7], [1, 2, 3, 4, 5]);

var seen = {}
iterate all map1 values and mark is as seen
iterate all map2 values and unmark if already seen, mark if not
add all seen values to new array
return array

function diffArray(arr1, arr2) {
 var newArr = [];

  arr1.forEach(function(val){
   if(arr2.indexOf(val) < 0) newArr.push(val);
  });

  arr2.forEach(function(val){
   if(arr1.indexOf(val) < 0) newArr.push(val);
  });

  return newArr;
}

function diffArray(a, b) {
    return   a.filter(function(val) { // reveal the unique values in a
        return b.indexOf(val) < 0;
    }.concat(b.filter(function(val) { // concat those with the unique values in b
        return a.indexOf(val) < 0;
    }
}

function getUniqueValues(src, other) {
    return src.filter(function(val) {
        return other.indexOf(val) < 0;
    }
}

function diffArr(a, b) {
    return getUniqueValues(a, b).concat(getUniqueValues(b, a));
}

function diffArr(a, b) {
    a.sort();
    b.sort();
    ...
}

function diffArray(arr1, arr2) {
   var set1 = new Set(arr1);
   var set2 = new Set(arr2);

   var arr = []

   set1.forEach(function(val) {
     if (!set2.has(val)) arr.push(val);
   });
   set2.forEach(function(val) {
     if (!set1.has(val)) arr.push(val);
   });

   return arr;
}

function diffArray(arr1, arr2) {
  return [...arr1.filter(el => !arr2.includes(el)), ...arr2.filter(el => !arr1.includes(el))]
}

diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]);

diff([1,2,3], [1,2,3,4,5]) // => [4,5]

function diff(firstArray: any[], secondArray: any[]): any[] {
  return firstArray.concat(secondArray).filter((val) => {
    return !(firstArray.includes(val) && secondArray.includes(val));
  });
};