let a = [1, 1, 2, 3, 3, 4, 4];
//undefined
Array.from(new Set(a));
// [1, 2, 3, 4]

function unique(arr) {
  var newArr = [];
  for (var i = 0; i < arr.length; i++) {
    if (newArr.indexOf(arr[i]) == -1) {
      newArr.push(arr[i]);
    }
  }
  return newArr;
}

var arr = [1, 2, 2, 3, 4, 4, 5, 1, 3];
var newArr = unique(arr);
console.log(newArr);

function unique(arr) {
  return arr.filter((item, index, array) => {
    return array.indexOf(item) === index;
  });
}

var arr = [1, 2, 2, 3, 4, 4, 5, 1, 3];
var newArr = unique(arr);
console.log(newArr);