Quick Inquiry

Unable to monkey patch array using uniq in JS [beginner] in node js.

You need to declare narr
Your condition in your for loop makes no sense i < 0. You need the length of the array this.length
Typo include its includes

 Array.prototype.uniq = function() {
  let narr = [];
  for (let i = 0; i < this.length; i++) {
    if (!narr.includes(this[i])) {
      narr.push(this[i]);
    }
  }
  return narr;
}

console.log([1, 2, 2, 3, 3, 3].uniq());

7 0