dupes.js

/**
* @param {number[]} nums
* @return {number}
*/
const XXXremoveDuplicates = (nums) => { //{{{
  if (nums.length === 0) return 0;

  let last_item = nums[0];
  let last_item_index = 0;

  for (let i = 1; i < nums.length; i++) {
    const current_item = nums[i];

    if (last_item === current_item) {
      console.log("dup", current_item, last_item, last_item_index, nums);
      continue;
    }

    nums[last_item_index + 1] = current_item;
    last_item_index++;
    last_item = current_item;
    console.log("loop", current_item, last_item, last_item_index, nums);
  }

  nums.length = last_item_index;

  return nums.length;
}; //}}}

const removeDuplicates = (nums) => {
  if (!nums.length) return 0;

  let len = 1;
  const lastNum = () => nums[len - 1];

  for (let i = 1; i < nums.length; i++) {
    const cn = nums[i];

    // skip dupes
    if (cn === lastNum()) continue;

    // move successor number to end of list (eol) and update eol
    nums[len++] = cn;
  }

  // Not sure if this needs to happen in the prompt
  nums.length = len;

  return len;
};

const rd = removeDuplicates;
let xs = [1, 2, 3, 4];

console.log(rd(xs));
console.log(xs);

xs = [1, 1, 2, 2, 2, 2, 2, 2, 3, 4, 5, 5, 5];

console.log(rd(xs));
console.log(xs);