JSFiddle - React, Tailwind, and code Playground

JavaScript

/**
 * @param {number[]} nums
 * @return {number}
 */
const longestConsecutive = function(nums) {
  if (!nums || !nums.length) {
    return 0;
  }

  var obj = {};

  for (let val of nums) {
    obj[val] = true;
  }

  let max = 1;

  for (let val of nums) {
    if (!((val - 1) in obj)) {
      let currLen = 1;
      let curr = val;
      while ((curr + 1) in obj) {
        curr++;
        currLen++;
      }
      max = Math.max(max, currLen);
    }
  }

  return max;
};


console.log(longestConsecutive([1000, 1001, 1002, 1003, 2001, 2, 3]));
// 4