Solution.js

const containsDuplicate = function (nums) {
  let set = new Set();
  nums.forEach((e) => {
    set.add(e);
  });
  return set.size !== nums.length;
};
var containsDuplicate2 = function (nums) {
  return new Set(nums).size !== nums.length;
};
var containsDuplicate3 = function (nums) {
  const map = {}; // key => number, value => count

  for (let i = 0; i < nums.length; i++) {
    if (map[nums[i]] !== undefined) {
      return true;
    } else {
      map[nums[i]] = 1;
    }
  }

  return false;
};