Answer 1

严格来说then方法 是同步执行的，then方法的参数（回调函数）是 异步执行的

Answer 2

一个哈希表一趟循环就可以了

function twoSum(nums, target) {
  const map = {};
  for (let i = 0; i < nums.length; i++) {
    map[nums[i]] = i;
    const index = map[target - nums[i]];
    if (index) return [index, i];
  }
}

Answer 3

let a = new Set([1, 2, 3]);
let b = new Set([4, 3, 2]);
let intersect = new Set([...a].filter(x => b.has(x)));
// set {2, 3}