for...in、for...of、for await...of

处世应当谦虚，切忌轻人傲世。——佚名

简单说下区别：

for...in遍历出来的是key

var obj = {a:1, b:2, c:3};

for (var prop in obj) {
  console.log("obj." + prop + " = " + obj[prop]);
}

// Output:
// "obj.a = 1"
// "obj.b = 2"
// "obj.c = 3"

for...of遍历出来的是元素

const array1 = ['a', 'b', 'c'];

for (const element of array1) {
  console.log(element);
}

// Expected output: "a"
// Expected output: "b"
// Expected output: "c"

for await...of和for...of差不多，但支持遍历由Promise组成的可迭代对象(如数组)，使用时会自动await直到Promise执行结束才执行

var asyncIterable = {
  [Symbol.asyncIterator]() {
    return {
      i: 0,
      next() {
        if (this.i < 3) {
          return Promise.resolve({ value: this.i++, done: false });
        }

        return Promise.resolve({ done: true });
      }
    };
  }
};

(async function() {
   for await (num of asyncIterable) {
     console.log(num);
   }
})();

// 0
// 1
// 2