async/await实现异步

  async function async_test() {
    return 'hello world'
  }
  async_test();
  // console.log(async_test())   // Promise {: "hello world"}
  async_test().then(result => {
      console.log(result);  // 这样就可以得到async_test的值了，即hello world
  })
  console.log('虽然在后面，但是我先执行');

3s后得到result的值，即60

  function async_test(num) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(2 * num)   
        }, 3000);
    })
  }
  async function testResult() {
    let result = await async_test(30);
    console.log(result);   // 60
    console.log(222)
  }
  testResult();

3s后会先糖出alert，在这之前并没有打印出2222，等alert执行完后，才会看到打印出了2222

  function await_push() {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve(
          sessionStorage.setItem('async_await','testdata'),
          alert(111111111)
        )
      }, 3000);
    })
  };
  async function async_push() {
    await this.await_push()
    console.log(22222)
  };
  async_push()