Promise

const asyncImg = (url) => (
  new Promise((resolve, reject) => {
    const image = new Image();
    image.onload = () => {
      resolve(image);
    };
    image.onerror = (err) => {
      reject(err);
    };
    image.src = url
  })
)
asyncImg('https://imgm.gmw.cn/attachement/jpg/site2/20200501/f832e46f786d20163c0c37.jpg');

const getJson = (url) => (
  new Promise((resolve, reject) => {
    const handler = function () {
      if (this.readyState !== 4) {
        return;
      }
      if (this.status === 200) {
        resolve(this.response);
      } else {
        reject(new Error(this.statusText))
      }
    };
    const client = new XMLHttpRequest();
    client.open('GET', url);
    client.onreadystatechange = handler;
    client.responseType = 'json';
    client.setRequestHeader('Accept', 'application/json');
    client.send();
  })
);
getJson('http://api.qingyunke.com/api.php?key=free&appid=0&msg=鹅鹅鹅');

// JSONP 处理跨域
const handler = (res) => {
  console.log(res);
}
const JSONP = (url) => {
  const script = document.createElement('script');
  script.src = `${url}&callback=${handler}`;
  document.body.insertBefore(script, document.body.firstChild);}
JSONP('http://api.qingyunke.com/api.php?key=free&appid=0&msg=鹅鹅鹅');

p1 的状态决定了 p2 的状态。如果p1的状态是 pending，那么 p2 的回调函数就会等待 p1 的状态改变；如果p1的状态已经是 Resolved 或者是 Rejected， 那么回调函数将会立刻执行。

const p1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(1);
  }, 3000);
});
const p2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(p1);
  }, 1000);
});
p2.then(res => console.log(res)).catch(err => console.log('err', err));