# Promises chaining

```javascript
new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000); // (*)

}).then(function(result) { // (**)

  alert(result); // 1
  return result * 2;

}).then(function(result) { // (***)

  alert(result); // 2
  return result * 2;

}).then(function(result) {

  alert(result); // 4
  return result * 2;

});
```

![](/files/-Li1Tims76acG3MVw_Of)

```javascript
new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000);

}).then(function(result) {

  alert(result);
  return result * 2; // <-- (1)

}) // <-- (2)
// .then…

// (1) 返回的值是一個 promise，這是為什麼可以在 (2) 加上 .then。
```

```javascript
// 新手常犯的錯誤
let promise = new Promise(function(resolve, reject) {
  setTimeout(() => resolve(1), 1000);
});

promise.then(function(result) {
  alert(result); // 1
  return result * 2;
});

promise.then(function(result) {
  alert(result); // 1
  return result * 2;
});

promise.then(function(result) {
  alert(result); // 1
  return result * 2;
});
```

![](/files/-Li1VJHv71UBty2v8pNk)

## Returning promises

```javascript
// 一般來說 .then 返回的值會立即傳入下一個 .then，但返回的值是 promise，會等 promise 執行完
// 再傳入
new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000);

}).then(function(result) {

  alert(result); // 1

  return new Promise((resolve, reject) => { // (*)
    setTimeout(() => resolve(result * 2), 1000);
  });

}).then(function(result) { // (**)

  alert(result); // 2

  return new Promise((resolve, reject) => {
    setTimeout(() => resolve(result * 2), 1000);
  });

}).then(function(result) {

  alert(result); // 4

});
```

## Example: loadScript

```javascript
loadScript("/article/promise-chaining/one.js")
  .then(function(script) {
    return loadScript("/article/promise-chaining/two.js");
  })
  .then(function(script) {
    return loadScript("/article/promise-chaining/three.js");
  })
  .then(function(script) {
    // use functions declared in scripts
    // to show that they indeed loaded
    one();
    two();
    three();
  });

// 較簡潔，依序加載腳本
loadScript("/article/promise-chaining/one.js")
  .then(script => loadScript("/article/promise-chaining/two.js"))
  .then(script => loadScript("/article/promise-chaining/three.js"))
  .then(script => {
    // scripts are loaded, we can use functions declared there
    one();
    two();
    three();
  });
  
// 回調地獄，要使用鏈(.then 返回 promise)解決。
loadScript("/article/promise-chaining/one.js").then(script1 => {
  loadScript("/article/promise-chaining/two.js").then(script2 => {
    loadScript("/article/promise-chaining/three.js").then(script3 => {
      // this function has access to variables script1, script2 and script3
      one();
      two();
      three();
    });
  });
});

// .then 可以返回任意的 “thenable” 物件，并且会被当做一个 promise 来对待，
// “thenable” 物件指拥有 .then 方法的任意物件
class Thenable {
  constructor(num) {
    this.num = num;
  }
  then(resolve, reject) {
    alert(resolve); // function() { native code }
    // resolve with this.num*2 after the 1 second
    setTimeout(() => resolve(this.num * 2), 1000); // (**)
  }
}

new Promise(resolve => resolve(1))
  .then(result => {
    return new Thenable(result); // (*)
  })
  .then(alert); // shows 2 after 1000ms
```

## Bigger example: fetch

```javascript
// promise 经常被用来网络请求
let promise = fetch(url);

fetch('/article/promise-chaining/user.json')
  // 当远程服务器开始响应时，下面的 .then 执行
  .then(function(response) {
    // 当结束下载时，response.text() 会返回一个新的 resolved promise，该 promise 拥有全部响应文字
    return response.text();
  })
  .then(function(text) {
    // ...这是远程文件内容
    alert(text); // {"name": "iliakan", isAdmin: true}
  });
  
// 同上，但是使用 response.json() 把远程内容解析为 JSON
fetch('/article/promise-chaining/user.json')
  .then(response => response.json())
  .then(user => alert(user.name)); // iliakan
  
// 多发一个请求到 github
// 发一个 user.json 请求
fetch('/article/promise-chaining/user.json')
  // 作为 json 加载
  .then(response => response.json())
  // 发一个请求到 github
  .then(user => fetch(`https://api.github.com/users/${user.name}`))
  // 响应作为 json 加载
  .then(response => response.json())
  // 显示头像图片（githubUser.avatar_url）3 秒（也可以加上动画效果）
  .then(githubUser => {
    let img = document.createElement('img');
    img.src = githubUser.avatar_url;
    img.className = "promise-avatar-example";
    document.body.append(img);

    setTimeout(() => img.remove(), 3000); // (*)
  }); 

// 我们如何能在头像结束显示并在移除之后做点什么?

// 为了使链可扩展，在头像结束显示时返回一个 resolved 状态的 promise，
// 一个异步动作应该永远返回一个 promise
fetch('/article/promise-chaining/user.json')
  .then(response => response.json())
  .then(user => fetch(`https://api.github.com/users/${user.name}`))
  .then(response => response.json())
  .then(githubUser => new Promise(function(resolve, reject) {
    let img = document.createElement('img');
    img.src = githubUser.avatar_url;
    img.className = "promise-avatar-example";
    document.body.append(img);

    setTimeout(() => {
      img.remove();
      resolve(githubUser);
    }, 3000);
  }))
  // triggers after 3 seconds
  .then(githubUser => alert(`Finished showing ${githubUser.name}`));

function loadJson(url) {
  return fetch(url)
    .then(response => response.json());
}

function loadGithubUser(name) {
  return fetch(`https://api.github.com/users/${name}`)
    .then(response => response.json());
}

function showAvatar(githubUser) {
  return new Promise(function(resolve, reject) {
    let img = document.createElement('img');
    img.src = githubUser.avatar_url;
    img.className = "promise-avatar-example";
    document.body.append(img);

    setTimeout(() => {
      img.remove();
      resolve(githubUser);
    }, 3000);
  });
}

// 使用它们
loadJson('/article/promise-chaining/user.json')
  .then(user => loadGithubUser(user.name))
  .then(showAvatar)
  .then(githubUser => alert(`Finished showing ${githubUser.name}`));
  // ...
```

![](/files/-Li1lyQo7gNdUjMi7Cmn)


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