> For the complete documentation index, see [llms.txt](https://mistborn.gitbook.io/til-coding/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mistborn.gitbook.io/til-coding/javascript/untitled-2.md).

# Introduction: callbacks

```javascript
// JavaScript 有許多函式都是異步
function loadScript(src) {
  let script = document.createElement('script');
  script.src = src;
  document.head.append(script);
}

// loads and executes the script
loadScript('/my/script.js');

// 新增 js 檔到 HTML 完成這行指令需要時間，js 會繼續執行其他程式嗎
loadScript('/my/script.js');
// the code below loadScript doesn't wait for the script loading to finish
// ...

// 因為加載腳本需要時間，如果立即使用腳本內的程式碼會出現錯誤，我們不知道何時腳本才加載完畢
loadScript('/my/script.js'); // the script has "function newFunction() {…}"
newFunction(); // no such function!

// 新增 callback 函式，當腳本加載完成後執行
function loadScript(src, callback) {
  let script = document.createElement('script');
  script.src = src;

  script.onload = () => callback(script);

  document.head.append(script);
}

// 如果要使用腳本內程式碼，要加在 callback 裡。
loadScript('/my/script.js', function() {
  // the callback runs after the script is loaded
  newFunction(); // so now it works
  ...
});

// 當動作執行完，執行第二個參數(函式)
function loadScript(src, callback) {
  let script = document.createElement('script');
  script.src = src;
  script.onload = () => callback(script);
  document.head.append(script);
}

loadScript('https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.2.0/lodash.js', script => {
  alert(`Cool, the ${script.src} is loaded`);
  alert( _ ); // function declared in the loaded script
});
```

## Callback in callback

```javascript
// 如何依序加載腳本，把加載第 2 個腳本放到 callback，加載完第 1 個會加載第 2 個。
loadScript('/my/script.js', function(script) {

  alert(`Cool, the ${script.src} is loaded, let's load one more`);

  loadScript('/my/script2.js', function(script) {
    alert(`Cool, the second script is loaded`);
  });

});

// 要加載一些腳本可以，若要加載很多腳本不適合
loadScript('/my/script.js', function(script) {

  loadScript('/my/script2.js', function(script) {

    loadScript('/my/script3.js', function(script) {
      // ...continue after all scripts are loaded
    });

  })

});
```

## Handling errors

```javascript
// 加載可能會失敗，因此要追蹤錯誤
function loadScript(src, callback) {
  let script = document.createElement('script');
  script.src = src;

  script.onload = () => callback(null, script);
  script.onerror = () => callback(new Error(`Script load error for ${src}`));

  document.head.append(script);
}

// 使用
loadScript('/my/script.js', function(error, script) {
  if (error) {
    // handle error
  } else {
    // script loaded successfully
  }
});
```

## Pyramid of Doom

```javascript
// 當我們加載很多腳本
loadScript('1.js', function(error, script) {

  if (error) {
    handleError(error);
  } else {
    // ...
    loadScript('2.js', function(error, script) {
      if (error) {
        handleError(error);
      } else {
        // ...
        loadScript('3.js', function(error, script) {
          if (error) {
            handleError(error);
          } else {
            // ...continue after all scripts are loaded (*)
          }
        });

      }
    })
  }
});

// 巢狀函式變多，程式碼越深層，越難維護，被稱為回調地獄
```

![](https://2384301684-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LS3vj0v55HiJ9Pkdigx%2F-Li0sN5CtAr4FCW2w1Jr%2F-Li0yLEmy6BhNA7mTauO%2Fcallback-hell%402x.png?alt=media\&token=e64f9a74-be31-470e-a8c3-dd21c02fb4f8)

```javascript
// 透過每一個動作編寫一個獨立函式解決這個問題，但程式碼很分散可讀性差，step* 都是單獨使用，
// 為了避免回調地獄產生的函數，只使用一次，造成函數命名混亂。
loadScript('1.js', step1);

function step1(error, script) {
  if (error) {
    handleError(error);
  } else {
    // ...
    loadScript('2.js', step2);
  }
}

function step2(error, script) {
  if (error) {
    handleError(error);
  } else {
    // ...
    loadScript('3.js', step3);
  }
}

function step3(error, script) {
  if (error) {
    handleError(error);
  } else {
    // ...continue after all scripts are loaded (*)
  }
};
```