> For the complete documentation index, see [llms.txt](https://jen-hsuan-hsieh.gitbook.io/javascript-node-js/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jen-hsuan-hsieh.gitbook.io/javascript-node-js/chapter1/12javascript-quan-gong-lve-ff1a-ke-fu-js-de-qi-guai-bu-fen/121zhi-xing-huan-jing-yu-ci-hui-huan-jing/12110guan-yu-fei-tong-bu-hui-hu.md).

# 1.2.1.10.關於非同步回呼

* 非同步 (Asynchronous)
  * 在一個時間點不只一段程式碼執行
* The browser
  * JavaScript engine裡面的等待列稱為Event queue

    * 存放可能要發生的事件, 事件通知
    * 當有一個需要被通知的物件在JavaScript engine的某處, 會被放到queue, 我們就可以監聽這個事件 然後用函式回應
    * 舉例來說, 如果有一個按鈕按下去後, **當執行堆是空的時候, JavaScript才會注意到event queue**, 如果有函數對應到這個事件, 他會看這個函數是否可被此事件觸發, **如果可以則會建立新的Execution context給此函數**
      * 1.長時間執行的函式會干擾事件
      * 2.依照事件發生的順序處理
      * 3.**這是發生在JavaScript engine之外的非同步**

    ![](/files/-M4M0Mx6ZeBDaxrpeFYK) ![](/files/-M4M0Mx88ThF0junOoUs)
  * Rendering engine
  * HTTP request 


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