Asynchronously Loading Js Files

The rendering engine and the Js script engine in the browser are mutually exclusive. When the browser starts rendering a page, if it encounters a <script> tag, it will stop rendering the current page. In other words, during the process of script loading and execution, the rendering of the page will be blocked. In an environment with poor network speed, the browser page may appear to be unresponsive. This is also why <script> files should be placed after the <body> tag as much as possible. Asynchronously loading Js files allows the browser to load external Js script files without blocking the rendering thread. This is known as non-blocking loading. Of course, the parsing and execution of the Js script after loading must be mutually exclusive with the rendering engine, and the timing of parsing and executing the Js script depends on the way the Js is asynchronously loaded.

defer

defer is an attribute supported by early versions of IE and is now supported by mainstream browsers. This attribute causes the browser to open a thread to load the Js and execute it after the DOM structure is parsed.

  • defer only applies to external scripts.
  • If there are multiple scripts declared with defer, they will be downloaded and executed in order.
  • defer scripts will be executed before onDOMContentLoaded, and of course, also before the onload event.
<!-- Compatibility https://caniuse.com/#feat=script-defer -->
<script type="text/javascript" defer="defer" src="https://cdn.jsdelivr.net/npm/jquery@3/dist/jquery.min.js" ></script>

async

HTML5 defines the async attribute for the <script> element. This attribute causes the browser to open a thread to load the Js and execute it immediately after downloading.

  • async only applies to external scripts.
  • If there are multiple scripts declared with async, their downloading and execution will also be asynchronous, and the order of execution cannot be guaranteed.
  • async will be executed before the onload event, but it cannot guarantee the sequence of execution relative to DOMContentLoaded.
<!-- Compatibility https://caniuse.com/#feat=script-async -->
<script type="text/javascript" async="async" src="https://cdn.jsdelivr.net/npm/jquery@3/dist/jquery.min.js" ></script>

Script DOM Element

The approach of the Script DOM Element is to dynamically insert a <script> tag to load an external Js script file, and use it as a callback function for the onload event, so that it will not block the rendering thread until the page is loaded.

window.onload = function(){
    var script= document.createElement('script'); 
    script.type = 'text/javascript'; 
    script.src ="https://cdn.jsdelivr.net/npm/jquery@3/dist/jquery.min.js"; 
    var body= document.getElementsByTagName('body')[0]; 
    body.appendChild(script);
}

Daily Question

https://github.com/WindrunnerMax/EveryDay

References

https://segmentfault.com/a/1190000006778717 https://www.cnblogs.com/jiasm/p/7683930.html https://blog.csdn.net/zyj0209/article/details/79698430 https://blog.csdn.net/qq_41245969/article/details/82428464
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