How JavaScript is loaded in the browser

How to load JavaScript in the browser: 1. Parse the web page; 2. Introduce external css through link; 3. Introduce external js through script; 4. Create a thread for asynchronous loading; 5. Continue to parse the document; 6. Execute defer scripts in sequence; 7. Convert to event-driven stage, etc.

The operating environment of this article: Windows 7 system, Dell G3 computer, javascript version 1.8.5.

Today I will share with you the loading process in JavaScript. It has certain reference value and I hope it will be helpful to everyone.

JavaScript loading starts from the moment it starts until the browser follows the instructions. Until everything is completed in sequence, the approximate steps can be divided into creating the document object ==> document parsing completed ==> document loading and execution, which will be introduced in detail in the article.

Loading process

(1) Parsing the web page means parsing HTML elements and their text content, adding objects and text to the document, and the document is loading Medium

Document Analysis

(2) Introduce external css through link, create threads, and load asynchronously.

(3) Introduce external js through script, and async and defer are not set. The browser loads synchronously and blocks etc.

(4) Wait for js to be loaded and then set async and defer for it. , the browser creates a thread for asynchronous loading, in which async is executed immediately after the script is loaded (Note: asynchronous loading should prohibit the use of document.write())

(5) When encountering attributes with src such as img, The dom structure should be parsed normally first, and then the browser will load the src asynchronously and continue to parse the document. If the tag is seen, the dom tree will be generated directly without waiting for the img to load the scr.

(6) After the document parsing is completed, all scripts set with defer will be executed in order. (Note the difference from async, but the use of document.write() is also prohibited);

(7) The document object triggers the DOMContentLoaded event, which also marks the transformation of program execution from the synchronous script execution stage to the event-driven stage .

(8) When all async scripts are loaded and executed, img, etc. are loaded (after all the pages are loaded), document.readyState = 'complete', and the window object triggers the load event.

(9) From now on, user input, network events, etc. will be processed in an asynchronous response manner

Note

It is best when we write a program The script tag is written below. Although writing it above can not only operate the div, but also process it immediately after the dom is parsed, which makes it more efficient, but it is best to write it at the bottom

<div style="width:100px;height:100px;background: pink;">
<script>
var div=document.getElementsByTagName("div")[0]
div.onclick=function(){
this.style.background="hotpink";
}
</script>

Summary: The above is the entire content of this article. I hope that through this article, everyone will have a certain understanding of JavaScript loading.

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