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The execution process of javascript in the browser (picture and text)_javascript skills

WBOYOriginal: 2016-05-16 17:53:47842browse

1. The components of most browsers are as shown in the figure

The three components at the bottom are network, UI backend and js interpreter. The functions are as follows:
(1) Network - used to complete network calls, such as http requests. It has a platform-independent interface and can work on different platforms
(2) UI backend - used to draw similar combination selections Basic components such as boxes and dialog boxes have a common interface that is not specific to a certain platform. The bottom layer uses the user interface of the operating system
(3) JS interpreter - used to interpret and execute JS code

ps: The above pictures and knowledge points are mainly from "HOW BROWSERS WORK: BEHIND THE SCENES OF MODERN WEB BROWSERS". Students who want to know more about it can focus on it.
2. Most browsers (such as Chrome) use a single thread to execute javascript and update the user interface. This thread, often called the "browser UI thread", can only perform one of these operations at a time, which means that the user interface cannot respond to input while the Javascript code is executing, and vice versa. This is done because the function of the javascript code is to operate the DOM and update the user interface. It can be more efficient to use the same thread to do these two things
3. The work of the browser UI thread is based on a simple queue system, and the tasks will be Save to queue until process is idle. Once free, the next task in the queue is re-extracted and run. These tasks either run JavaScript code or perform UI updates, including redrawing and reflowing.
4. It is important to emphasize that JavaScript runs in a single thread. Don’t be confused by functions such as setTimeout() and setInterVal() and mistakenly think that it is multi-threaded.
ok, the basic points have been explained, let us get to the point and explain the execution process of javascript in the browser.
1. Principle
Generally speaking, every time the <script> tag appears, it will overbearingly make the page wait for the parsing and execution of the script. Regardless of whether the current Javascript is embedded or contains an external link file, the page Both downloading and rendering must stop and wait for script execution to complete. This is necessary in the life cycle of the page, because the page content may be modified during script execution. A typical example is using document.write() in the page. <br>When the javascript code is embedded in html, this is relatively easy to understand, but when the javascript is an external link file, it is a little bit more loaded, because there is a loading process, and the browser often loads the js file. Cache it. <br>First, we use the following example to illustrate the caching problem<br><div class="codetitle"> <span><a style="CURSOR: pointer" data="13076" class="copybut" id="copybut13076" onclick="doCopy('code13076')"><u>Copy the code</u></a></span> The code is as follows:</div> <div class="codebody" id="code13076"> <br><html> <br><head> <br><script type='text/javascript' src='js/f2.js'></script>

When opening the page for the first time:

When opening the page for the second time:

It can be clearly seen from the above example that high-version browsers such as Chrome will cache js files, which is self-evident and reduces network requests.

Secondly, the second question is whether when a javascript file is loaded, it will block the loading of other javascript files or other files. The book "High-Performance Javascript" provides a better answer to this question: the processing of low versions of various browsers is that when a javascript file is loading, it will also block the loading of other files on the page (including other javascript files) , but IE8, Firfox3.5, Safari 4 and Chrome 2 all allow parallel downloading of javascript files, but unfortunately, the javascript download process will still block the download of other resources, such as pictures. Although the downloading processes of javascript scripts do not affect each other, the page still has to wait for all javascript codes to be downloaded and executed before it can continue.

A digression here: browsers also have limits on the number of concurrent links under the same domain name. Some other parameters are as follows:

2. Tips
1. Script location
Since scripts will block the download of other resources on the page, it is recommended to place all