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The browser must parse the Web page before it can be presented to the user. If the browser encounters an external script that is blocked by the system during parsing, it must stop parsing and download the JavaScript. Each time this happens, the browser adds a network round trip, which results in a delay in the first time the page is rendered.
SuggestionsIt is recommended that you handle the JavaScript required to render the first screen area inline and have the JavaScript delay loading required to add additional functionality to the Web page until the first screen is finished sending. Note that to shorten the load time in this way, you must also optimize the CSS sending process .
Embedded with smaller JavaScriptIf the external scripts are small, you can add them directly to the HTML document. Embedding smaller files in this way allows the browser to continue rendering the Web page. For example, if the HTML document looks like this:
<scripttype="Text/javascript"src="Small.js"></script> <body> <div>Hello, world!. </div> </body>
resources small.js is as follows:
/* Contents of a small JavaScript file */
Then you can embed the script as follows:
<scripttype="Text/javascript"> / * Contents of a small JavaScript file * / </script> <body> <div>Hello, world!. </div> </body>
This allows you to small.js embed it in an HTML document, eliminating external requests for it.
To prevent JavaScript from blocking page loading, we recommend that you use HTML async properties when you load JavaScript. For example:
<script async src="my.js">
If your JavaScript resource is using document.write, it is unsafe to use asynchronous loading. We recommend that you rewrite the script that uses document.write to use other techniques instead.
Also, when loading JavaScript asynchronously, it is important to be cautious if your Web pages load interdependent scripts to ensure that your app loads the script in the appropriate order of dependency.
Frequently Asked QuestionsTo remove JavaScript that prevents rendering
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