The same process applies to CSS files, so combining the assets of a website from multiple files to one could increase speed and offer a better user experience and also could improve ranking to your SEO strategy by improving core web vitals of Google Search Console.
Immediately users will browse faster as page loading time will be minimized, this is an important factor in improving ranking to SEO and user navigation. A technical guide to doing this is the following.
Technical guide for optimizing assets
Closure Compiler for combining JS files
To combine multiple js files and compress them, there is a variety of tools. One of them is the Closure Compiler. It's written in Java, so it's a requirement to have Java installed in your development environment. You can find the compiler in https://developers.google.com/closure/compiler . For CSS compression you can use the yahoo compressor http://yui.github.io/yuicompressor, Of course, we can use these tools individually.
For example, to combine 2 js files can be done with the following command:
java -jar compiler.jar --js_output_file=all-files.js jquery.maskedinput.js jquery.jgrowl.js
Assets in Yii project
Let's see this process in the context of a yii2 project. Firstly in the project, we create a template by pressing the following command:
php yii asset/template assets.php
This will create a file in the root of the project which is the Configuration file for the asset command. After that, after you have to set correctly the directory path in @webroot and the URL in this file asset.php @web.
We assume that already have download the Closure Compiler, renamed to compiler.jar and put it in the same folder as the assets.php. Then we do the same with the Yahoo compiler. We rename it to yuicompressor.jar and place it in the same directory. Finally, we type:
yii asset assets.php config/assets-prod.php
The CSS and JS will be combined and compressed, and the new configuration will be created with name assets-prod.php in the directory config. Finally, in the web config/web.php we have to replace the bundles with the following command:
'bundles' => require(__DIR__.'/assets-prod.php')
technical optimization of speed of the website can be easy but in hard tasks always can be done with the assistance of a php developer as you see before this technique can be done easily in the last phase of yii development based project.
Delivery of Assets
<script async src="/js/jquery.min.js"></script>
with async the files are downloaded asynchronously and then in the first chance script is executed
<script defer src="/js/jquery.min.js" defer></script>
with defer, the documented is download asynchronously after DOM is fully parsed from the browser.
There a lot of ways to implement code splitting very easily and without the need for complex tools like webpack. Just use the loadJS library a tiny script from filamentgroup with Intersection Observer API.
Measure how fast you site loads
There are various tools on the web to measure the loading in your website is
Page Speed Insights
The best tool in measuring load time is Page Speed Insights, using this you will have details about the factors that affect the speed and give tips on how to improve it.
Another popular tool is GTmetrix that will provide a detailed report in the waterfall format and also will give a score for speed.