We’ll cover the best practices listed for accomplishing both goals, including considerations for third-party JavaScript that you may not have as much control over.įor the following, it’s important to make sure your user base has browser support for the methods implemented. Load non-critical JS components asynchronously.Split JS files to deliver essential components.Self-host third-party JS (or use a service worker).Minify all JS resources, and use minified third-party JS.Reduce how much JS you use to build your page.So how can we reduce how much page loading time is associated with JavaScript delivery? There are two ways, with best practices for each: When you consider that GZIP compression can reduce delivered text file size by 70 to 90%, 400 KB really becomes 1.3 to 4.0 MB. Byte for byte, JavaScript is more expensive than any other file type to process on the page in terms of CPU and time. When you consider that the first connection opened to load a page only transfers 14 KB of data, it becomes obvious that reducing JS file size is an important optimization to make.īut 400 KB of JavaScript is not equal to 400 KB of an image loading, partly because JavaScript processing takes place when the file is uncompressed. For desktop users, this equates to about 400 KB per page visited, and 360 KB for mobile users. The impact of JS file sizeĪcross the millions of sites measured by HTTP archive, JavaScript accounts for 15% of page weight for desktop pages and 16% for mobile pages. Optimizing JavaScript processing is also incredibly important to the way modern websites load, but I’ll be addressing it in a separate post.

This post is going to cover delivery optimization for JavaScript: why you should care, considerations, and best practices, along with some links to relevant resources. This makes JavaScript processing more costly than processing other file types. JavaScript is also a dynamic language, and compiles on the fly once the browser receives it. The size of JavaScript files along with the way these files are delivered both affect the load time. Like any other file building a web page, JavaScript needs to load before it can function. JavaScript is an essential aspect of building engaging sites, but it can drastically affect the way pages load.

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