The goal of the syntax parser is to raise JavaScript syntax errors as soon as possible. When all the bytes of one JavaScript source are received we run a syntax parser. The web page’s startup lasts beyond the first paint, it goes beyond the point when all resources are loaded, and can even last a few seconds longer after the page feels ready to be used. Most of the code of these libraries is not used (~70%) while the web page is starting up. JavaScript libraries are collections of code that are quite wide in terms of scope and usage.

It makes them more interactive, it can request dynamically loaded content, and it can improve the way web pages are programmed with libraries. JavaScript is a general purpose programming language that is used to improve web pages. This code is transferred to the browser from the network, from the network cache, or from a service worker. When Firefox loads a web page, it is likely that this web page will need to run some JavaScript code. In exchange for faster page-load, we store extra data in a cache. As with many optimizations, this is a trade-off between speed and memory. The JavaScript Startup Bytecode Cache (JSBC) is an optimization made to improve the time to fully load a page. We want to make Firefox load pages as fast as possible, to make sure that you can get all the goods from the loaded pages available as soon as possible on your screen.

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