High-performance JavaScript-loading and execution

The performance of JavaScript in the browser is considered to be the most important availability issue for developers. This issue is complicated due to the blocking feature of JavaScript, that is to say, other tasks cannot be processed by the browser when JavaScript is running. In fact, most browsers use a single process to process multiple tasks such as UI update and JavaScript running, and only one task can be executed at a time. How long does JavaScript run? How long does the browser wait before it is idle to respond to user input. Basically speaking, this means that the appearance of the <script> tag makes the entire page wait for parsing and running scripts. No matter whether the actual JavaScript code is inline or contained in an unrelated external file, the page download and parsing process must be stopped. Wait until the script completes the processing before it can continue, it is also an essential part of the page lifecycle, because the script may modify the page content during running. During JavaScript loading, page parsing and user interaction are completely blocked. The script location HTML 4 document specifies that a <script> tag can be placed in the