Detailed explanation of examples of synchronous loading and asynchronous loading in JavaScript

Synchronous loading

The most commonly used form of synchronous loading is this:

Synchronous mode, also known as blocking mode, will prevent the browser from subsequent processing. Stops subsequent parsing, thus stopping subsequent file loading (such as images), rendering, and code execution. The reason why js needs to be executed synchronously is because there may be behaviors such as outputting document content, modifying dom, redirection, etc. in js, so synchronous execution is safe by default. The general recommendation in the past was to place