Synchronous loading: Synchronous mode, also known as blocking mode, means that when a process is executing a request, if the request takes a while to return information, then the process will wait until it receives the return information before continuing to execute Go on;
that is, synchronous loading will prevent subsequent processing by the browser and stop subsequent parsing, and therefore stop subsequent file loading (such as images), rendering, and code execution.
Asynchronous loading: Asynchronous loading, also called non-blocking, means that the process does not need to wait forever, but continues to perform the following operations, regardless of the status of other processes. When a message is returned, the system will notify the process for processing, which can improve the efficiency of execution.