同步（Sync）When issuing a function call, you must do it one by one, and wait for the previous one to finish before you can do the next thing. 异步（Async）When an asynchronous procedure call is issued, the caller can continue to perform subsequent operations before getting a result.
In summary, the difference between synchronous and asynchronous: after the request is issued, do you need to wait for the result before continuing to perform other operations.
