An async function can contain an await expression that pauses the execution of the async function and waits for the passed Promise's resolution, and then resumes the asyncfunction's execution and returns the resolved value.
Remember, the await keyword is only valid inside async functions. If you use it outside of an async function's body, you will get a SyntaxError.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
async 函數的調用方式：
await調用：同步調用；
常規調用：遇到內部await結點直接返回。