async and await are two keywords introduced in ES2017 (ES8) to solve the problem of asynchronous programming in JavaScript. async is used to define an asynchronous function, and await is used to wait for an asynchronous operation to complete.
Specifically, an async function will automatically return a Promise object, and you can use the .then() method or the await keyword to get the return value of the function. The await keyword can only be used in async functions, and is used to wait for the completion of an asynchronous operation returning a Promise object and obtain its result.
When using async/await, if the await keyword is encountered, the JavaScript engine will suspend the execution of the current function, wait for the asynchronous operation to complete and return the result, and then continue to execute the following code. This avoids the problem of callback hell and makes asynchronous code clearer and more concise.
When using async/await, you need to pay attention to the following points:
Only asynchronous functions can use the await keyword.
The await keyword can only wait for the completion of the asynchronous operation that returns the Promise object. If it is not a Promise object, it will be automatically converted into a Promise object.
The await keyword can only be used in async functions.
The await keyword blocks the execution of the current function until the asynchronous operation completes.
The await keyword can be used with try/catch to catch exceptions from asynchronous operations.
To sum up, async/await is a simple and intuitive way of asynchronous programming, which can make asynchronous code clearer and easier to maintain.