使用 async/await 与使用 Promise 有何不同？

Question

Recently I started using (Async & Await). Before this, I used Promise to make my process Asynchronous. Like:

example.firstAsyncRequest()
.then(firstResponse => {
    return example.secondAsyncRequest(firstResponse)
})
.then(secondResponse => {
    return example.thirdAsyncRequest(secondResponse)
})
.then(result => {
    console.log(result)
})
.catch(err => {
    console.log(err)
})

Now I am achieving this like:

  try {
    const firstResponse = await example.firstAsyncRequest();
    const secondResponse = await example.secondAsyncRequest(firstResponse);
    const thirdAsyncRequest = await example.thirdAsyncRequest(secondResponse);
    console.log(thirdAsyncRequest)
  }
  catch (error) {
    // Handle error
  }

In both, the code-block method executes one after another and finally throws an error if any and gets caught by catch block. My question is, is this only a difference of syntax? Please explain or suggest me any link to understand this better.

Thank You

Answer 1

Is there only difference of syntax?

Yes. Your examples are functionally equivalent except that your second example is missing

console.log(thirdAsyncRequest);

...after the third await . (That variable really should be result to match the first code block, or thirdResponse to match the other two response variables).

async / await is syntactic sugar around promise creation and consumption. It's really helpful sugar, but that's all it is. async functions return promises. await expressions let you wait for a promise to settle, either getting the fulfillment value if it's fulfilled or throwing an error if it's rejected. (Since rejections are errors, you can use try / catch to handle them.) An uncaught error in an async function (whether a synchronous error or a promise rejection observed by await ) causes the promise the async function returned to be rejected with that error. Otherwise, the function's promise is resolved to whatever the function's code returns: if it returns a non-thenable, the promise is fulfilled with that value; if it returns a thenable, the promise is fulfilled or rejected based on the settlement of that thenable.

(Re "resolve" vs. "fulfill" and such, I've written up this post on promise terminology .)