Using async/await you can write the above code in synchronous manner without any .then. Once a Promise is fulfilled or rejected, the respective handler function (onFulfilled or onRejected) will be called asynchronously (scheduled in the current thread loop). Callbacks added with then() even after the success or failure of the asynchronous operation, will be called, as above.
The behavior of the handler function follows a specific set of rules. If the value returned inside the .then is a promise, the promise returned by then () will "adopt the state" of that promise and resolve/reject just as the returned promise does. By clicking “Post Your Answer”, you agree to our To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 故then方法与catch方法均会返回一个Promise对象（对，即使return 为某个值，或者throw error，或者不返回值） 我们来看看MDN的定义，这里可能为了严谨而说得有点乱七八糟的 If you haven’t already created an account, you will be prompted to do so after signing in. The The source for this interactive demo is stored in a GitHub repository. // using a resolved promise, the 'then' block will be triggered instantly, // but its handlers will be triggered asynchronously as demonstrated by the console.logs const resolvedProm = Promise.resolve(33); let thenProm = resolvedProm.then(value => { console.log("this gets called after the end of the main stack. Multiple callbacks may be added by calling then() several times. For example:
A bit too much code hence first the example above.The rule is, if the function that is in the Quoting from the Promises/A+ spec:The promise resolution procedure is an abstract operation taking as input a promise and a value, which we denote as This treatment of thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliant then method. 一.catch为then的语法糖. I figured I should add a short one.The following things are identical with So the following are all identical for a promise or plain value X:And it's no surprise, the promises specification is based on the The only difference is that you're creating an unnecessary promise when you do Thanks for contributing an answer to Stack Overflow!But To learn more, see our Required, but never shownRequired, but never shown 3. Return Data From Promise using ES6 Async/Await. Stack Overflow for Teams is a private, secure spot for you and Callbacks will never be called before the completion of the current run of the JavaScript event loop.
The whole thing works, because a call to promise.then returns a promise, so that we can call the next .then on it. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.The key thing to notice here is this line:if link: In simple terms, inside a Read more on this topic on the Both of your examples should behave pretty much the same.A value returned inside a In your first example, you return In your second example, you return a promise that is immediately resolved with the value The outcome is the same.If you can show us an example that actually exhibits different behavior, we can tell you why that is happening.You already got a good formal answer. When a handler returns a value, it becomes the result of that promise, so the next .then is called with it. By using our site, you acknowledge that you have read and understand our Q&A for Work A value returned inside a then () handler becomes the resolution value of the promise returned from that then (). the value received and returned is: " + value); return value; }); // instantly logging the value of thenProm …
It allows you to associate handlers with an asynchronous action's eventual success value or failure reason. 以下是在学习Promise关于catch与then的疑惑总结. JavaScript ES6 provides a new feature called async/await which can used as an alternative to Promise.then.
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