How to use promise

"Promise" is a programming concept used to handle asynchronous operations. It can be used to represent the final result of an asynchronous operation. Promise objects have three states: pending (in progress), fulfilled (successful) and rejected (failed). The usage of Promise mainly includes constructors, instance methods (then, catch, finally) and state transitions.

In programs, "promise" is a programming concept used to handle asynchronous operations. It can be used to represent the final result of an asynchronous operation. Promise objects have three states: pending (in progress), fulfilled (successful) and rejected (failed).

The usage of Promise mainly includes the following aspects:

1. Constructor of Promise: You can use the "new Promise" keyword to create a Promise object.

const  promise  =  new  Promise((resolve,  reject)  =>  {   
   //  异步操作   });

In the constructor, the first parameter is an executor function, which is used to handle asynchronous operations. The second parameter is a resolve function that changes the Promise's status from pending to fulfilled. The third parameter is a reject function, used to change the state of the Promise from pending to rejected.

2. Instance methods of Promise: Promise objects have two commonly used instance methods, namely "Promise.prototype.then" and "Promise.prototype.catch".

• then: The callback function used to handle the success of Promise.

promise   
   .then((value)  =>  {   
     //  异步操作成功时的回调   
   })   
   .catch((error)  =>  {   
     //  异步操作失败时的回调   
   });

• catch: The callback function used to handle Promise failure.

promise   
   .then((value)  =>  {   
     //  异步操作成功时的回调   
   })   
   .catch((error)  =>  {   
     //  异步操作失败时的回调   
   });

3. The finally method of Promise: a callback function used to execute when the Promise succeeds or fails.

promise   
   .then((value)  =>  {   
     //  异步操作成功时的回调   
   })   
   .catch((error)  =>  {   
     //  异步操作失败时的回调   
   })   
   .finally(()  =>  {   
     //  无论成功或失败都会执行的回调   
   });

4. State transition of Promise: The state of a Promise object can only change from pending to fulfilled or from pending to rejected. Once the state is converted, it cannot be converted again.

promise   
   .then((value)  =>  {   
     //  异步操作成功时的回调   
   })   
   .catch((error)  =>  {   
     //  异步操作失败时的回调   
   });//  以下代码会报错，因为  promise  的状态已经从  pending  变为  fulfilled  或  rejected，不能再转换   promise.then((value)  =>  {   
   //  异步操作成功时的回调   });

In short, Promise is a programming concept used to handle asynchronous operations. It can be used to represent the final result of an asynchronous operation. Promise objects have three states: pending (in progress), fulfilled (successful) and rejected (failed). The usage of Promise mainly includes constructors, instance methods (then, catch, finally) and state transitions.

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