What functions does promise have?

Promise functions include Promise constructor, then function, catch function, finally function, all function, race function, etc. Detailed introduction: 1. Promise constructor, which accepts a function as a parameter. This function has two parameters, namely resolve and reject; 2. then function, used to specify the callback function when the Promise state changes; 3. catch function, used The callback function when an error occurs in the specified Promise, etc.

Operating system for this tutorial: Windows 10 system, Dell G3 computer.

Promise is an object in JavaScript used to handle asynchronous operations. It provides a more elegant and reliable way to handle asynchronous code, avoiding the problem of callback hell. The following are several functions commonly used in Promise:

Promise constructor: The Promise constructor accepts a function as a parameter, which has two parameters, namely resolve and reject. The resolve function is used to change the state of Promise from pending to fulfilled, and pass the result to the subsequent then function. The reject function is used to change the state of the Promise from pending to rejected, and pass the error information to the subsequent catch function.

then function: Then function is a method of the Promise object, used to specify the callback function when the Promise state changes. The then function accepts two parameters, a callback function on success and a callback function on failure. The callback function on success will receive the return value of the Promise as a parameter, and the callback function on failure will receive the error message as a parameter.

catch function: The catch function is a method of the Promise object, used to specify the callback function when an error occurs in the Promise. The catch function accepts one parameter, which is the callback function in case of error. It is equivalent to the second parameter of the then function.

finally function: The finally function is a method of the Promise object, used to specify the callback function that will be executed whether the Promise succeeds or fails. The finally function does not accept any parameters.

all function: all function is a static method of Promise object, used to wrap multiple Promise objects into a new Promise object. When all Promises become fulfilled, the new Promise object will become fulfilled, and the return values ​​​​of all Promise will be passed to the then function as an array. If any Promise becomes rejected, the new Promise object will immediately become rejected, and the error information of the first rejected Promise will be passed to the catch function.

race function: The race function is a static method of the Promise object. Similar to the all function, it also wraps multiple Promise objects into a new Promise object. But the difference is that as long as a Promise changes to the fulfilled or rejected state in the race function, the new Promise object will change to the corresponding state, and the return value or error information of the first completed Promise will be passed to the subsequent then or catch function.

These are several functions commonly used in Promise. They can handle asynchronous operations more conveniently and make the code more concise and readable. Of course, Promise also has some other functions, such as resolve function, reject function, allSettled function, etc., which can be used according to actual needs.

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