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Today, the editor will share with you the relevant knowledge points about how to use es6's promise. The content is detailed and the logic is clear. I believe most people still know too much about this knowledge, so share this article for your reference. I hope you can get something after reading this article. Let's take a look at it.

In es6, promise means "promise", which represents the result of asynchronous operation and is a new asynchronous programming solution, which is represented as an object in the code, which is mainly used to solve callback region problems. The syntax is "new Promise (function (resolve, reject) {.})".

This tutorial operating environment: windows10 system, ECMAScript version 6. 0, Dell G3 computer.

What does es6's promise mean?

Promise means commitment, and the core idea behind it is that commitment represents the result of asynchronous operations.

Is a new asynchronous programming solution for es6 that is represented as an object in the code.

Promise is a solution provided by js asynchronous programming, which is mainly used to solve the callback region problem.

Promise has three states, which are

Pending (in progress) initial state

Fulfilled (successful) means the operation is successful

Rejected (failed) means that the operation failed

Note: the three states of the Promise object are not affected by the outside world, and only the events stored in the promise that will end in the future will be affected. That is, only the result of the asynchronous operation can determine which state the current state is, and no other operation can change this state.

Once the Promise state is changed, it is irreversible

The Pendding state can be transitioned to the Fulfilled state

The Pendding state can be transitioned to the rejected state

There are only two cases in which the state changes in promise. Once these two state changes occur, the state is solidified and the result will be maintained all the time.

Basic usage

Syntax:

New Promise (function (resolve, reject) {...} / * executor * /)

Principle:

When building a Promise object, you need to pass in an executor function, and the main business processes are executed in the executor function.

Call the executor functions, resolve and reject immediately when the Promise constructor executes

Two functions are passed as arguments to executor,resolve and reject

When the function is called, change the state of promise to fulfilled (complete) or rejected (failed), respectively. Once the state changes, it will not change again, and this result can be obtained at any time.

When the resolve function is called in the executor function, the callback function set by promise.then is triggered, and reject is called.

Function, the callback function set by promise.catch is triggered.

As shown in the following figure:

Examples are as follows:

Create a new Promise object

A callback function needs to be passed. The callback function takes two parameters, which represent resolve (resolve) and reject (reject), and both parameters are functions.

If neither parameter is called, the default pending state

Let promise=new Promise (function (resolve,reject) {}); / / pending status

Call the resolve function, which represents the state of Promise, and will change from pending== > fulfilled

Let promise=new Promise (function (resolve,reject) {resolve ();}); / / fulfilled status

Call the reject function, which represents the state of Promise, and will change from pending== > rejected

Let promise=new Promise (function (resolve,reject) {reject ();}); / / the above rejected status is all the content of the article "how to use es6's promise". Thank you for reading! I believe you will gain a lot after reading this article. The editor will update different knowledge for you every day. If you want to learn more knowledge, please pay attention to the industry information channel.

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