Introduction to promise module in nodejs

1、Promise brief introduction

Promise Is a solution to asynchronous programming , More than traditional solutions —— Callback functions and events —— More reasonable and more powerful . It was first proposed and implemented by the community ,ES6 It's written into the language standard , Unify usage , Native provides Promise object .

So-called Promise, It's simply a container , It holds an event that will end in the future （ This is usually an asynchronous operation ） Result . grammatically ,Promise It's an object , From it you can get messages for asynchronous operations .Promise Provide uniform API, Various asynchronous operations can be handled in the same way .

Promise Objects have the following two characteristics .

（1） The state of the object is not affected by the outside world .Promise The object represents an asynchronous operation , There are three states ：pending（ Have in hand ）、fulfilled（ Have succeeded ） and rejected（ Failed ）. Only results of asynchronous operations , You can decide which state you are in , No other operation can change this state . This is also Promise The origin of the name , It means the same thing in English “ promise ”, Says nothing else can be changed .

（2） Once the status changes , It won't change , You can get that at any time .Promise Object state changes , There are only two possibilities ： from pending Turn into fulfilled And from the pending Turn into rejected. As long as those two things happen , The state is frozen , It won't change , It's going to stay that way , This is called resolved（ Have to finalize the design ）. If change has already happened , You again to Promise Object to add a callback function , And you'll get that immediately . With the event （Event） Completely different , The characteristic of the event is , If you missed it , To listen again , You don't get results .

30.2、Promise Basic use of

New Promise It's a container , It can store asynchronous operations , You can also store synchronous operations

const fs = require("fs");
const path = require("path");
​
let filePath = path.join(__dirname, "files", "3.txt");
​
//  Asynchronous operations may succeed or fail 
//  First parameter resolve ,  Function executed when successful 
//  The second parameter reject ,  The function executed when it fails 
let p1 = new Promise((resolve, reject)=>{
    //1、 Synchronization code 
    console.log(" Synchronization code ");
​
    // pending（ Have in hand ）、fulfilled（ Have succeeded ） and rejected（ Failed ）
    //2、 Asynchronous code 
    fs.readFile(filePath,"utf-8",(error1, data1)=>{
        if(error1){
            // What you do when you fail 
            reject(error1);    //  Call back p1.then The second function of 
        }
        // What to do after reading 
        resolve(data1);    //  Call back p1.then The second function of 
    })
});
​
p1.then((data1)=>{
    console.log(" Read successful ", data1);
},(error1)=>{
    console.log(" Read failed ", error1);
});

30.3、Promise Of then Characteristics of chain call

Characteristics of chain call ：1、 first then The second one will be executed after execution then2、then The return value of the function inside , Will be next then The formal parameters of are received 3、 If you return a promise object , next then The formal parameter of received is not this promise object , It's this promise Call... Inside the object resolve Actual parameters at time

const fs = require("fs");
const path = require("path");
​
let filePath = path.join(__dirname, "files", "3.txt");
​
//  New Promise It's a container , It can store asynchronous operations , You can also store synchronous operations 
​
//  Asynchronous operations may succeed or fail 
//  First parameter resolve ,  Function executed when successful 
//  The second parameter reject ,  The function executed when it fails 
let p1 = new Promise((resolve, reject)=>{
    //1、 Synchronization code 
    // console.log(" Synchronization code ");
​
    // pending（ Have in hand ）、fulfilled（ Have succeeded ） and rejected（ Failed ）
    //2、 Asynchronous code 
    fs.readFile(filePath,"utf-8",(error1, data1)=>{
        if(error1){
            // What you do when you fail 
            reject(error1)
        }
        // What to do after reading 
        resolve("resolve The parameter of ")
    })
});
​
// The industry puts promise then The way of writing is called driving a train 
​
p1.then((data1)=>{
    return p1
},(error1)=>{
    console.log(" Read failed ", error1);
    return error1
}).then((data)=>{
    console.log(data);   // “resolve("resolve The parameter of ")
});