A case of using promise to read and write files in nodejs

31、 Use promise Implement the case of file reading before

In reference to util Module adoption ： Follow common error first callback style functions （ Also is to  (err, value) => ...  Callback as the last parameter ）, And return a return promise The object method of

And in chain splicing return The parameter that returns the parameter actually acts on the second chained formal parameter

Characteristics of chain call ：

1、 first then The second one will be executed after execution then

2、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

31.1、 Basic Edition

// Basic Edition 
const fs = require("fs");
const path = require("path");
​
let filePath1 = path.join(__dirname, "files", "1.txt");
let filePath2 = path.join(__dirname, "files", "2.txt");
let filePath3 = path.join(__dirname, "files", "3.txt");
​
​
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(filePath1,"utf-8",(error1, data1)=>{
        if(error1){
            // What you do when you fail 
            reject(error1);
        }
        // What to do after reading 
        resolve(data1)
    })
});
let p2 = 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(filePath2,"utf-8",(error1, data1)=>{
        if(error1){
            // What you do when you fail 
            reject(error1);
        }
        // What to do after reading 
        resolve(data1)
    })
});
let p3 = 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(filePath3,"utf-8",(error1, data1)=>{
        if(error1){
            // What you do when you fail 
            reject(error1)
        }
        // What to do after reading 
        resolve(data1)
    })
});
​
let str1 = "";
​
p1.then((data)=>{
    str1+=data;
    return p2
},(error1)=>{
    console.log(" Read the file 1 Failure ", error1);
    return error1
}).then((data)=>{
    str1+=data;
    return p3;
}).then((data)=>{
    str1+=data;
    console.log(str1);
});
​

32.2、 Encapsulated function version

const fs = require("fs");
const path = require("path");
​
let filePath1 = path.join(__dirname, "files", "1.txt");
let filePath2 = path.join(__dirname, "files", "2.txt");
let filePath3 = path.join(__dirname, "files", "3.txt");
​
function readFilePromise(filePath){
    return new Promise((resolve, reject)=>{
​
        fs.readFile(filePath,"utf-8",(error1, data1)=>{
            if(error1){
                // What you do when you fail 
                reject(error1);
            }
            // What to do after reading 
            resolve(data1)
        })
    });
}
​
let str1 = "";
​
readFilePromise(filePath1).then((data)=>{
    str1+=data;
    return readFilePromise(filePath2)
},(error1)=>{
    console.log(" Read the file 1 Failure ", error1);
    return error1
}).then((data)=>{
    str1+=data;
    return readFilePromise(filePath3);
}).then((data)=>{
    str1+=data;
    console.log(str1);
});

33.3、util edition

node There is one of them. util Under tool module , There is one promisify Method , This method is equivalent to encapsulating a return promise Object function .

Document URL ：util Utilities | Node.js API file

Pass in a function that follows the common error first callback style （ That is to  (err, value) => ...  Callback as the last parameter ）, And return a return promise Version of .

let readFilePromise = util.promisify(fs.readFile);  // This sentence of code is equivalent to the following code of the whole function 
// function readFilePromise(filePath){
//     return new Promise((resolve, reject)=>{
//
//         fs.readFile(filePath,"utf-8",(error1, data1)=>{
//             if(error1){
//                 // What you do when you fail 
//                 reject(error1);
//             }
//             // What to do after reading 
//             resolve(data1)
//         })
//     });
// }
​
//  summary  ：util.promisify(fs.readFile)  Get one promise object 

Complete code ：

const fs = require("fs");
const path = require("path");
const util = require("util");
​
let filePath1 = path.join(__dirname, "files", "1.txt");
let filePath2 = path.join(__dirname, "files", "2.txt");
let filePath3 = path.join(__dirname, "files", "3.txt");
let filePath4 = path.join(__dirname, "files", "data.txt");
​
let readFilePromise = util.promisify(fs.readFile); 
let writeFilePromise = util.promisify(fs.writeFile);
​
let str1 = "";
​
readFilePromise(filePath1).then((data)=>{
    str1+=data;
    return readFilePromise(filePath2)
},(error1)=>{
    console.log(" Read the file 1 Failure ", error1);
    return error1
}).then((data)=>{
    str1+=data;
    return readFilePromise(filePath3);
}).then((data)=>{
    str1+=data;
    console.log(str1);
    writeFilePromise(filePath4, str1);
});