Promise optimized callback hell

promise

callback hell- Back to hell

var fs = require('fs')
fs.readFile('./data/a.text','uft8',function(err,data){
    
    if(err){
    
        throw err
    }
    console.log(data);
})
fs.readFile('./data/b.text','uft8',function(err,data){
    
    if(err){
    
        throw err
    }
    console.log(data);
})
fs.readFile('./data/c.text','uft8',function(err,data){
    
    if(err){
    
        throw err
    }
    console.log(data);
})

Code order cannot be guaranteed , It's not necessarily who's ahead and who's behind

The order is guaranteed by nesting callbacks

var fs = require('fs')
fs.readFile('./data/a.text','uft8',function(err,data){
    
    if(err){
    
        throw err
    }
    console.log(data);
    fs.readFile('./data/b.text','uft8',function(err,data){
    
    if(err){
    
        throw err
    }
    console.log(data);
	    fs.readFile('./data/c.text','uft8',function(err,data){
    
	    if(err){
    
	        throw err
	    }
	    console.log(data);
		})
	})
})

When nested too deeply , Difficult to maintain
In order to solve the problem solved by the above coding method, so es6 There's a new API

promise

Commitment is not asynchronous , Content （ Mission ） It's asynchronous
Basic grammar

//  Create a container 
// 1、 Give someone a promise 
var p1 = new Promise(function (resolve, reject) {
    
    fs.readFile('./data/a.text', 'utf8', function (err,data) {
    
        if (err) {
    
            //  A change of state 
            reject(err)
        } else {
    
            //  success 
            resolve(data)
        }
    })
})
// p1 It's commitment 
// then Method of acceptance funtion It's in the container resolve
p1
    .then(function(data){
    
        console.log(data);
    },function(err){
    
        console.log(' Read failed ');
    })

example

var fs = require('fs')
var p1 = new Promise(function (resolve, reject) {
    
    fs.readFile('./a.text', 'utf8', function (err, data) {
    
        if (err) {
    
            //  A change of state 
            reject(err)
        } else {
    
            //  success 
            resolve(data)
        }
    })
})

var p2 = new Promise(function (resolve, reject) {
    
    fs.readFile('./b.text', 'utf8', function (err, data) {
    
        if (err) {
    
            //  A change of state 
            reject(err)
        } else {
    
            //  success 
            resolve(data)
        }
    })
})

var p3 = new Promise(function (resolve, reject) {
    
    fs.readFile('./c.text', 'utf8', function (err, data) {
    
        if (err) {
    
            //  A change of state 
            reject(err)
        } else {
    
            //  success 
            resolve(data)
        }
    })
})
p1
    .then(function (data) {
    
        console.log(data);
        //  Returns the object 
        return p2;
    }, function (err) {
    
        console.log(' Read failed ');
    })
    .then(function (data) {
    // Here it is p2 Of resolve function 
        console.log(data);
        return p3;
    })
    .then(function (data) {
    // Here it is p3 Of resolve function 
        console.log(data);
    })