异步编程

题：读取若干个文件并将文件中的内容拼串

1. async串行读取

const async = require('async');
const fs = require('fs');

let files = ['01.txt', '02.txt', '03.txt']
let arr = [];
let str = '';
for (let index = 0; index < files.length; index++) {

    arr[index] = function(callback) {
        fs.readFile('../data/' + files[index], (err, data) => {
            if (err) {
                // console.error('errMsg: ' + err);
                callback(`read files[${index}] -> readFile ${files[index]} wrong!`, str);
            } else {
                str += data;
                callback(null);
            }
        });
    };

}

async.waterfall(arr, (err, result) => {
    if (err) {
        console.log(err);
        console.log('result: ' + result);
        return;
    }
    console.log('result: ' + str);
});

2. async并行读取

const async = require('async');
const fs = require('fs');

let files = ['01.txt', '02.txt', '03.txt'];
let arr = []; //存函数
let all = []; //存数据
for (let index = 0; index < files.length; index++) {

    arr[index] = function(callback) {
        fs.readFile('../data/' + files[index], (err, data) => {
            if (err) {
                console.error('errMsg: ' + err);
            }
            all[index] = data;
            callback(null,data);
        });
    };
}

async.parallel(arr,(err)=>{
    if (err) {
        console.log(err);
    }
    console.log('result:'+all.toString());
});

3. Promise并行读取

const fs = require('fs');
const files = ['01.txt', '02.txt', '03.txt', '04.txt'];
const arr = [];
const all = [];

for (let index = 0; index < files.length; index++) {
    arr[index] =
        new Promise((resolve, reject) => {
            fs.readFile('../data/' + files[index], (err, data) => {
                if (err) {
                    reject(`read ${files[index]} error`);
                }
                all[index] = data;
                resolve();
            });
        });
}

Promise.all(arr).then(() => {
    console.log(all.join(''));
});