Node 回调嵌套

一、Node 回调嵌套

需求：使用 异步 方法读取文件，按顺序分别读取文件 A、B、C，之后将 A 、B、C 的内容拼接起来输出。在不借助第三方变量的情况下。

1.1 原始嵌套

const fs = require('fs');
const path = require('path');
fs.readFile(path.join(__dirname, '../file/A.txt'), 'utf-8', (error, dataA) => {
    if (error) throw error;
    fs.readFile(path.join(__dirname, '../file/B.txt'), 'utf-8', (error, dataB) => {
        if (error) throw error;
        fs.readFile(path.join(__dirname, '../file/C.txt'), 'utf-8', (error, dataC) => {
            if (error) throw error;
            console.log(dataA + dataB + dataC);
        })
    })
})

代码的可阅读性极差，维护起来也是相当困难

1.2 使用 ES6 的 promise

const fs = require('fs');
const path = require('path');
new Promise((resolve) => {
    fs.readFile(path.join(__dirname, '../file/A.txt'), 'utf-8', (error, dataA) => {
        if (error) throw error;
        resolve(dataA);
    })
}).then((dataA) => {
    return new Promise((resolve)=>{
        fs.readFile(path.join(__dirname, '../file/B.txt'), 'utf-8', (error, dataB) => {
            if (error) throw error;
            resolve (dataA + dataB);
        })
    })
}).then((dataAB)=>{
    fs.readFile(path.join(__dirname, '../file/C.txt'), 'utf-8', (error, dataC) => {
        if (error) throw error;
        console.log(dataAB + dataC);
    })
})

1.3 用第三方模块 async

$ npm i async

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

async.series([
        (callback) => {
            fs.readFile(path.join(__dirname, '../file/A.txt'), 'utf-8', (error, data) => {
                if (error) throw error;
                callback(null, data);
            })
        },
        (callback) => {
            fs.readFile(path.join(__dirname, '../file/B.txt'), 'utf-8', (error, data) => {
                if (error) throw error;
                callback(null, data);
            })
        },
        (callback) => {
            fs.readFile(path.join(__dirname, '../file/C.txt'), 'utf-8', (error, data) => {
                if (error) throw error;
                callback(null, data);
            })
        }
    ],
    (err, results) => {
        //results 为一个数组，里面包含了上面所有回调的 data
        console.log(results.toString());
    }
);

1.4 使用 js 的 async/await

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

function readA() {
    return new Promise((resolve) => {
        fs.readFile(path.join(__dirname, '../file/A.txt'), 'utf-8', (error, dataA) => {
            if (error) throw error;
            resolve(dataA);
        })
    })
}

function readB() {
    return new Promise((resolve) => {
        fs.readFile(path.join(__dirname, '../file/B.txt'), 'utf-8', (error, dataB) => {
            if (error) throw error;
            resolve(dataB);
        })
    })
}

function readC() {
    return new Promise((resolve) => {
        fs.readFile(path.join(__dirname, '../file/C.txt'), 'utf-8', (error, dataC) => {
            if (error) throw error;
            resolve(dataC);
        })
    })
}

// 调用
async function readABC() {
    const aTxt = await readA();
    const bTxt = await readB();
    const cTxt = await readC();
    console.log(aTxt + bTxt + cTxt);
}
readABC();

未完待续