使用fs.readFile的简单nodejs回调示例

Question

我正在尝试学习异步编程,并且正在努力学习nodeschool.io的第4课,并实现了带回调的异步io.

基本上,我正在尝试使用fs.readFile来计算使用回调的文件中的换行符数.

这是我的代码:

var fs = require('fs');
var pathToFile = process.argv[2];

function counter(callback) {
    var buffer = fs.readFile(pathToFile, function (err, data) {
    var bufferString = buffer.toString();
    var bufferStringSplit = bufferString.split('\n');
  });
  callback();
}

function logMyNumber() {
  console.log(bufferStringSplit.length-1);
}

counter(logMyNumber);

我知道一旦代码行完成执行就会执行回调,所以不应该执行

var bufferString = buffer.toString();
var bufferStringSplit = bufferString.split('\n');

在fs.readFile()完成从磁盘读取文件后调用？

最后,callback()调用logMyNumber,它应该只输出文件所具有的行数.

Answer 1

您有几个问题正在进行中,我将尝试尽可能地概述它们

问题1:可变范围

var fs = require('fs');
var pathToFile = process.argv[2];

function counter(callback) {
  var buffer = fs.readFile(pathToFile, function (err, data) { 
    // Both of the following variables are scoped to the callback of fs.readFile
    var bufferString = buffer.toString(); 
    var bufferStringSplit = bufferString.split('\n'); 
  });
  callback();
}

function logMyNumber() {
  // Because the variables are in a closure above, bufferStringSplit is null here
  console.log(bufferStringSplit.length-1);
}

counter(logMyNumber);

解:

声明模块范围内的变量:

var fs = require('fs');
var pathToFile = process.argv[2];

// These can now be accessed from anywhere within the module
var bufferString, bufferStringSplit;

function counter(callback) {
  fs.readFile(pathToFile, function (err, data) {
    bufferString = data.toString(); 
    bufferStringSplit = bufferString.split('\n'); 
    callback();
  });
}

// bufferStringSplit should no longer return null here
function logMyNumber() {
  console.log(bufferStringSplit.length-1);
}

问题2:回调

function counter(callback) {
  fs.readFile(pathToFile, function (err, data) {
    bufferString = buffer.toString(); 
    bufferStringSplit = bufferString.split('\n'); 

    // Place the callback WITHIN the other callback, otherwise they run in parallel
    callback();
  });
}

问题3:fs.readFile API

fs.readFile不会返回任何内容,因此buffer下面的变量是null

function counter(callback) {      
  var buffer = fs.readFile(pathToFile, function (err, data) {
    bufferString = buffer.toString(); 
    bufferStringSplit = bufferString.split('\n'); 
  });
  callback();
}

解:

function counter(callback) {      
  fs.readFile(pathToFile, function (err, data) {
    // The data argument of the fs.readFile callback is the data buffer
    bufferString = data.toString(); 
    bufferStringSplit = bufferString.split('\n'); 
  });
  callback();
}

最后,代码应如下所示:

var fs = require('fs');
var pathToFile = process.argv[2];

var bufferString, bufferStringSplit;

function counter(callback) {
  fs.readFile(pathToFile, function (err, data) {
    bufferString = data.toString(); 
    bufferStringSplit = bufferString.split('\n'); 
    callback();
  });
}

function logMyNumber() {
  console.log(bufferStringSplit.length-1);
}

counter(logMyNumber);