How to copy files in Nodejs_node.js

Every front-end kid knows that JavaScript does not have permission to operate disk files, and server kids have always despised it. But nodejs can be said to make our front-end proud. I have been learning node recently, and its powerful functions make people very excited and excited. Today I learned how it reads and writes files.

First you need to introduce the fs module, which comes with nodejs.

var fs=require("fs");
For details, please refer to Nodejs API: http://www.w3cfuns.com/tools.php?mod=booknodejs

There are two main methods used:

1. fs.readFile(filename, [encoding], [callback])

This is asynchronous reading of files, filename is the file path, encoding is the encoding format, and callback is the callback function.

Asynchronously read all the contents of a file, the example is as follows:

fs.readFile('/etc/passwd', function (err, data) {
 if (err) throw err;
 console.log(data);
});

Here I am using a local test file:

function readFile(){
  console.log('--------开始读取文件--------');
  var fs = require('fs');
  fs.readFile('test.txt', 'utf-8', function(err, data) {
    if (err) {
      console.log("读取失败");
    } else {
      console.log(data);
      return data;
    }
  });
  console.log('--------读取结束--------');
}

2. fs.writeFile(filename, data, encoding='utf8', [callback])
Write file:

function writeFile(data){
  fs.writeFile("test2.txt",data,function(error){
    if(error){
      throw error;
    }else{
      console.log("文件已保存");  
    }
  });
}

Error code: copyFile.js file

var fs=require("fs");
function readFile(){
  console.log('--------开始读取文件--------');
  var fs = require('fs');
  fs.readFile('test.txt', 'utf-8', function(err, data) {
    if (err) {
      console.log("读取失败");
    } else {
      console.log(data);
      return data;
    }
  });
  console.log('--------读取结束--------');
}

function writeFile(data){
  fs.writeFile("test2.txt",data,function(error){
    if(error){
      throw error;
    }else{
      console.log("文件已保存");  
    }
  });
}
function copyFile(){
  var txt=readFile();
  writeFile(txt);
}
copyFile();

The result of running node copyFile.js in the terminal is as follows:

Note:

1. File encoding. At the beginning, I directly created a new txt document locally. When I read it, I found that the result was always aaaaaaa. It turned out that when I opened it with editor, it was garbled. Secondly, it is best to bring encoding. , otherwise it will read according to the buffer.

2. Synchronous execution problem.

There is a problem with the above code. I wrote the file reading and writing methods separately. I originally wanted to copy the contents of the test.txt file to test2.txt, but reading the file is executed asynchronously. , that is to say, no one knows when it will be executed, so the result of test.txt is undefined.

The correct method should be to write the file after reading:

var fs=require("fs");
function copyFile(){
  console.log('--------开始读取文件--------');
  var fs = require('fs');
  fs.readFile('test.txt', 'utf-8', function(err, data) {
    if (err) {
      console.log("读取失败");
    } else {
      writeFile(data)
      return data;
    }
  });
  console.log('--------读取结束--------');
}

function writeFile(data){
  console.log(data);
  fs.writeFile("test2.txt",data,'utf8',function(error){
    if(error){
      throw error;
    }else{
      console.log("文件已保存");  
    }
  });
}

copyFile();

The above is the entire content of this article, I hope it will be helpful to everyone’s study.