How to modify file content in nodejs

Nodejs method to modify file content: 1. Use "fs.writeFile(path,data,callback:(err)=>void)" method; 2. Use "fs.open(path,(err) ,fd)=>{})" to open the file, and then write data through the file descriptor fd.

The operating environment of this tutorial: windows7 system, nodejs version 14.15.4, DELL G3 computer.

In nodejs, you can use the built-in method of the file system module (fs for short) to modify the file content.

Modify the file content

Write data to the file. The data can be a string or buffer: fs.writeFile(path,data,callback:(err )=>void)

fs.writeFile('message.txt', data, (err) => {
  if (err) throw err;
});

There is another way to write a file through the file descriptor fd:

fs.open(path,(err,fd)=>{
    //针对拿到的fd 进行操作：将buffer内容写如fd对应的文件里
    //position为文件的起点
    //length为待写的长度
    //offset为缓存区起写的位置
    fs.write(fd,buffer,offset,length,position,(err,bytesWrittenLen,buffer)=>{

    })
    //关闭文件
    fs.close(fd, (err) => {
        if (err) throw err;
    });
})

Example: read the file and modify the file Content

const fs = require('fs');
const path = require('path');
const newList = [];
fs.readFile(path.join(__dirname, './json/hp_mph.json'), 'utf8', function (err, data) {
    if (err) throw err;
    let list = JSON.parse(data);
    // list.forEach((item,index)=>{
    //     let value = item.properties;
    //     let result = {};  
    //     result.ID = index + 1;
    //     result.TYPE = value.FLAG_A;
    //     result.X = value.X;
    //     result.Y = value.Y;
    //     newList.push(result);
    // })
 
    for (let i = 0; i < list.length; i++) {
        let result = {};
        let value = list[i].properties;
        result.ID = i + 1;
        result.TYPE = value.FLAG_A;
        result.X = value.X;
        result.Y = value.Y;
        newList.push(result);
    }
 
    let newContent = JSON.stringify(newList, null, 4);
    fs.writeFile(&#39;result.json&#39;, newContent, &#39;utf8&#39;, (err) => {
        if (err) throw err;
        console.log(&#39;success done&#39;);
    });
});

[Recommended learning: "nodejs tutorial"]

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