This is the section that contains the code for the above.

console.log is logging the data properly before the writeFile operation.

const fs = require('fs');
    const path = require('path');
    
    const p = path.join(
        path.dirname(require.main.filename),
        'data',
        'userdata.json'
    );
    
    module.exports=class userdata{
        constructor(username,phoneNo,mothersName,fathersName,address){
            this.username=username
            this.phoneNo=phoneNo
            this.fathersName=fathersName
            this.mothersName=mothersName
            this.address=address
        }
        save() {
            
            this.id = Math.random().toString();
            fs.readFile(p,(err,data)=>{
                let userData = []
                if(!err){
                    userData=JSON.parse(data)
                    console.log(userData)
    
                }
                userData.push(this)
                
                console.log("pushed")
            })
            
            fs.writeFile(p, JSON.stringify(userData),(err)=>{
                if(err){
                console.log(err)
               }
              })
            }
    

please ignore this fetchAll function

static fetchAll(){
        fs.readFile(p,(err,data)=>{
            if(err)
            return []
            return JSON.parse(data)
        }
        )
        
    }
}

Reference from NodeJS - The Complete Guide (MVC, REST APIs, GraphQL, Deno) by Maximilian Schwarzmüller udemy.

CodePudding user response：

fs.readFile is asynchronous, but you execute fs.writeFile synchronously, without waiting for the fs.readFile to complete. Put the fs.writeFile command into the callback for fs.readFile:

fs.readFile(p,(err,data)=>{
  let userData = []
  if(!err){
    userData=JSON.parse(data)
    console.log(userData)
    return
  }
  userData.push(this)
  console.log("pushed")
  fs.writeFile(p, JSON.stringify(userData),(err)=>{...})
})