javascript - Write / add data in JSON file using Node.js

Question

I am trying to write JSON file using node from loop data, e.g.:

let jsonFile = require('jsonfile');

for (i = 0; i < 11; i++) {
    jsonFile.writeFile('loop.json', "id :" + i + " square :" + i * i);
}

outPut in loop.json is:

id :1 square : 1

but I want output file like this (below) and also if I run that code again it should add that new output as elements in same existing JSON file:

{
   "table":[
      {
         "Id ":1,
         "square ":1
      },
      {
         "Id ":2,
         "square ":3
      },
      {
         "Id ":3,
         "square ":9
      },
      {
         "Id ":4,
         "square ":16
      },
      {
         "Id ":5,
         "square ":25
      },
      {
         "Id ":6,
         "square ":36
      },
      {
         "Id ":7,
         "square ":49
      },
      {
         "Id ":8,
         "square ":64
      },
      {
         "Id ":9,
         "square ":81
      },
      {
         "Id ":10,
         "square ":100
      }
   ]
}

I want to use same file that I created 1st time but whenever I run that code new elements should add in that same file

const fs = require('fs');

let obj = {
    table: []
};

fs.exists('myjsonfile.json', function(exists) {

    if (exists) {

        console.log("yes file exists");

        fs.readFile('myjsonfile.json', function readFileCallback(err, data) {

            if (err) {
                console.log(err);
            } else {
                obj = JSON.parse(data);

                for (i = 0; i < 5; i++) {
                    obj.table.push({
                        id: i,
                        square: i * i
                    });
                }

                let json = JSON.stringify(obj);
                fs.writeFile('myjsonfile.json', json);
            }
        });
    } else {

        console.log("file not exists");

        for (i = 0; i < 5; i++) {
            obj.table.push({
                id: i,
                square: i * i
            });
        }

        let json = JSON.stringify(obj);
        fs.writeFile('myjsonfile.json', json);
    }
});

Answer 1

If this JSON file won't become too big over time, you should try:

1. Create a JavaScript object with the table array in it

   var obj = {
      table: []
   };
   

2. Add some data to it, for example:

   obj.table.push({id: 1, square:2});
   

3. Convert it from an object to a string with JSON.stringify

   var json = JSON.stringify(obj);
   

4. Use fs to write the file to disk

   var fs = require('fs');
   fs.writeFile('myjsonfile.json', json, 'utf8', callback);
   

5. If you want to append it, read the JSON file and convert it back to an object

   fs.readFile('myjsonfile.json', 'utf8', function readFileCallback(err, data){
       if (err){
           console.log(err);
       } else {
       obj = JSON.parse(data); //now it an object
       obj.table.push({id: 2, square:3}); //add some data
       json = JSON.stringify(obj); //convert it back to json
       fs.writeFile('myjsonfile.json', json, 'utf8', callback); // write it back 
   }});
   

This will work for data that is up to 100 MB effectively. Over this limit, you should use a database engine.

UPDATE:

Create a function which returns the current date (year+month+day) as a string. Create the file named this string + .json. the fs module has a function which can check for file existence named fs.stat(path, callback). With this, you can check if the file exists. If it exists, use the read function if it's not, use the create function. Use the date string as the path cuz the file will be named as the today date + .json. the callback will contain a stats object which will be null if the file does not exist.