Write Data to Local JSON File using Node.js

November 11th, 2016

First, we need to have a JSON file with empty array or existing array with objects. Suppose this users.json:

{
	"users": []
}

For append new content to users.json file we need to read the file and convert it contents to an JavaScript object using JSON.parse() method:

var fs = require('fs')

fs.readFile('./users.json', 'utf-8', function(err, data) {
	if (err) throw err

	var arrayOfObjects = JSON.parse(data)
})

Now arrayOfObjects variable contains a object from users.json file:

console.log(arrayOfObjects)
// Output: { users: [] }

Now we can write the data into an users array using the push() method is as follows:

var fs = require('fs')

fs.readFile('./users.json', 'utf-8', function(err, data) {
	if (err) throw err

	var arrayOfObjects = JSON.parse(data)
	arrayOfObjects.users.push({
		name: "Mikhail",
		age: 24
	})

	console.log(arrayOfObjects)
})

Now we have an array with an object:

console.log(arrayOfObjects)
// Output: { users: [ { name: 'Mikhail', age: 24 } ] }

You can appended any objects to users array.

After we have data in variable we need to write this into file:

fs.writeFile('./users.json', JSON.stringify(arrayOfObjects), 'utf-8', function(err) {
	if (err) throw err
	console.log('Done!')
})

You may have noticed that here uses JSON.stringify(arrayOfObjects). JSON.stringify() method convert the contents of arrayOfObjects into JSON.

Now users.json looks like this:

{
	"users": [
		{
			"name": "Mikhail",
			"age": 24
		}
	]
}

Full Code:

index.js

var fs = require('fs')

fs.readFile('./users.json', 'utf-8', function(err, data) {
	if (err) throw err

	var arrayOfObjects = JSON.parse(data)
	arrayOfObjects.users.push({
		name: "Mikhail",
		age: 24
	})

	console.log(arrayOfObjects)

	fs.writeFile('./users.json', JSON.stringify(arrayOfObjects), 'utf-8', function(err) {
		if (err) throw err
		console.log('Done!')
	})
})

users.json

{
	"users": []
}

Mikhail

I am Mikhail Evdokimov, a Hobbyist Self Taught Programmer