How to write data to a JSON file using Javascript

Question

You have to be clear on what you mean by "JSON".

Some people use the term JSON incorrectly to refer to a plain old JavaScript object, such as [{a: 1}]. This one happens to be an array. If you want to add a new element to the array, just push it, as in

var arr = [{a: 1}];
arr.push({b: 2});

< [{a: 1}, {b: 2}]

The word JSON may also be used to refer to a string which is encoded in JSON format:

var json = '[{"a": 1}]';

Note the (single) quotation marks indicating that this is a string. If you have such a string that you obtained from somewhere, you need to first parse it into a JavaScript object, using JSON.parse:

var obj = JSON.parse(json);

Now you can manipulate the object any way you want, including push as shown above. If you then want to put it back into a JSON string, then you use JSON.stringify:

var new_json = JSON.stringify(obj.push({b: 2}));
'[{"a": 1}, {"b": 1}]'

JSON is also used as a common way to format data for transmission of data to and from a server, where it can be saved (persisted). This is where ajax comes in. Ajax is used both to obtain data, often in JSON format, from a server, and/or to send data in JSON format up to to the server. If you received a response from an ajax request which is JSON format, you may need to JSON.parse it as described above. Then you can manipulate the object, put it back into JSON format with JSON.stringify, and use another ajax call to send the data to the server for storage or other manipulation.

You use the term "JSON file". Normally, the word "file" is used to refer to a physical file on some device (not a string you are dealing with in your code, or a JavaScript object). The browser has no access to physical files on your machine. It cannot read or write them. Actually, the browser does not even really have the notion of a "file". Thus, you cannot just read or write some JSON file on your local machine. If you are sending JSON to and from a server, then of course, the server might be storing the JSON as a file, but more likely the server would be constructing the JSON based on some ajax request, based on data it retrieves from a database, or decoding the JSON in some ajax request, and then storing the relevant data back into its database.

Do you really have a "JSON file", and if so, where does it exist and where did you get it from? Do you have a JSON-format string, that you need to parse, mainpulate, and turn back into a new JSON-format string? Do you need to get JSON from the server, and modify it and then send it back to the server? Or is your "JSON file" actually just a JavaScript object, that you simply need to manipulate with normal JavaScript logic?

JSON can be written into local storage using the JSON.stringify to serialize a JS object. You cannot write to a JSON file using only JS. Only cookies or local storage

    var obj = {"nissan": "sentra", "color": "green"};

localStorage.setItem('myStorage', JSON.stringify(obj));

And to retrieve the object later

var obj = JSON.parse(localStorage.getItem('myStorage'));

Unfortunatelly, today (September 2018) you can not find cross-browser solution for client side file writing.

For example: in some browser like a Chrome we have today this possibility and we can write with FileSystemFileEntry.createWriter() with client side call, but according to the docu:

This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

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For IE (but not MS Edge) we could use ActiveX too, but this is only for this client.

If you want update your JSON file cross-browser you have to use server and client side together.

The client side script

On client side you can make a request to the server and then you have to read the response from server. Or you could read a file with FileReader too. For the cross-browser writing to the file you have to have some server (see below on server part).

var xhr = new XMLHttpRequest(),
    jsonArr,
    method = "GET",
    jsonRequestURL = "SOME_PATH/jsonFile/";

xhr.open(method, jsonRequestURL, true);
xhr.onreadystatechange = function()
{
    if(xhr.readyState == 4 && xhr.status == 200)
    {
        // we convert your JSON into JavaScript object
        jsonArr = JSON.parse(xhr.responseText);

        // we add new value:
        jsonArr.push({"nissan": "sentra", "color": "green"});

        // we send with new request the updated JSON file to the server:
        xhr.open("POST", jsonRequestURL, true);
        xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        // if you want to handle the POST response write (in this case you do not need it):
        // xhr.onreadystatechange = function(){ /* handle POST response */ };
        xhr.send("jsonTxt="+JSON.stringify(jsonArr));
        // but on this place you have to have a server for write updated JSON to the file
    }
};
xhr.send(null);

Server side scripts

You can use a lot of different servers, but I would like to write about PHP and Node.js servers.

By using searching machine you could find "free PHP Web Hosting*" or "free Node.js Web Hosting". For PHP server I would recommend 000webhost.com and for Node.js I would recommend to see and to read this list.

PHP server side script solution

The PHP script for reading and writing from JSON file:

<?php

// This PHP script must be in "SOME_PATH/jsonFile/index.php"

$file = 'jsonFile.txt';

if($_SERVER['REQUEST_METHOD'] === 'POST')
// or if(!empty($_POST))
{
    file_put_contents($file, $_POST["jsonTxt"]);
    //may be some error handeling if you want
}
else if($_SERVER['REQUEST_METHOD'] === 'GET')
// or else if(!empty($_GET))
{
    echo file_get_contents($file);
    //may be some error handeling if you want
}
?>

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Node.js server side script solution

I think that Node.js is a little bit complex for beginner. This is not normal JavaScript like in browser. Before you start with Node.js I would recommend to read one from two books:

• Learning Node: Moving to the Server-Side
• Node.js Web Development: Server-side development

The Node.js script for reading and writing from JSON file:

var http = require("http"),
    fs = require("fs"),
    port = 8080,
    pathToJSONFile = '/SOME_PATH/jsonFile.txt';

http.createServer(function(request, response)
{
    if(request.method == 'GET')
    {
        response.writeHead(200, {"Content-Type": "application/json"});
        response.write(fs.readFile(pathToJSONFile, 'utf8'));
        response.end();
    }
    else if(request.method == 'POST')
    {
        var body = [];

        request.on('data', function(chunk)
        {
            body.push(chunk);
        });

        request.on('end', function()
        {
            body = Buffer.concat(body).toString();
            var myJSONdata = body.split("=")[1];
            fs.writeFileSync(pathToJSONFile, myJSONdata); //default: 'utf8'
        });
    }
}).listen(port);

Related links for Node.js:

• How to Develop Web Application using pure Node.js (HTTP GET and POST, HTTP Server) (detailed video tutorial)
• Anatomy of an HTTP Transaction
• How to handle POST request in Node.js
• How do you extract POST data in Node.js?