How do I add to an existing json file in node.js

Question

I am new to Node.js and JavaScript. I have a results.json file that I want to keep a running log of results from a script that pulls images from the web. However, my current script only overwrites the existing result. How do I build upon or add to the results.json so each subsequent result is logged in the results.json file? I would like it to be valid json.

Here is general example:

var currentSearchResult = someWebSearchResult
var fs = require('fs');
var json = JSON.stringify(['search result: ' + currentSearchResult + ': ', null, "\t");
fs.writeFile("results.json", json);

And the results.json:

[
    "search result: currentSearchResult"
]

Possible duplicate of [How to append to a file in Node?](http://stackoverflow.com/questions/3459476/how-to-append-to-a-file-in-node) — peteb, Mar 18 '16 at 19:53
do you want the resulting file to contain valid json? if so, just appending to the file isn't going to be good enough. — Kevin B, Mar 18 '16 at 19:54
@KevinB yes, that would be ideal but honestly I am not sure append is the correct terminology, so I edited my question. — mmryspace, Mar 18 '16 at 19:59

Daniel Diekmeier · Accepted Answer · 2018-09-04T14:24:51.080

44

If you want the file to be valid JSON, you have to open your file, parse the JSON, append your new result to the array, transform it back into a string and save it again.

var fs = require('fs')

var currentSearchResult = 'example'

fs.readFile('results.json', function (err, data) {
    var json = JSON.parse(data)
    json.push('search result: ' + currentSearchResult)

    fs.writeFile("results.json", JSON.stringify(json))
})

edited Sep 04 '18 at 14:24

answered Mar 18 '16 at 19:54

Daniel Diekmeier

3,401
18
33

2

`json`is an Array, and you can append a new item to an array with `.push`. – Daniel Diekmeier Jan 22 '18 at 11:10
3

In this example, "json" is not an array tho – Moe kanan Jan 23 '18 at 14:36
The results.json in the original question contains an array. With `fs.readFile`, we get a string, and after `JSON.parse`, we have an array of strings. Then we push another string to the array. – Daniel Diekmeier Jan 23 '18 at 16:42
1

You're correct, I didn't notice the actual result is an array. – Moe kanan Jan 24 '18 at 04:33
6

I downvote because this will not work in case of multiple call in a a short time with no delay to lock the file. – Sylvain Martin Apr 29 '20 at 22:44
The `writeFile` needed callback, Otherwise get error: `TypeError [ERR_INVALID_CALLBACK]: Callback must be a function. Received undefined` be like this answer: https://stackoverflow.com/a/36093068/1407491 – Nabi K.A.Z. Mar 09 '22 at 19:04
Also won't work with large files. There is a reason you may want to append data, and not read the file itself... – Douglas Gaskell Dec 05 '22 at 22:31

score 15 · Answer 2 · 2016-03-18T20:00:41.333

15

In general, If you want to append to file you should use:

fs.appendFile("results.json", json , function (err) {
   if (err) throw err;
   console.log('The "data to append" was appended to file!');
});

Append file creates file if does not exist.

But ,if you want to append JSON data first you read the data and after that you could overwrite that data.

fs.readFile('results.json', function (err, data) {
    var json = JSON.parse(data);
    json.push('search result: ' + currentSearchResult);    
    fs.writeFile("results.json", JSON.stringify(json), function(err){
      if (err) throw err;
      console.log('The "data to append" was appended to file!');
    });
})

edited Mar 18 '16 at 20:00

answered Mar 18 '16 at 19:48

1

To clarify, I do not want to overwrite data. I want to add additional data to existing results – mmryspace Mar 18 '16 at 20:32
@filmplane The first example append data without making valid json. The second example append data also, but json structure is not destroyed. – Mar 18 '16 at 20:36
We have to read the data, parse, append, and then overwrite the original file because of the JSON format. – SaundersB Sep 14 '20 at 22:38

score 1 · Answer 3 · answered Oct 05 '22 at 11:20

Promise based solution [Javascript (ES6) + Node.js (V10 or above)]

const fsPromises = require('fs').promises;
fsPromises.readFile('myFile.json', 'utf8') 
        .then(data => { 
                let json = JSON.parse(data);
                json.myArr.push({name: "Krishnan", salary: 5678});

                fsPromises.writeFile('myFile.json', JSON.stringify(json))
                        .then(  () => { console.log('Append Success'); })
                        .catch(err => { console.log("Append Failed: " + err);});
            })
        .catch(err => { console.log("Read Error: " +err);});

If your project supports Javascript ES8 then you could use asyn/await instead of native promise.

score 0 · Answer 4 · answered Nov 29 '21 at 15:16

0

I have created data. Js file to maintain data displaying in the table fomat. Including text box to read data from user and how to display data enteted through text box in table

answered Nov 29 '21 at 15:16

priyanka bajantri

1
1

1

This does not provide an answer to the question. Once you have sufficient [reputation](https://stackoverflow.com/help/whats-reputation) you will be able to [comment on any post](https://stackoverflow.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/30461741) – camille Nov 29 '21 at 22:48

How do I add to an existing json file in node.js

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