write into json file with fs.append

Question

I am working on a project using node.js and am struggling to fit an JSON object at the right position into my already existing data. My file currently looks like this:

[
  {
    "id": "001",
    "name": "Paul,
    "city": "London"
  },
  {
    "id": "002",
    "name": "Peter,
    "city": "New York"
  },
...
]

I tried to arrange my data like:

var data = { id: id, name: name, city: city };

having the respective data stored in those variables. Then I used var json = JSON.stringify(data) and tried

fs.appendFile("myJSONFile.json", json, function (err) {
        if (err) throw err;
        console.log('Changed!');
    });

The file changes indeed but the new entry is positioned after the square bracket.

[
  {
    "id": "001",
    "name": "Paul,
    "city": "London"
  },
  {
    "id": "002",
    "name": "Peter,
    "city": "New York"
  },
...
]{"id":"004","name":"Mark","city":"Berlin"}

How do I get it alongside the previous entries? Any help would be truly appreciated!

Azamat Abdullaev · Accepted Answer · 2021-10-22T05:19:09.253

3

You need to first read the file, in your case you are storing an array in the file. So, you need to push your object to the array that you read from the file, and write the result back to the file (not append):

const fs = require('fs/promises');

// ...

const addToFile = async data => {
  let fileContents = await fs.readFile('myJSONFile.json', { encoding: 'utf8' });
  fileContents = JSON.parse(fileContents);
  fileContents.push(data);
  await fs.writeFile('myJSONFile.json', JSON.stringify(fileContents, null, 2), { encoding: 'utf8' });
};

edited Oct 22 '21 at 05:19

answered Oct 15 '21 at 10:48

Azamat Abdullaev

726
9
24

I get in the last line of the function `TypeError [ERR_INVALID_ARG_TYPE]: The "cb" argument must be of type function. Received an instance of Object`. I call it with `addtofile(data)`. – Timo May 30 '22 at 14:49
1

@Timo seems like you are using the callback variant of the `fs` module, make sure your require statement is `'fs/promises'`, instead of `'fs'` like in line 1. – Azamat Abdullaev May 30 '22 at 18:21
Two further things: the function head can be rewritten to `function addToFile(data){}`? The advantage of the promise to the sync function is that the promise can run other code until the promise finished, so is more efficient and non-blocking? – Timo May 30 '22 at 21:07

score 1 · Answer 2 · answered Oct 15 '21 at 10:49

You need to read current file, parse JSON content, modify it and then save the modified content:

const jsonString = fs.readFileSync(path);
const jsonObject = JSON.parse(jsonString);
jsonObject.push(item);
fs.writeFileSync(path, JSON.stringify(jsonObject));

write into json file with fs.append

2 Answers2