Writing JSON object to a JSON file with fs.writeFileSync

Question

I am trying to write a JSON object to a JSON file. The code executes without errors, but instead of the content of the object been written, all that gets written into the JSON file is:

[object Object]

This is the code that actually does the writing:

fs.writeFileSync('../data/phraseFreqs.json', output)

'output' is a JSON object, and the file already exists. Please let me know if more information is required.

fs.writeFileSync('../data/phraseFreqs.json', JSON.stringify(output)) — Daniel, Feb 11 '17 at 17:41
Possible duplicate of [Write objects into file with Node.js](https://stackoverflow.com/questions/21976567/write-objects-into-file-with-node-js) — Neeraj Sewani, Nov 02 '18 at 12:23

score 215 · Accepted Answer · edited Sep 22 '17 at 03:16

215

You need to stringify the object.

fs.writeFileSync('../data/phraseFreqs.json', JSON.stringify(output));

edited Sep 22 '17 at 03:16

LightBender

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answered Sep 22 '17 at 02:43

Kamal

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Welcome to SO, before answering a question, try to review the existing answer. If your answer has already been suggested, upvote that answer instead. See the [community guide](https://stackoverflow.com/help/how-to-answer) for writing a good answer. – LightBender Sep 22 '17 at 03:07
55

I like that this answers the question without opinions about whether or not to use synchronous vs async operations. – Brian Duncan Mar 30 '18 at 18:48
4

For readability purposes, you can use the space parameter of the JSON.stringify method: `fs.writeFileSync('../data/phraseFreqs.json', JSON.stringify(output, null, 2));` More: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify – Binh Jan 09 '20 at 09:36

score 54 · Answer 2 · edited Apr 11 '19 at 06:07

54

I don't think you should use the synchronous approach, asynchronously writing data to a file is better also stringify the output if it's an object.

Note: If output is a string, then specify the encoding and remember the flag options as well.:

const fs = require('fs');
const content = JSON.stringify(output);

fs.writeFile('/tmp/phraseFreqs.json', content, 'utf8', function (err) {
    if (err) {
        return console.log(err);
    }

    console.log("The file was saved!");
});

Added Synchronous method of writing data to a file, but please consider your use case. Asynchronous vs synchronous execution, what does it really mean?

const fs = require('fs');
const content = JSON.stringify(output);

fs.writeFileSync('/tmp/phraseFreqs.json', content);

edited Apr 11 '19 at 06:07

codejockie

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answered Feb 11 '17 at 17:42

Akinjide

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If it's being done in a short script or something, synchronous is fine. If it's part of a server request or something, then it should be asynchronous. – Hilton Shumway Feb 23 '18 at 19:21
1

Not necessarily, I/O bound processes should be made asynchronous, but depending on the short script complexity you might opt in for synchronous. – Akinjide Feb 23 '18 at 21:55
5

This is not an answer to the question. – Stephan Bijzitter Mar 06 '18 at 12:52
8

User specifically asked for synchronous method – Anthony Apr 02 '18 at 22:50
this is generating a empty object: `{}` – fccoelho Oct 18 '18 at 12:19
what do you have has `output` in `const content = JSON.stringify(output);` i.e. `const content = JSON.stringify({ 'hello': 'world' });` – Akinjide Oct 18 '18 at 13:54
9

Please stop saying async good. And implying sync bad. If you are worried about speed, your webpack should do that optimization for you. You are not an optimizer. Reason: sync file writing is needed for json command-line tools. Which must close any files they have open before piping data to the next app in the chain. – TamusJRoyce Nov 01 '18 at 04:11
Synchronous is completely fine. There's no reason it shouldn't be accepted and supported. – Jason Rice May 05 '20 at 02:24

score 38 · Answer 3 · edited Apr 11 '19 at 06:09

38

Make the json human readable by passing a third argument to stringify:

fs.writeFileSync('../data/phraseFreqs.json', JSON.stringify(output, null, 4));

edited Apr 11 '19 at 06:09

codejockie

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answered Nov 17 '17 at 09:49

Timelot

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score 1 · Answer 4 · edited Apr 11 '19 at 06:10

When sending data to a web server, the data has to be a string (here). You can convert a JavaScript object into a string with JSON.stringify(). Here is a working example:

var fs = require('fs');

var originalNote = {
  title: 'Meeting',
  description: 'Meeting John Doe at 10:30 am'
};

var originalNoteString = JSON.stringify(originalNote);

fs.writeFileSync('notes.json', originalNoteString);

var noteString = fs.readFileSync('notes.json');

var note = JSON.parse(noteString);

console.log(`TITLE: ${note.title} DESCRIPTION: ${note.description}`);

Hope it could help.

score 1 · Answer 5 · answered Nov 11 '19 at 11:00

1

Here's a variation, using the version of fs that uses promises:

const fs = require('fs');

await fs.promises.writeFile('../data/phraseFreqs.json', JSON.stringify(output)); // UTF-8 is default

answered Nov 11 '19 at 11:00

Flimm

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Writing JSON object to a JSON file with fs.writeFileSync

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