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.

Solution 1

You need to stringify the object.

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

Solution 2

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);

Solution 3

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

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

Solution 4

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.

Solution 5

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

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Comments

[LightBender]

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 for writing a good answer.

[Hilton Shumway]

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.

[Akinjide]

Not necessarily, I/O bound processes should be made asynchronous, but depending on the short script complexity you might opt in for synchronous.

[Stephan Bijzitter]

This is not an answer to the question.

[Brian Duncan]

I like that this answers the question without opinions about whether or not to use synchronous vs async operations.

[Anthony]

User specifically asked for synchronous method

[fccoelho]

this is generating a empty object: {}

[Akinjide]

what do you have has output in const content = JSON.stringify(output); i.e. const content = JSON.stringify({ 'hello': 'world' });

[TamusJRoyce]

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.

[Binh]

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: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

[Jason Rice]

Synchronous is completely fine. There's no reason it shouldn't be accepted and supported.