write a javascript object to a file in node

Question

How do I use Node to take a JS object (e.g. var obj = {a: 1, b: 2}) and create a file that contains that object?

For example, the following works:

var fs = require('fs')

fs.writeFile('./temp.js', 'var obj = {a: 1, b: 2}', function(err) {
  if(err) console.log(err)
  // creates a file containing: var obj = {a: 1, b: 2}
})

But this doesn't work:

var obj = {a: 1, b: 2}

fs.writeFile('./temp.js', obj, function(err) {
  if(err) console.log(err)
  // creates a file containing: [object Object]
})

Update: JSON.stringify() will create a JSON object (i.e. {"a":1,"b":2}) and not a Javascript object (i.e. {a:1,b:2})

Possible duplicate of [Write objects into file with Node.js](http://stackoverflow.com/questions/21976567/write-objects-into-file-with-node-js) — Stephan Bijzitter, Feb 09 '17 at 09:12
I don't believe stringify is the answer. This will create a JSON string (i.e. `{"a":1,"b":2}`) and not a JS object (i.e. `{a:1,b:2}`) — Ed Williams, Feb 09 '17 at 09:48

score 17 · Accepted Answer · edited May 23 '17 at 12:24

17

Thanks Stephan Bijzitter for the link

My problem can be solved like so:

var fs = require('fs')
var util = require('util')
var obj = {a: 1, b: 2}

fs.writeFileSync('./temp.js', 'var obj = ' + util.inspect(obj) , 'utf-8')

This writes a file containing: var obj = { a: 1, b: 2 }

edited May 23 '17 at 12:24

Community

1
1

answered Feb 09 '17 at 10:09

Ed Williams

2,447
3
15
21

3

This is unfortunately not a proper answer, as `util.inspect` will truncate arrays and integers, and not result in the expected object. – Steve Dodier-Lazaro Jun 04 '20 at 08:51

score 3 · Answer 2 · answered Feb 09 '17 at 09:14

Its because of arguments that writeFile requires.

https://nodejs.org/api/fs.html#fs_fs_writefile_file_data_options_callback

It wants data to be <String> | <Buffer> | <Uint8Array>

So your first example works because it's a string.

In order to make second work just use JSON.stringify(obj);

Like this

fs.writeFile('file.txt', JSON.stringify(obj), callback)

Hope this helps.

Emeke Ajeh · Answer 3 · 2021-02-02T18:01:07.343

1

const fs = require('fs')
const util = require('util')
const obj = {a: 1, b: 2}

fs.writeFileSync(
  './temp.js',
  util.formatWithOptions({compact: false}, 'var obj = %o', obj),
  'utf-8'
)

Formatted output:

// ./temp.js
var obj = {
  a: 1,
  b: 2
}

edited Feb 02 '21 at 18:01

answered Feb 02 '21 at 17:49

Emeke Ajeh

933
1
10
16

score 0 · Answer 4 · answered Feb 09 '17 at 09:13

0

You need to stringify your object:

var obj = {a: 1, b: 2}

fs.writeFile('./temp.js', JSON.stringify(obj), function(err) {
  if(err) console.log(err)
})

answered Feb 09 '17 at 09:13

Robby Cornelissen

91,784
22
134
156

write a javascript object to a file in node

4 Answers4