node.js - how to write an array to file

Question

I have a sample array as follows

var arr = [ [ 1373628934214, 3 ],
  [ 1373628934218, 3 ],
  [ 1373628934220, 1 ],
  [ 1373628934230, 1 ],
  [ 1373628934234, 0 ],
  [ 1373628934237, -1 ],
  [ 1373628934242, 0 ],
  [ 1373628934246, -1 ],
  [ 1373628934251, 0 ],
  [ 1373628934266, 11 ] ]

I would like to write this array to a file such as I get a file as follows

1373628934214, 3 
1373628934218, 3
1373628934220, 1
......
......

mak · Accepted Answer · 2013-07-12T12:29:02.613

136

If it's a huuge array and it would take too much memory to serialize it to a string before writing, you can use streams:

var fs = require('fs');

var file = fs.createWriteStream('array.txt');
file.on('error', function(err) { /* error handling */ });
arr.forEach(function(v) { file.write(v.join(', ') + '\n'); });
file.end();

edited Jul 12 '13 at 12:29

answered Jul 12 '13 at 11:46

mak

13,267
5
41
47

3

Why not put `file.end();` after the loop instead of testing the index? – Jul 12 '13 at 12:22
Thanks! My arrays are in the order of 1000+ so this method works perfectly – dopplesoldner Jul 12 '13 at 14:24
18

odd, this is giving me error: `TypeError: v.join is not a function`, to troubleshoot, i logged `typeof(v)` within the loop and it is a string, [coderade's solution](https://stackoverflow.com/a/51362713) doesn't seem to produce error though. – user1063287 Jul 30 '19 at 10:10
2

@user1063287 Double check that you're passing a multi-dimensional array (as in the OP) – Jose A Sep 11 '20 at 17:14

score 53 · Answer 2 · answered Apr 04 '15 at 12:00

53

Remember you can access good old ECMAScript APIs, in this case, JSON.stringify().

For simple arrays like the one in your example:

require('fs').writeFile(

    './my.json',

    JSON.stringify(myArray),

    function (err) {
        if (err) {
            console.error('Crap happens');
        }
    }
);

answered Apr 04 '15 at 12:00

Jairo

863
7
14

Not related to ArrayBuffers – Jimmy Obonyo Abor Nov 20 '17 at 20:40
4

if `myArray` is huge, `JSON.stringify(myArray)` will become a problem. – Thịnh Phạm Dec 07 '17 at 04:42
6

For better readability, you can use `JSON.stringify(myArray, null, 1)` – hb20007 Feb 27 '19 at 10:40
@ThịnhPhạm How huge is too huge? – Lemon Mar 26 '22 at 07:52

valdeci · Answer 3 · 2020-11-04T17:28:55.653

To do what you want, using the fs.createWriteStream(path[, options]) function in a ES6 way:

const fs = require('fs');
const writeStream = fs.createWriteStream('file.txt');
const pathName = writeStream.path;
 
let array = ['1','2','3','4','5','6','7'];
  
// write each value of the array on the file breaking line
array.forEach(value => writeStream.write(`${value}\n`));

// the finish event is emitted when all data has been flushed from the stream
writeStream.on('finish', () => {
   console.log(`wrote all the array data to file ${pathName}`);
});

// handle the errors on the write process
writeStream.on('error', (err) => {
    console.error(`There is an error writing the file ${pathName} => ${err}`)
});

// close the stream
writeStream.end();

Denys Séguret · Answer 4 · 2017-05-03T11:52:46.543

18

A simple solution is to use writeFile :

require("fs").writeFile(
     somepath,
     arr.map(function(v){ return v.join(', ') }).join('\n'),
     function (err) { console.log(err ? 'Error :'+err : 'ok') }
);

edited May 03 '17 at 11:52

answered Jul 12 '13 at 11:42

Denys Séguret

372,613
87
782
758

If you do, don't forget the callback with err: http://nodejs.org/api/fs.html#fs_fs_writefile_filename_data_options_callback – bryanmac Jul 12 '13 at 11:49
4

Not sure what's amiss, but my system pukes on this code as written. `TypeError: v.join is not a function` – zipzit Mar 07 '16 at 21:00
Missing a `)` after `'ok'` – eozzy Sep 02 '16 at 16:48
@3zzy thanks, fixed. Please note that you can edit answers on SO – Denys Séguret Sep 02 '16 at 16:49
1

syntax error need to add arr.map(function(v){ return v.join(', ') >>} – Gurjinder Singh May 03 '17 at 11:50
@GurjinderSingh yes – Denys Séguret May 03 '17 at 11:52
@zipzit might be a bit late, but make sure items inside your array are arrays and not strings, otherwise, you can't use join() function on a string. – nicoolaslb Mar 24 '22 at 22:14

score 4 · Answer 5 · answered Apr 04 '21 at 11:21

async function x(){
var arr = [ [ 1373628934214, 3 ],
[ 1373628934218, 3 ],
[ 1373628934220, 1 ],
[ 1373628934230, 1 ],
[ 1373628934234, 0 ],
[ 1373628934237, -1 ],
[ 1373628934242, 0 ],
[ 1373628934246, -1 ],
[ 1373628934251, 0 ],
[ 1373628934266, 11 ] ];
await fs.writeFileSync('./PATH/TO/FILE.txt', arr.join('\n'));
}

score 3 · Answer 6 · answered Jun 23 '19 at 13:44

3

We can simply write the array data to the filesystem but this will raise one error in which ',' will be appended to the end of the file. To handle this below code can be used:

var fs = require('fs');

var file = fs.createWriteStream('hello.txt');
file.on('error', function(err) { Console.log(err) });
data.forEach(value => file.write(`${value}\r\n`));
file.end();

\r\n

is used for the new Line.

\n

won't help. Please refer this

answered Jun 23 '19 at 13:44

KamalDeep

791
5
8

That depends on the OS where the script is being run. For Linux \n is a valid newline. For Windows you need \r\n – Rahul Kadukar May 19 '20 at 23:13

score 2 · Answer 7 · edited May 18 '22 at 01:56

2

Try with this

await fs.writeFileSync('arreglo.txt', arreglo.join('\n '));

This code works for me

edited May 18 '22 at 01:56

Roman Mahotskyi

4,576
5
35
68

answered May 13 '22 at 01:18

Roel Leal

49
2

Works great. Although the extra space isn't needed for the `.join('\n ')`, it adds a space at the beginning of each line starting at line 2. – synkro May 21 '22 at 19:13
Yeah, i added this space to separate data but this doesn't work for me hahaha, just add a space, I'm glad it works for you – Roel Leal May 22 '22 at 01:28

node.js - how to write an array to file

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