Javascript unknown undefined returned

Question

I keep getting undefined returned when I run this code:

var fs = require('fs'),
    path = require('path'),
    filepath = process.argv[2];

function listFilesByExtension(path_to_file, ext) {
    fs.readdir(path_to_file, function (err, files) {
        if (err) {
            console.log(err);
        } else {
            console.log(files);
        }
    });
}

console.log(listFilesByExtension(filepath, 'txt'));

the console.log(files) returns:

undefined
[ 'CHANGELOG.md',
  'LICENCE.md',
  'README.md',
  'api.html',
  'dat',
  'data.dat',
  'data.json',
  'learnyounode.dat',
  'learnyounode.sql',
  'learnyounode.txt',
  'md',
  'w00t.dat',
  'w00t.txt',
  'words.dat',
  'wrrrrongdat' ]

So the undefined is not from there, console.log(err) is also return null.

Can anybody explain to me why this is happen? thank you

`listFilesByExtension` doesn't return anything, so your `console.log` for this function call will output `undefined`. This line is the cause: `console.log(listFilesByExtension(filepath, 'txt'));` — Alexander O'Mara, Nov 04 '15 at 07:16
Try to remove the last console.log and just make a call to the function. — Rodmentou, Nov 04 '15 at 07:19

Alexander O'Mara · Accepted Answer · 2015-11-04T07:28:21.850

3

This is because your listFilesByExtension function does not return anything, so your console.log for this function call will output undefined.

This line is the cause:

console.log(listFilesByExtension(filepath, 'txt'));

If you remove the console.log form this line, you're unexpected undefined will go away.

listFilesByExtension(filepath, 'txt');

edited Nov 04 '15 at 07:28

answered Nov 04 '15 at 07:22

Alexander O'Mara

58,688
18
163
171

score 0 · Answer 2 · edited May 23 '17 at 12:03

The function listFilesByExtension() does return undefined because there isn't any return statement. There is also called asynchronous function so this code will never return what you want.

You should try synchronous function https://nodejs.org/api/fs.html#fs_fs_readdirsync_path

eg:

function listFilesByExtension(path, ext) {
    var filtered,
        dirContent = fs.readdirSync(path);

    // do your filtering by extension here.
    // The filtered will be an Array of paths filtered out from the dirContent
    return filtered;
}

NOTE: there isn't any error handling if the path does not exists

score -1 · Answer 3 · answered Nov 04 '15 at 07:21

-1

If you write in the browser console: console.log('Hello, World!'); it will print

Hello, World!
undefined

That's because console.log(); returns undefined.

answered Nov 04 '15 at 07:21

Marin Takanov

1,079
3
19
36

While all of this is true, they would only see this if they ran this code interactively, in which case they would see `undefined` twice. P.S. Not the downvoter. – Alexander O'Mara Nov 04 '15 at 07:24

Javascript unknown undefined returned

3 Answers3