returning undefined from readFile

Question

I'm trying to get the following code to work:

'use strict';

var fs = require('fs');

var fileName = 'readme.txt';
var str = fs.readFile(fileName, 'utf8', function (err, data) {
    if (err) {
        console.log(err);
        throw err;
    }
    return data;
});

console.log('result read: ' + str);

readme.txt:

console.log('working');

I'm trying to display the following:

result read: console.log('working');

This one of the most commonly asked things about asynchronous operations in Javascript. See the question this is marked a duplicate for a discussion of the options. You can't return a result directly from an async operation. In Javascript, you must use callbacks. — jfriend00, Jan 24 '16 at 20:00
@jfriend00 So I myself cannot turn a sync function into a async function (via js), is this somehow programmed on a lower level? — basickarl, Jan 24 '16 at 20:03
You can't turn an asynchronous function into a synchronous function or vice versa. They are what they are. In the case of the `fs` module, this is programmed in native code in the `fs` library. An async function will be non-blocking and the result will ONLY be available sometime later via a callback. A synchronous function will be blocking and the result be directly available when it finishes blocking. You can sort of make a synchronous function seem like it might be asynchronous by just sending the return result via a callback, but it will still be blocking while performing its operation. — jfriend00, Jan 24 '16 at 20:25
@jfriend00 You mention `native code`, can you be more specific (I will be checking it :))? — basickarl, Jan 24 '16 at 20:28
Here's the [node.js code on GitHub](https://github.com/nodejs/node) and [the native code for the `fs` module](https://github.com/nodejs/node/blob/master/src/node_file.cc) and here's [the JS interface](https://github.com/nodejs/node/blob/master/lib/fs.js) above it. — jfriend00, Jan 24 '16 at 20:31

score 22 · Accepted Answer · edited Sep 03 '18 at 02:03

22

readFile is executed asynchronously, so data can only be accessed inside the callback function, if you want it to be synchronous, you should use readFileSync

Async:

'use strict';

const fs = require('fs');

const fileName = 'readme.txt';
fs.readFile(fileName, 'utf8', function (err, data) {
   if (err)
      return console.log(err);
   console.log('result read: ' + data);
});

Sync:

var str = fs.readFileSync(fileName, 'utf8'); // This will block the event loop, not recommended for non-cli programs.
console.log('result read: ' + str);

UPDATE

You can use util.promisify to convert fs.readFile from callback API to promise API.

const fs = require('fs');
const { promisify } = require('util');
const readFile = promisify(fs.readFile);

(async() =>  {
   try {
      const result = await readFile('readme.txt', 'utf8');
      consle.log(result);
   } catch(e) {
      console.error(e);
   }
})();

In Node 10 you can use fs/promises and avoid util.promisify

const fs = require('fs').promises;

(async() =>  {
   try {
      const result = await fs.readFile('readme.txt', 'utf8');
      consle.log(result);
   } catch(e) {
      console.error(e);
   }
})();

edited Sep 03 '18 at 02:03

Boogie

750
7
17

answered Jan 24 '16 at 19:21

Marcos Casagrande

37,983
8
84
98

The "str" *must* be outside of the function. – basickarl Jan 24 '16 at 19:22
Use the syncronously approach then. – Marcos Casagrande Jan 24 '16 at 19:22
1

I was able to bake a Promise in the final product :) – basickarl Jan 24 '16 at 20:02
Well of course that's another solution ;), remember that node file system functions are async by default, and they do have they sync counterpart ;) – Marcos Casagrande Jan 24 '16 at 20:04
I'm learning as I go, I think I need to read up on how async functions are written in frameworks to fully understand this^^ I always thought if I bake a sync function in a promise that what an async function is, guess I'm wrong^^ – basickarl Jan 24 '16 at 20:05

score 3 · Answer 2 · answered Jan 24 '16 at 19:21

3

You're calling an asynchronous function. This means it does not wait to return when called and will, in this case, return undefined.

You can either use the readFileSync method to call it synchronously, or put your console.log in the callback

answered Jan 24 '16 at 19:21

Joseph Young

2,758
12
23

score 0 · Answer 3 · answered Jan 24 '16 at 19:21

0

readFile is an async method (thus the callback.) You need the console.log (or anything you want to do with the result) in the callback for async functions. This is pretty much the backbone of node.js - by allowing async methods to be put onto the event loop, it keeps your code from being blocked.

answered Jan 24 '16 at 19:21

Ahh thank you. I'll wrap it in a promise then with the co library. – basickarl Jan 24 '16 at 19:22

returning undefined from readFile

3 Answers3

Linked