How to readFile() in NodeJS?

Question

I'm just studying Node.JS, and i have a question over it.

I have to read a file. I do it with 'fs' node and 'fs.readFile()' func. Here is a code

const fs = require('fs');
let input;
fs.readFile('./'+config.file, (err, data) => { 
    input = data.toString('utf-8');
});

console.log(input) //!!! it gives me 'undefined' result but i expect file's data

I haven't any ideas how solve this problem. Only i know that this is the async func.

Hope, you will help me :3

@RolandStarke you don't know if there is such an error or not because it is broken asynchronous code. — Aluan Haddad, Feb 18 '18 at 09:29

score 1 · Accepted Answer · answered Feb 18 '18 at 09:34

1

Since your code is async, your variable will be logged before the reading of the file is complete.
You can use a function and return a Promise:

const fs = require('fs');

function myReadFile() {
    return new Promise((resolve, reject) => {
        fs.readFile('myfile.txt', (err, data) => {
            if (err) reject(err);
            resolve(data.toString());
        });
    })
}

myReadFile().then((contents) => {
    console.log(contents)
});

answered Feb 18 '18 at 09:34

Evya

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Thx a lot, buddy) – Vladimir Feb 18 '18 at 11:19
But id doesn't resolves my problem yet. I just want to use 'input' variable outside* this stuff(include .then() like sync code – Vladimir Feb 18 '18 at 11:30

score 0 · Answer 2 · answered Feb 18 '18 at 09:30

It is asynchronous so it will first execute the synchronous code that's why input is still undefined. Synchronous code is executed first then async code. Because fs.readFile is async it will get executed after the console.log() in the event loop.

const fs = require('fs');
let input;
fs.readFile('./'+config.file, (err, data) => { 
    input = data.toString('utf-8');
    console.log(input)
});

This should log your input

More on asynchronous code in this article

How to readFile() in NodeJS?

2 Answers2