Reading files with Node.js

The simplest way to read a file in Node.js is to use the fs.readFile() method, passing it the file path, encoding and a callback function that will be called with the file data (and the error):

const fs = require('node:fs');

fs.readFile('/Users/joe/test.txt', 'utf8', (err, data) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(data);
});

import fs from 'node:fs';

fs.readFile('/Users/joe/test.txt', 'utf8', (err, data) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(data);
});

Alternatively, you can use the synchronous version fs.readFileSync():

const fs = require('node:fs');

try {
  const data = fs.readFileSync('/Users/joe/test.txt', 'utf8');
  console.log(data);
} catch (err) {
  console.error(err);
}

import fs from 'node:fs';

try {
  const data = fs.readFileSync('/Users/joe/test.txt', 'utf8');
  console.log(data);
} catch (err) {
  console.error(err);
}

You can also use the promise-based fsPromises.readFile() method offered by the fs/promises module:

const fs = require('node:fs/promises');

async function example() {
  try {
    const data = await fs.readFile('/Users/joe/test.txt', { encoding: 'utf8' });
    console.log(data);
  } catch (err) {
    console.log(err);
  }
}
example();

import fs from 'node:fs/promises';

try {
  const data = await fs.readFile('/Users/joe/test.txt', { encoding: 'utf8' });
  console.log(data);
} catch (err) {
  console.log(err);
}

All three of fs.readFile(), fs.readFileSync() and fsPromises.readFile() read the full content of the file in memory before returning the data.

This means that big files are going to have a major impact on your memory consumption and speed of execution of the program.

In this case, a better option is to read the file content using streams.