Bisexual Pride

Last week, we learned how to asynchronously check if a file exists in Node.js. I mentioned that you shouldn’t use this technique before reading/writing files, and that I’d show you what to do instead. That’s what we’re gonna do today!

Reading files

The fs.readFile() method lets you asynchronously read the entire contents of a file. It accepts up to three arguments:

  1. The path to the file.
  2. An object literal of options or a string to specify the encoding.
  3. A callback function with error and data parameters.

In the following examples, I’m using the string 'utf8' as the encoding, which saves me from having to parse the data as a Buffer.

Callback API

import { readFile } from 'fs';

readFile('./package.json', 'utf8', (error, data) => {
if (error) {
throw error;
} else {
console.log(data);
}
});

Promises API

The fsPromises.readFile() method returns a promise that fulfills with the contents of the file.

import { readFile } from 'fs/promises';

readFile('./package.json', 'utf8')
.then(console.log)
.catch(console.error);

Writing files

The fs.writeFile() method lets you asynchronously write data to a file, replacing the file if it already exists. It accepts up to four arguments:

  1. The path to the file.
  2. The data to write to the file.
  3. An object literal of options or a string to specify the encoding.
  4. A callback function with an error parameter.

In the following examples, I’m again using the string 'utf8' as the encoding.

Callback API

import { writeFile } from 'fs';

const file = 'hello_world.txt';
const data = 'Hello, World!';

writeFile(file, data, 'utf8', error => {
if (error) {
throw error;
} else {
console.log(`Successfully wrote '${data}' to ${file}.`);
}
});

Promises API

The fsPromises.writeFile() method returns a promise that fulfills with undefined upon success. This means we don’t need to specify a parameter for our callback to the then() method.

import { writeFile } from 'fs/promises';

const file = 'hello_world.txt';
const data = 'Hello, World!';

writeFile(file, data, 'utf8')
.then(() => console.log(`Successfully wrote '${data}' to ${file}.`))
.catch(console.error);

What if I don’t want to overwrite the file?

The writeFile() method overwrites the file if it already exists. Next week, we’ll learn how to append data to a file using the appendFile() method.

All the code from this post is available as a Gist.