Working with files

Read and write files with the fs module, using async and await.

Lesson 4 of 110% of the track complete

What you'll learn

How to write text to a file and read it back using node:fs/promises.

Why it matters

Reading config, saving logs and processing uploads all mean touching the file system, something browsers are not allowed to do freely.

Explanation

Node's fs module has file functions. The promise version is the modern way, imported from node:fs/promises. It works nicely with async and await:

await writeFile(path, text): create or overwrite a file.
await readFile(path, "utf8"): read it as text.
await appendFile(path, text): add to the end.
await unlink(path): delete a file.

await pauses that function until the operation finishes, without freezing the rest of the program. At the top level of an ES module you can use await directly.

Example

Save as files.mjs:

Step-by-step walkthrough

writeFile creates note.txt containing Buy milk.
appendFile adds a second line.
readFile returns the whole file as a string. Printing it shows both lines.
We trim, split on newlines and count: 2.
unlink deletes the file so the script leaves no mess.

Try it yourself

Run it on your machine, then change the script so that it keeps the file and reads it again on the next run.

files.mjsNode.js

Ctrl+Enter to run

Common mistakes

Forgetting await, so you get a Promise instead of the file text.
Omitting "utf8" and getting a Buffer of bytes.
Reading a file that does not exist, which throws. Wrap in try / catch.
Relative path confusion. Paths are relative to where you ran node, not to the script.

Quick quiz

Question 1 of 3

Which function reads a file as text?

readFile(path, "utf8")openText(path)getFile(path)

Question 2 of 3

What does await do?

Pauses this function until the Promise settlesStops the whole programRuns the code twice

Question 3 of 3

What does writeFile do if the file exists?

Appends to itOverwrites itThrows an error

Summary

Use node:fs/promises with await.
writeFile, readFile, appendFile and unlink cover most needs.
Handle errors with try / catch.

Next: your first web server.

JavaScript

import { writeFile, readFile, appendFile, unlink } from "node:fs/promises";

await writeFile("note.txt", "Buy milk\n");
await appendFile("note.txt", "Call Ravi\n");

const text = await readFile("note.txt", "utf8");
console.log(text);

const lines = text.trim().split("\n");
console.log("Lines:", lines.length);

await unlink("note.txt");