Files and Async Code

Introduction

Reading a file or calling a network takes time compared with running code. If a program stopped and waited every time, it would be slow. Node is built around asynchronous code: start a job, carry on, and handle the result when it is ready.

Reading and writing files

The fs/promises module returns promises, which work with async and await:

"hello\nworld\n"
2

• writeFile creates or replaces a file. Be careful not to overwrite something you need.
• readFile with "utf8" gives you text. Without it you get raw bytes.
• await pauses this function until the promise finishes, without blocking the rest of the program.
• Top-level await works in ES modules.

Other useful functions include appendFile to add to the end, readdir to list a folder, mkdir to make folders and rm to remove files. Use rm with great care.

Building safe file paths

Use the path module instead of gluing strings together:

On Linux and macOS, path.join uses /. On Windows it uses \. It also handles extra slashes for you. Never build a path directly from user input without checking it, because someone could use ../ to reach other folders.

Handling errors

Files can be missing, locked or unreadable. Use try and catch:

ENOENT means "no such file or directory". Check the code for the cases you expect, and rethrow others so real problems are not hidden.

Why async?

Imagine a restaurant with one waiter. If the waiter stood at the kitchen door until each dish was ready, nobody else would be served. Instead the waiter takes an order, moves on, and returns when the kitchen rings. Node works like that waiter, which is why one Node process can handle thousands of connections.

Three styles exist, all doing the same job:

• Callbacks: the original style, which can become deeply nested
• Promises: objects representing a future result, with .then()
• async and await: promises written like normal code, which is the style to prefer

Doing several things at once

Start independent jobs together and wait for all of them:

If one fails, Promise.all rejects. Use Promise.allSettled when you want every result, success or failure.

Try it yourself

Write a script that reads a text file given on the command line, counts its lines and words, prints the results, and prints a friendly message if the file does not exist.

Summary

• fs/promises reads and writes files, and path builds file paths safely.
• Asynchronous code lets Node keep working while it waits, and async with await makes it readable.
• Use try and catch, check error codes such as ENOENT, and use Promise.all for independent jobs.