Programming

Promise-based filesystem APIs in Node 10

Your async life just got a little bit easier...
April 30, 2018
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User avatar
Adrian Perez
@blackxored

Almost one year ago, Node 8 was released. It was a major release and brought quite a bit of ESnext to the masses. Even then, back in June I recognized that one of the major pitfalls when working with Node are the callback-based APIs that (IMHO) pollute your code.

Somewhat hidden in that release, was util.promisify, a native way of converting an errback (or Node-style callback) into a Promise.

A Promise-based fs

As Node 10 just landed a few days ago, I was excited to see the most common case for such an util (dealing with filesystem APIs) was being addressed in some fashion.

While marked as experimental, it means that we can now use promises (and as you might have guessed async/await) when dealing with the filesystem.

Unfortunately, this hasn't landed in the original module, which means you need to change your imports to fs/promises. You will also get this warning:

(node:95830) ExperimentalWarning: The fs/promises API is experimental

A Simple Example

Here's an example, leveraging latest JavaScript (sans modules):

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

const readConfig = async (configFile) => {
  try {
      const config = await fs.readFile(configFile, 'utf-8');
      // do something with file contents
  } catch (e) {
    throw new Error(`Failed to load config file at ${configFile}`);
  }
} 

Supported APIs

You can access the documentation for the fs Promises API on this page, but if you're eager to see what's supported here's a list for your convenience (sort-of sorted by popularity):

  • readFile
  • writeFile
  • write
  • copyFile
  • open
  • read
  • rename
  • mkdir
  • rmdir
  • fstat
  • readlink
  • unlink
  • mkdtemp
  • access
  • truncate
  • ftruncate
  • fdatasync
  • fsync
  • readdir
  • symlink
  • lstat
  • stat
  • link
  • fchmod
  • chmod
  • lchmod
  • lchown
  • fchown
  • utimes
  • futimes
  • realpath
  • appendFile

Final Words

So, it took us quite a while, but finally we got Promises somewhere it matters. I can only hope that this extends more and more into Node core, and we might have a shot against callback hell. It only took 10 major versions, but I'm hopeful.

What's that? What about observables, you say? Well...

~ EOF ~

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