projectsgallery

Optimizing JS for Nix

In the Before Times, I would just throw my code somewhere on a server, edit some config in the index.js, add a systemd unit (or start screen if I was really in a hurry) and be done with it.
But everything changed when the fire nation attacked I started switching everything to NixOS. The old approach still 'worked', but Nix had so much more to offer.

Along the way I've accrued a set of changes to my software that makes the whole packaging/running easier.

Packaging

I pass my name, version and source attributes to a simple function that configures mkYarnPackage:

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# package.nix
common.node-application rec {
    pname = "woei";
    version = "v0.0.1";

    source = fetchgit {
        url = "https://git.pixie.town/f0x/${pname}";
        rev = "0fe28e90919929a88b2a35ed04aff00c395222fa";
        # sha256 = lib.fakeSha256;
        sha256 = "sha256-ohpjQIK9ZaZNrizacckTEN9CEdIIgOoB0pS1ijP6ILE=";
    };
}

# node-application.nix
{ pname, version, source }: mkYarnPackage {
    inherit pname;
    inherit version;
    src = source;
    packageJSON = "${source}/package.json";
    yarnLock = "${source}/yarn.lock";
}

This will take care of fetching the dependencies, but the files are kinda hidden in $out/libexec/$package/deps/$package. You have to use that complex path and pass it to pkgs.nodejs, not ideal.

It gets a lot easier with some small changes to the codebase:
add to package.json:

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  "bin": {
    "woei": "./index.js"
  },

index.js first line:

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#!/usr/bin/env node

chmod +x index.js
Nix will then automatically convert that shebang into whatever store path node is in, and now you can just run $out/bin/woei.

Configuration

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const config = {
    ...
};

Just hardcoding the config might be easy at first, but makes it real hard to configure from Nix, and tends to cause git merge conflicts. Multiple approaches:

Yargs

Throw in a command-line argument parser, in my case yargs, and use that to pass some config variables (as set in the Nix'd systemd unit ExecStart=).

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const yargs = require("yargs");

const args = yargs
    .option("port", {
        description: "listening port",
        default: 8080
    })
    .option("host", {
        description: "listening host",
        default: "localhost"
    })
    .help()
    .alias('help', 'h')
    .argv;

This works best when there's only a few options, and yargs does a decent job of parsing + doing stuff like generating a help overview. When there's too many options though that gets bothersome, and it's a lot nicer to have Nix generate the config as JSON, and let the software read from there instead.

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const yargs = require("yargs");
const path = require("path");

const args = yargs
    .option("config", {
        description: "config file (json)",
        default: __dirname + "/config.json"
    })
    .help()
    .alias('help', 'h')
    .argv;

const config = require(path.resolve(__dirname, args.config)); // automatically parses JSON

process.argv

Less robust, but maybe a little simpler:

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#!/usr/bin/env node

const configFile = process.argv[2]; // node file.js argument1
const config = require(path.resolve(__dirname, configFile));