Automatically 'npm install'

I implemented this at work recently and although it felt like a hack, I've come to like it and it's been very helpful to our many contributors.
As (Node) engineers, we know that you should keep your node_modules up-to-date by running npm install periodically or every time you git pull from the upstream. It could be that some package got upgraded last night since you git pulled last time.
But not everyone remembers to run npm install often enough. They might do git pull origin main && npm start and now the code that starts up depends on some latest version that was upgraded in package.json and package-lock.json.

How we solved it was that we added this script:

node script/cmp-files.js package-lock.json .installed.package-lock.json || npm install && cp package-lock.json .installed.package-lock.json

And it's hooked up as a script in package.json called prestart:

"scripts": {
  ...
  "prestart": "node script/cmp-files.js ...",
  ...
}

Now, every time you run npm start to start up the local development server, it will run that piece of bash. No more having to remember to run npm install after every git pull.

A note on performance

The npm install command is fast when all packages are already updated. You can see it with:

# First time
$ npm install

# Second time when nothing should happen
$ time npm install
...
2.53s user 0.37s system 134% cpu 2.166 total

So it only takes 2 seconds. Not bad.

$ time node script/cmp-files.js package-lock.json .installed.package-lock.json
...
0.08s user 0.03s system 100% cpu 0.110 total

But 0.08 seconds is better :)

The comparison script

The cmp-files.js script looks like this:

#!/usr/bin/env node

// Given N files. Exit 0 if they all exist and are identical in content.

import fs from 'fs'

import { program } from 'commander'

program.description('Compare N files').arguments('[files...]', '').parse(process.argv)

main(program.args)

function main(files) {
  if (files.length < 2) throw new Error('Must be at least 2 files')
  try {
    const contents = files.map((file) => fs.readFileSync(file, 'utf-8'))
    if (new Set(contents).size > 1) {
      process.exit(1)
    }
  } catch (error) {
    if (error.code === 'ENOENT') {
      process.exit(1)
    } else {
      throw error
    }
  }
}

The file .installed.package-lock.json file is added to the repo's .gitignore

Note; given how well this works for running before npm start we can probably add this to a post-checkout git hook too.