Automation for the people

If you're working in the JavaScript ecosystem, your project is likely based around a package.json file. This is used by NPM for defining the attributes of your package, including its "name" and "version". You may have noticed that one of the other options in the package file is "scripts". By default, this contains a single task:

{
    "...": "...",
    "scripts": {
        "test": "echo \"Error: no test specified\" && exit 1"
    }
}

This is an inauspicious start to what I think is a pretty powerful entry point to high-level developer automation.

Why automate?

It's hopefully not controversial to state that developer time is expensive and you only want to spend that expensive, finite resource on things that are valuable to the business. Automation can minimise the time spent on repeating simple tasks that happen many times a day, like linting or testing the code.

Package scripts provide a way to do this within the tooling your developers are likely already using on a daily basis. They don't need to learn a new tool and all of the existing NPM packages are available to help them out. It's also easy to expand when you need to, because they can start writing more complex processes as .js Node scripts or .sh shell scripts and still invoke them via npm run.

Using the package scripts as an entry point also allows you to keep a consistent "developer API" across multiple projects. For example, when I switched fauxauth over to TypeScript, using package scripts for all common tasks (running e.g. npm run lint rather than calling eslint directly) meant that I could change the meaning of the scripts in the package file without having to change the commands I was executing (or update the CI configuration). This could also be beneficial if your developers work across multiple technologies; npm run lint could call eslint for React in some projects and the Angular CLI's ng lint in others, but the developer experience would be consistent.

Sidebar: linting

One of the least valuable things for your developers to spend time on is what the code looks like, particularly in the JavaScript ecosystem where you rarely ship the code you're writing without some kind of transpilation or bundling. You also don't want them to spend time trading commits back and forth switching ' for " and vice versa; the smaller the diffs, the easier it is to figure out what's meaningfully changed when reviewing a commit, the better (for the same reason I think trailing commas are a good thing!)

In general, the less they have to think about, the better; although the ASI rules are fairly straightforward, for example, using semicolons everywhere means they never have to consider it. Modern tools can generally do this for them; individuals can write the code how they want to, then auto-fix on type, save or lint to the agreed shared rules. The team can now focus more of its time on delivering the things that matter to your users.

I also don't think you should have any warnings when you run linters; things you actually care about get lost in the noise. Every rule should either be an error or ignored completely, so it's unambiguous what's important, and any error should fail the build.

Hints and tips

  • "task:step": this is a very common convention for writing scripts, using a colon in the script name to indicate some kind of sub-step or configuration option. For example:

    {
        "...": "...",
        "scripts": {
            "test": "...",
            "test:cover": "npm run test -- --coverage"
        }
    }
    

    or

    {
        "...": "...",
        "scripts": {
            "start": "npm run start:compile && npm run start:watch",
            "start:compile": "...",
            "start:watch": "..."
        }
    }
    
  • pre and post: all package scripts get a "free" pre- and post- script hook. All you need to do is include another entry with the same name prefixed with pre or post and this script will get run at the appropriate point in the lifecycle, assuming everything so far has exited zero.

    {
        "...": "...",
        "scripts": {
            "test": "...",
            "posttest": "./collect-coverage.sh"
        }
    }
    
  • -- <args>: any arguments to npm run thing are passed to npm run, not thing. To pass arguments to thing you need to include --, to indicate the end of the arguments to npm run. For example, to pass the argument --port=3000 to the serve script, you'd do:

    $ npm run serve -- --port=3000
    

Helpful libraries

Here are a few great "glue" libraries I've used frequently in other projects:

  • concurrently: run multiple processes at once. Very helpful for a local dev setup with watching processes on both the server and client builds, for example.

  • cross-env: set environment variables in your package scripts, cross-platform. If you're externalising configuration to env vars and want your project to run correctly on both Windows and *nix, this is a must.

    cross-env was also the target of the first package discovered in the hacktask account, a set of malicious typo-squatting packages that sent the user's environment variables to a remote server. Be careful to double-check that you're installing what you think you are.

  • husky: turn package scripts into git hooks. Never again will you accidentally break the build by pushing code that doesn't pass the tests!

  • rimraf: rm -rf for Node. Handy for clearing out output directories to ensure a clean state before running a build.

  • wait-on: wait for things to be ready. I've used this to run E2E tests against a local dev server, making sure the server is actually spun up and providing responses before kicking off the test suite.

Downsides

There are a couple of downsides I've noticed to doing automation predominantly through the package file.

One is simply length; as you add more complexity to your automation, either you have very long lines, or lots of sub-steps, or sometimes both. For example, this project that I worked on has 43 scripts, steps and pre-/post- hooks for performing various tasks. This led to having a separate wiki page listing what they are and what they do, as putting comments in a JSON file is not straightforward (see "How do I add comments to package.json for npm install?" for one option). Now there is a risk that, as the scripts are updated and added to, the wiki page does not stay up-to-date with what they currently do. You could move the steps out to shell or Node scripts, or use an additional tool like nps, but that's yet another thing to think about.

Another is the difficulty of handling arguments when combining scripts. As mentioned above you can split a complex process into multiple steps and call each of them in turn:

{
    "...": "...",
    "scripts": {
        "process:first": "...",
        "process:second": "...",
        "process": "npm run process:first && npm run process:second"
    }
}

But what if you want to pass an argument down to one of the sub-steps? npm run process -- --something won't do it, so you often end up writing several scripts to call the same processes with different arguments.

Bonus round: Yarn

The other major packaging option in the Node ecosystem is Yarn. This uses the same package.json file as NPM, so switching over is relatively straightforward. I won't go into the pros and cons here, but one thing to note is that running scripts and passing arguments is a little simpler in Yarn than in NPM; to run a script and pass arguments, instead of:

$ npm run thing -- --arg

you can just do:

$ yarn thing --arg

Further reading

Here are some useful articles and references:

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