# Npm on Nix.

This is an example of how to install and debug an npm package on nix.
We'll install `serve` from `npm`. This package was chosen because it currently
breaks, so we'll have figure out how to debug and correct it.

In general, JS ecosystem leaves more to be desired when it comes to deterministic
builds. `npm` moves in the direction of handling packages in a more and more
imperative way, which makes it harder to achieve reliable and reproduciable packaging.
[Even Facebook wrote about it](https://code.facebook.com/posts/1840075619545360).

There are still [discussions](http://lists.science.uu.nl/pipermail/nix-dev/2016-June/020837.html) about how to approach JS packages in Nix. Tools to handle JS packages are
being actively developed and reengineered (e.g., `npm2nix` is being replaced by
`node2nix`). This is one of the reasons why you won't find most JS libraries
in Nixpkgs.

However, you can easily install them yourself and not wait until the moment
they might land in Nixpkgs.


## Packaging `serve` for Nix

First, we want to specify packages that we want available in our environment.
We'll put them in `node-packages.json`. It will be later parsed by `node2nix`
that is going to generate a nix expression containing those packages. It
will not install ALL those packages at the same time; instead, you'll be able
to choose which ones you want to be installed.

```
>>> cat node-packages.json
[
  "serve"
]
```

Then, we need to generate the nix expression for those packages:

`node2nix -i node-packages.json`

This will generate a bunch of `.nix` files. Specific information about each package
and its dependencies will be stored in `node-packages.nix`. But, as always, by convention
`default.nix` is the "entry point" file that we'll pass to `nix-env`:

`nix-env -f default.nix -iA serve`

The package was installed successfully, but it will break at runtime when we try to run it:

```
/nix/store/clq0pzzjvp9blr2vknj1jy8hk4nn940c-node-serve-2.0.0/lib/node_modules/serve/dist/bin/serve:163
      const directoryPath = [...pathParts];
                             ^^^

SyntaxError: Unexpected token ...
```

Okay, this looks like it tries to use the spread syntax that was introduced in ES6. So, it probably uses an old version of NodeJS.

On [`node2nix`'s Github page](https://github.com/svanderburg/node2nix/blob/14819200b2de849850ccb98c5b03bf48634421b4/README.md#generating-packages-for-nodejs-5x)  we can find that passing `-5` arg will make it use NodeJS 5:

```
>>> node2nix -5 -i node-packages.json
>>> nix-env -f default.nix -iA serve
installing ‘node-serve-2.0.0’
error: attribute ‘nodejs-5_x’ missing, at /home/cyr/packages/serve/default.nix:5:48
```

Hm, it can't find NodeJS 5. We can look inside `default.nix` and check where it looks for it. On line 5
we can see that it tries to find it in `pkgs."nodejs-5_x" `

```
  }, system ? builtins.currentSystem, nodejs ? pkgs."nodejs-5_x"}:
```

Let's jump into `nix-repl` and figure out what's going on:

```
>>> nix-repl "<nixpkgs>"
Welcome to Nix version 1.11.4. Type :? for help.

Loading ‘<nixpkgs>’...
Added 6642 variables.

nix-repl> pkgs.nodejs<TAB>
pkgs.nodejs      pkgs.nodejs-4_x  pkgs.nodejs-6_x
nix-repl> pkgs.nodejs.version
"4.6.0"
```

Tab completion tells us that there is only Node.js 4 and 6.
There is also `nodejs`, but checking its version tells us that
it points to `nodejs-4_x`.

We can either add Node.js 5 or try the 6th one, which already
exists in nixpkgs. The latter approach will be simpler,
so this is what we are going to do by opening `default.nix`
and changing `nodejs-5_x` to `nodejs-6_x`. This gives us the
following:

```
>>> cat default.nix | head -5
# This file has been generated by node2nix 1.1.0. Do not edit!

{pkgs ? import <nixpkgs> {
    inherit system;
  }, system ? builtins.currentSystem, nodejs ? pkgs."nodejs-6_x"}:
```

Let's try to install it again:

```
>>> nix-env -f default.nix -iA serve
```

This time it installs successfully. Let's try running it:

```
>>> serve                                                                              ⏎
Running on http://localhost:3000

```