##### Pitfalls {#javascript-yarn2nix-pitfalls}

- If version is missing from upstream package.json, yarn will silently install nothing. In that case, you will need to override package.json as shown in the [package.json section](#javascript-upstream-package-json)
- Having trouble with `node-gyp`? Try adding these lines to the `yarnPreBuild` steps:

  ```nix
  {
    yarnPreBuild = ''
      mkdir -p $HOME/.node-gyp/${nodejs.version}
      echo 9 > $HOME/.node-gyp/${nodejs.version}/installVersion
      ln -sfv ${nodejs}/include $HOME/.node-gyp/${nodejs.version}
      export npm_config_nodedir=${nodejs}
    '';
  }
  ```

  - The `echo 9` steps comes from this answer: <https://stackoverflow.com/a/49139496>
  - Exporting the headers in `npm_config_nodedir` comes from this issue: <https://github.com/nodejs/node-gyp/issues/1191#issuecomment-301243919>
- `offlineCache` (described [above](#javascript-yarn2nix-preparation)) must be specified to avoid [Import From Derivation](#ssec-import-from-derivation) (IFD) when used inside Nixpkgs.

#### Yarn Berry v3/v4 {#javascript-yarn-v3-v4}
Yarn Berry (v3 / v4) have similar formats, they start with blocks like these:

```yaml
__metadata:
  version: 6
  cacheKey: 8[cX]
```

```yaml
__metadata:
  version: 8
  cacheKey: 10[cX]
```

For these packages, we have some helpers exposed under the respective `yarn-berry_3` and `yarn-berry_4` packages:

- `yarn-berry-fetcher`
- `fetchYarnBerryDeps`
- `yarnBerryConfigHook`

It's recommended to ensure you're explicitly pinning the major version used, for example by capturing the `yarn-berry_Xn` argument and then re-defining it as a `yarn-berry` `let` binding.

```nix
{
  stdenv,
  nodejs,
  yarn-berry_4,
}:

let
  yarn-berry = yarn-berry_4;
in

stdenv.mkDerivation (finalAttrs: {
  pname = "foo";
  version = "0-unstable-1980-01-01";

  src = {
    #...
  };

  nativeBuildInputs = [
    nodejs
    yarn-berry.yarnBerryConfigHook
  ];

  offlineCache = yarn-berry.fetchYarnBerryDeps {
    inherit (finalAttrs) src;
    hash = "...";
  };
})
```

##### `yarn-berry_X.fetchYarnBerryDeps` {#javascript-fetchYarnBerryDeps}
`fetchYarnBerryDeps` runs `yarn-berry-fetcher fetch` in a fixed-output-derivation. It is a custom fetcher designed to reproducibly download all files in the `yarn.lock` file, validating their hashes in the process. For git dependencies, it creates a checkout at `${offlineCache}/checkouts/<40-character-commit-hash>` (relying on the git commit hash to describe the contents of the checkout).

To produce the `hash` argument for `fetchYarnBerryDeps` function call, the `yarn-berry-fetcher prefetch` command can be used:

```console
$ yarn-berry-fetcher prefetch </path/to/yarn.lock> [/path/to/missing-hashes.json]
```

This prints the hash to stdout and can be used in update scripts to recalculate the hash for a new version of `yarn.lock`.