# ...
  };
}
```

In this example, `prePnpmInstall` will be run by both `pnpm.configHook` and by the `pnpm.fetchDeps` builder.

#### PNPM `fetcherVersion` {#javascript-pnpm-fetcherVersion}

This is the version of the output of `pnpm.fetchDeps`, if you haven't set it already, you can use `1` with your current hash:

```nix
{
  # ...
  pnpmDeps = pnpm.fetchDeps {
    # ...
    fetcherVersion = 1;
    hash = "..."; # you can use your already set hash here
  };
}
```

After upgrading to a newer `fetcherVersion`, you need to regenerate the hash:

```nix
{
  # ...
  pnpmDeps = pnpm.fetchDeps {
    # ...
    fetcherVersion = 2;
    hash = "..."; # clear this hash and generate a new one
  };
}
```

This variable ensures that we can make changes to the output of `pnpm.fetchDeps` without breaking existing hashes.
Changes can include workarounds or bug fixes to existing PNPM issues.

##### Version history {#javascript-pnpm-fetcherVersion-versionHistory}

- 1: Initial version, nothing special
- 2: [Ensure consistent permissions](https://github.com/NixOS/nixpkgs/pull/422975)

### Yarn {#javascript-yarn}

Yarn based projects use a `yarn.lock` file instead of a `package-lock.json` to pin dependencies.

To package yarn-based applications, you need to distinguish by the version pointers in the `yarn.lock` file. See the following sections.

#### Yarn v1 {#javascript-yarn-v1}

Yarn v1 lockfiles contain a comment `# yarn lockfile v1` at the beginning of the file.

Nixpkgs provides the Nix function `fetchYarnDeps` which fetches an offline cache suitable for running `yarn install` before building the project. In addition, Nixpkgs provides the hooks:

- `yarnConfigHook`: Fetches the dependencies from the offline cache and installs them into `node_modules`.
- `yarnBuildHook`: Runs `yarn build` or a specified `yarn` command that builds the project.
- `yarnInstallHook`: Runs `yarn install --production` to prune dependencies and installs the project into `$out`.

An example usage of the above attributes is:

```nix
{
  lib,
  stdenv,
  fetchFromGitHub,
  fetchYarnDeps,
  yarnConfigHook,
  yarnBuildHook,
  yarnInstallHook,
  nodejs,
}:

stdenv.mkDerivation (finalAttrs: {
  pname = "...";
  version = "...";

  src = fetchFromGitHub {
    owner = "...";
    repo = "...";
    rev = "v${finalAttrs.version}";
    hash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
  };

  yarnOfflineCache = fetchYarnDeps {
    yarnLock = finalAttrs.src + "/yarn.lock";
    hash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
  };

  nativeBuildInputs = [
    yarnConfigHook
    yarnBuildHook
    yarnInstallHook
    # Needed for executing package.json scripts
    nodejs
  ];

  meta = {
    # ...
  };
})
```

##### `yarnConfigHook` arguments {#javascript-yarnconfighook}

By default, `yarnConfigHook` relies upon the attribute `${yarnOfflineCache}` (or `${offlineCache}` if the former is not set) to find the location of the offline cache produced by `fetchYarnDeps`. To disable this phase, you can set `dontYarnInstallDeps = true` or override the `configurePhase`.

##### `yarnBuildHook` arguments {#javascript-yarnbuildhook}

This script by default runs `yarn --offline build`, and it relies upon the project's dependencies installed at `node_modules`. Below is a list of additional `mkDerivation` arguments read by this hook:

- `yarnBuildScript`: Sets a different `yarn --offline` subcommand (defaults to `build`).
- `yarnBuildFlags`: Single string list of additional flags to pass the above command, or a Nix list of such additional flags.

##### `yarnInstallHook` arguments {#javascript-yarninstallhook}

To install the package `yarnInstallHook` uses both `npm` and `yarn` to cleanup project files and dependencies. To disable this phase, you can set `dontYarnInstall = true` or override the `installPhase`. Below is a list of additional `mkDerivation` arguments read by this hook:

- `yarnKeepDevDeps`: Disables the removal of devDependencies from `node_modules` before installation.