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


### 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.

#### yarn2nix {#javascript-yarn2nix}

WARNING: The `yarn2nix` functions have been deprecated in favor of `yarnConfigHook`, `yarnBuildHook` and `yarnInstallHook` (for Yarn v1) and `yarn-berry_*.*` tooling (Yarn v3 and v4). Documentation for `yarn2nix` functions still appears here for the sake of the packages that still use them. See also a tracking issue [#324246](https://github.com/NixOS/nixpkgs/issues/324246).

##### Preparation {#javascript-yarn2nix-preparation}

You will need at least a `yarn.lock` file. If upstream does not have one you need to generate it and reference it in your package definition.

If the downloaded files contain the `package.json` and `yarn.lock` files they can be used like this:

```nix
{
  offlineCache = fetchYarnDeps {
    yarnLock = src + "/yarn.lock";
    hash = "....";
  };
}
```

##### mkYarnPackage {#javascript-yarn2nix-mkYarnPackage}

`mkYarnPackage` will by default try to generate a binary. For package only generating static assets (Svelte, Vue, React, WebPack, ...), you will need to explicitly override the build step with your instructions.