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+# Bower {#sec-bower}
+
+[Bower](https://bower.io) is a package manager for web site front-end components. Bower packages (comprising of build artefacts and sometimes sources) are stored in `git` repositories, typically on Github. The package registry is run by the Bower team with package metadata coming from the `bower.json` file within each package.
+
+The end result of running Bower is a `bower_components` directory which can be included in the web app's build process.
+
+Bower can be run interactively, by installing `nodePackages.bower`. More interestingly, the Bower components can be declared in a Nix derivation, with the help of `nodePackages.bower2nix`.
+
+## bower2nix usage {#ssec-bower2nix-usage}
+
+Suppose you have a `bower.json` with the following contents:
+
+### Example bower.json {#ex-bowerJson}
+
+```json
+  "name": "my-web-app",
+  "dependencies": {
+    "angular": "~1.5.0",
+    "bootstrap": "~3.3.6"
+  }
+```
+
+Running `bower2nix` will produce something like the following output:
+
+```nix
+{ fetchbower, buildEnv }:
+buildEnv { name = "bower-env"; ignoreCollisions = true; paths = [
+  (fetchbower "angular" "1.5.3" "~1.5.0" "1749xb0firxdra4rzadm4q9x90v6pzkbd7xmcyjk6qfza09ykk9y")
+  (fetchbower "bootstrap" "3.3.6" "~3.3.6" "1vvqlpbfcy0k5pncfjaiskj3y6scwifxygfqnw393sjfxiviwmbv")
+  (fetchbower "jquery" "2.2.2" "1.9.1 - 2" "10sp5h98sqwk90y4k6hbdviwqzvzwqf47r3r51pakch5ii2y7js1")
+];
+```
+
+Using the `bower2nix` command line arguments, the output can be redirected to a file. A name like `bower-packages.nix` would be fine.
+
+The resulting derivation is a union of all the downloaded Bower packages (and their dependencies). To use it, they still need to be linked together by Bower, which is where `buildBowerComponents` is useful.
+
+## buildBowerComponents function {#ssec-build-bower-components}
+
+The function is implemented in [pkgs/development/bower-modules/generic/default.nix](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/bower-modules/generic/default.nix).
+
+### Example buildBowerComponents {#ex-buildBowerComponents}
+
+```{=docbook}
+<programlisting language="nix">
+bowerComponents = buildBowerComponents {
+  name = "my-web-app";
+  generated = ./bower-packages.nix; <co xml:id="ex-buildBowerComponents-1" />
+  src = myWebApp; <co xml:id="ex-buildBowerComponents-2" />
+};
+</programlisting>
+```
+
+In ["buildBowerComponents" example](#ex-buildBowerComponents) the following arguments are of special significance to the function:
+
+```{=docbook}
+<calloutlist>
+  <callout arearefs="ex-buildBowerComponents-1">
+    <para>
+      <varname>generated</varname> specifies the file which was created by <command>bower2nix</command>.
+    </para>
+    </callout>
+      <callout arearefs="ex-buildBowerComponents-2">
+    <para>
+      <varname>src</varname> is your project's sources. It needs to contain a <filename>bower.json</filename> file.
+    </para>
+  </callout>
+</calloutlist>
+```
+
+`buildBowerComponents` will run Bower to link together the output of `bower2nix`, resulting in a `bower_components` directory which can be used.
+
+Here is an example of a web frontend build process using `gulp`. You might use `grunt`, or anything else.
+
+### Example build script (gulpfile.js) {#ex-bowerGulpFile}
+
+```javascript
+var gulp = require('gulp');
+
+gulp.task('default', [], function () {
+  gulp.start('build');
+});
+
+gulp.task('build', [], function () {
+  console.log("Just a dummy gulp build");
+  gulp
+    .src(["./bower_components/**/*"])
+    .pipe(gulp.dest("./gulpdist/"));
+});
+```
+
+### Example Full example — default.nix {#ex-buildBowerComponentsDefaultNix}
+
+```{=docbook}
+<programlisting language="nix">
+{ myWebApp ? { outPath = ./.; name = "myWebApp"; }
+, pkgs ? import &lt;nixpkgs&gt; {}
+}:
+
+pkgs.stdenv.mkDerivation {
+  name = "my-web-app-frontend";
+  src = myWebApp;
+
+  buildInputs = [ pkgs.nodePackages.gulp ];
+
+  bowerComponents = pkgs.buildBowerComponents { <co xml:id="ex-buildBowerComponentsDefault-1" />
+    name = "my-web-app";
+    generated = ./bower-packages.nix;
+    src = myWebApp;
+  };
+
+  buildPhase = ''
+    cp --reflink=auto --no-preserve=mode -R $bowerComponents/bower_components . <co xml:id="ex-buildBowerComponentsDefault-2" />
+    export HOME=$PWD <co xml:id="ex-buildBowerComponentsDefault-3" />
+    ${pkgs.nodePackages.gulp}/bin/gulp build <co xml:id="ex-buildBowerComponentsDefault-4" />
+  '';
+
+  installPhase = "mv gulpdist $out";
+}
+</programlisting>
+```
+
+A few notes about [Full example — `default.nix`](#ex-buildBowerComponentsDefaultNix):
+
+```{=docbook}
+<calloutlist>
+  <callout arearefs="ex-buildBowerComponentsDefault-1">
+    <para>
+      The result of <varname>buildBowerComponents</varname> is an input to the frontend build.
+    </para>
+  </callout>
+  <callout arearefs="ex-buildBowerComponentsDefault-2">
+    <para>
+      Whether to symlink or copy the <filename>bower_components</filename> directory depends on the build tool in use. In this case a copy is used to avoid <command>gulp</command> silliness with permissions.
+    </para>
+  </callout>
+  <callout arearefs="ex-buildBowerComponentsDefault-3">
+    <para>
+      <command>gulp</command> requires <varname>HOME</varname> to refer to a writeable directory.
+    </para>
+  </callout>
+  <callout arearefs="ex-buildBowerComponentsDefault-4">
+    <para>
+      The actual build command. Other tools could be used.
+    </para>
+  </callout>
+</calloutlist>
+```
+
+## Troubleshooting {#ssec-bower2nix-troubleshooting}
+
+### ENOCACHE errors from buildBowerComponents
+
+This means that Bower was looking for a package version which doesn't exist in the generated `bower-packages.nix`.
+
+If `bower.json` has been updated, then run `bower2nix` again.
+
+It could also be a bug in `bower2nix` or `fetchbower`. If possible, try reformulating the version specification in `bower.json`.