Adding dependency override example

doc/languages-frameworks/javascript.section.md

@@ -195,10 +195,46 @@ configurePhase = "ln -s $node_modules node_modules";
 
 this will generate a derivation including the node_modules. If you have to build a derivation for an integrated web framework (rails, phoenix..), this is probably the easiest way. [Plausible](https://github.com/NixOS/nixpkgs/blob/master/pkgs/servers/web-apps/plausible/default.nix#L39) offers a good example of how to do this.
 
+#### Overriding dependency behavior
+
+In the `mkYarnPackage` record the property `pkgConfig` can be used to override packages when you encounter problems building.
+
+For instance, say your package is throwing errors when trying to invoke node-sass: `ENOENT: no such file or directory, scandir '/build/source/node_modules/node-sass/vendor'`
+
+To fix this we will specify different versions of build inputs to use, as well as some post install steps to get the software built the way we want:
+
+```nix
+mkYarnPackage rec {
+  pkgConfig = {
+    node-sass = {
+      buildInputs = with final;[ python libsass pkgconfig ];
+      postInstall = ''
+        LIBSASS_EXT=auto yarn --offline run build
+        rm build/config.gypi
+      '';
+    };
+  };
+}
+```
+
 #### 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>
+
 ## Outside of nixpkgs {#javascript-outside-nixpkgs}
 
 There are some other options available that can't be used inside nixpkgs. Those other options are written in nix. Importing them in nixpkgs will require moving the source code into nixpkgs. Using [Import From Derivation](https://nixos.wiki/wiki/Import_From_Derivation) is not allowed in hydra at present. If you are packaging something outside nixpkgs, those can be considered