diff --git a/packages/gatsby-plugin-netlify-cms/src/gatsby-node.js b/packages/gatsby-plugin-netlify-cms/src/gatsby-node.js
index bd805360bb81c..daaf7600ab858 100644
--- a/packages/gatsby-plugin-netlify-cms/src/gatsby-node.js
+++ b/packages/gatsby-plugin-netlify-cms/src/gatsby-node.js
@@ -1,5 +1,5 @@
 import path from "path"
-import { get, mapValues, isPlainObject, trim } from "lodash"
+import { mapValues, isPlainObject, trim } from "lodash"
 import webpack from "webpack"
 import HtmlWebpackPlugin from "html-webpack-plugin"
 import HtmlWebpackExcludeAssetsPlugin from "html-webpack-exclude-assets-plugin"
@@ -7,23 +7,17 @@ import MiniCssExtractPlugin from "mini-css-extract-plugin"
 // TODO: swap back when https://github.com/geowarin/friendly-errors-webpack-plugin/pull/86 lands
 import FriendlyErrorsPlugin from "@pieh/friendly-errors-webpack-plugin"
 
-/**
- * Deep mapping function for plain objects and arrays. Allows any value,
- * including an object or array, to be transformed.
- */
+// Deep mapping function for plain objects and arrays. Allows any value,
+// including an object or array, to be transformed.
 function deepMap(obj, fn) {
-  /**
-   * If the transform function transforms the value, regardless of type,
-   * return the transformed value.
-   */
+  // If the transform function transforms the value, regardless of type, return
+  // the transformed value.
   const mapped = fn(obj)
   if (mapped !== obj) {
     return mapped
   }
 
-  /**
-   * Recursively deep map arrays and plain objects, otherwise return the value.
-   */
+  // Recursively deep map arrays and plain objects, otherwise return the value.
   if (Array.isArray(obj)) {
     return obj.map(value => deepMap(value, fn))
   }
@@ -33,6 +27,42 @@ function deepMap(obj, fn) {
   return obj
 }
 
+function replaceRule(value) {
+  // If `value` does not have a `test` property, it isn't a rule object.
+  if (!value || !value.test) {
+    return value
+  }
+
+  // when javascript we exclude node_modules
+  if (
+    value.type === `javascript/auto` &&
+    value.exclude &&
+    value.exclude instanceof RegExp
+  ) {
+    return {
+      ...value,
+      exclude: new RegExp(
+        [value.exclude.source, `node_modules|bower_components`].join(`|`)
+      ),
+    }
+  }
+
+  // Manually swap `style-loader` for `MiniCssExtractPlugin.loader`.
+  // `style-loader` is only used in development, and doesn't allow us to pass
+  // the `styles` entry css path to Netlify CMS.
+  if (
+    typeof value.loader === `string` &&
+    value.loader.includes(`style-loader`)
+  ) {
+    return {
+      ...value,
+      loader: MiniCssExtractPlugin.loader,
+    }
+  }
+
+  return value
+}
+
 exports.onCreateDevServer = ({ app, store }, { publicPath = `admin` }) => {
   const { program } = store.getState()
   const publicPathClean = trim(publicPath, `/`)
@@ -49,7 +79,7 @@ exports.onCreateDevServer = ({ app, store }, { publicPath = `admin` }) => {
 }
 
 exports.onCreateWebpackConfig = (
-  { store, stage, getConfig, plugins, pathPrefix },
+  { store, stage, getConfig, plugins, pathPrefix, loaders },
   {
     modulePath,
     publicPath = `admin`,
@@ -79,26 +109,11 @@ exports.onCreateWebpackConfig = (
       path: path.join(program.directory, `public`, publicPathClean),
     },
     module: {
-      /**
-       * Manually swap `style-loader` for `MiniCssExtractPlugin.loader`.
-       * `style-loader` is only used in development, and doesn't allow us to
-       * pass the `styles` entry css path to Netlify CMS.
-       */
-      rules: deepMap(gatsbyConfig.module.rules, value => {
-        if (
-          typeof get(value, `loader`) === `string` &&
-          value.loader.includes(`style-loader`)
-        ) {
-          return { ...value, loader: MiniCssExtractPlugin.loader }
-        }
-        return value
-      }),
+      rules: deepMap(gatsbyConfig.module.rules, replaceRule),
     },
     plugins: [
-      /**
-       * Remove plugins that either attempt to process the core Netlify CMS
-       * application, or that we want to replace with our own instance.
-       */
+      // Remove plugins that either attempt to process the core Netlify CMS
+      // application, or that we want to replace with our own instance.
       ...gatsbyConfig.plugins.filter(
         plugin =>
           ![`MiniCssExtractPlugin`, `GatsbyWebpackStatsExtractor`].find(
@@ -122,49 +137,37 @@ exports.onCreateWebpackConfig = (
           },
         }),
 
-      /**
-       * Use a simple filename with no hash so we can access from source by
-       * path.
-       */
+      // Use a simple filename with no hash so we can access from source by
+      // path.
       new MiniCssExtractPlugin({
         filename: `[name].css`,
       }),
 
-      /**
-       * Auto generate CMS index.html page.
-       */
+      // Auto generate CMS index.html page.
       new HtmlWebpackPlugin({
         title: htmlTitle,
         chunks: [`cms`],
         excludeAssets: [/cms.css/],
       }),
 
-      /**
-       * Exclude CSS from index.html, as any imported styles are assumed to be
-       * targeting the editor preview pane. Uses `excludeAssets` option from
-       * `HtmlWebpackPlugin` config.
-       */
+      // Exclude CSS from index.html, as any imported styles are assumed to be
+      // targeting the editor preview pane. Uses `excludeAssets` option from
+      // `HtmlWebpackPlugin` config.
       new HtmlWebpackExcludeAssetsPlugin(),
 
-      /**
-       * Pass in needed Gatsby config values.
-       */
+      // Pass in needed Gatsby config values.
       new webpack.DefinePlugin({
         __PATH__PREFIX__: pathPrefix,
         CMS_PUBLIC_PATH: JSON.stringify(publicPath),
       }),
     ].filter(p => p),
 
-    /**
-     * Remove common chunks style optimizations from Gatsby's default
-     * config, they cause issues for our pre-bundled code.
-     */
+    // Remove common chunks style optimizations from Gatsby's default
+    // config, they cause issues for our pre-bundled code.
     mode: stage === `develop` ? `development` : `production`,
     optimization: {
-      /**
-       * Without this, node can get out of memory errors
-       * when building for production.
-       */
+      // Without this, node can get out of memory errors when building for
+      // production.
       minimizer: stage === `develop` ? [] : gatsbyConfig.optimization.minimizer,
     },
     devtool: stage === `develop` ? `cheap-module-source-map` : `source-map`,