Add config options for eslint directories and extensions

defaults.yml

@@ -302,3 +302,19 @@ styleguide:
   root: 'styleguide'
   # Command to compile the style guide assets, for use during the build and artifact steps.
   command: 'yarn default'
+
+# Configuration for linting custom javascript.
+#
+# Update these properties to reflect the directories and extensions used for
+# your project's custom javascript.
+# @see https://eslint.org/docs/latest/use/core-concepts
+eslint:
+  # Run eslint on custom modules AND themes, this must be a single value.
+  # @see: https://eslint.org/docs/latest/use/command-line-interface#run-the-cli
+  #
+  # If you need mulstiple directories that don't match a single pattern, you
+  # may need to add multiple build targets for eslint using each directory.
+  directory: "${drupal.root}/**/custom/"
+  # Run eslint on files with .es6.js extension.
+  # @see: https://eslint.org/docs/latest/use/command-line-interface#--ext
+  extensions: ".es6.js"