Configure Babel proposal plugins via Gatsby Node API

Configured the installed Babel proposal plugins through Gatsby's Node
API (1) using the provided `setBabelPlugin` action (2).

References:

  (1) https://www.gatsbyjs.org/docs/node-apis
  (2) https://www.gatsbyjs.org/docs/actions

GH-29

.gatsby/onCreateBabelConfig.js

@@ -0,0 +1,85 @@
+/*
+ * Copyright (C) 2018-present Arctic Ice Studio <[email protected]>
+ * Copyright (C) 2018-present Sven Greb <[email protected]>
+ *
+ * Project:    Nord Docs
+ * Repository: https://github.com/arcticicestudio/nord-docs
+ * License:    MIT
+ */
+
+/**
+ * @file Implementation of the Gatsby Node `onCreateBabelConfig` API.
+ * Allows to let plugins extend/mutate the project's Babel configuration.
+ * @author Arctic Ice Studio <[email protected]>
+ * @author Sven Greb <[email protected]>
+ * @see https://gatsbyjs.org/docs/node-apis/#onCreateBabelConfig
+ * @see https://babeljs.io
+ * @since 0.1.0
+ */
+
+/**
+ * Implementation of the Gatsby Node `onCreateBabelConfig` API.
+ *
+ * @method onCreateBabelConfig
+ * @param  {object} actions Collection functions provided by Gatsby used to manipulate the state of the build process.
+ * @see https://gatsbyjs.org/docs/node-apis/#onCreateBabelConfig
+ * @see https://gatsbyjs.org/docs/actions
+ * @since 0.1.0
+ */
+const onCreateBabelConfig = ({ actions }) => {
+  /*
+   * Allows to use the "ES Class Fields & Static Properties" proposal to transforms static class properties as well as
+   * properties declared with the experimental property initializer syntax.
+   *
+   * References:
+   * - https://github.com/tc39/proposal-class-fields
+   * - https://babeljs.io/docs/en/babel-plugin-proposal-class-properties
+   */
+  actions.setBabelPlugin({
+    name: "@babel/plugin-proposal-class-properties",
+    options: {
+      loose: true
+    }
+  });
+
+  /*
+   * Allows to use the experimental `export { default} from "mod"` proposal syntax.
+   *
+   * References:
+   * - https://github.com/tc39/proposal-export-default-from
+   * - https://babeljs.io/docs/en/babel-plugin-proposal-export-default-from
+   */
+  actions.setBabelPlugin({ name: "@babel/plugin-proposal-export-default-from" });
+
+  /*
+   * Allows to use the "Nullish Coalescing" proposal trhough the experimental `??` operator to combine with the
+   * "Optional Chaining" proposal operator.
+   *
+   * References:
+   * - https://github.com/tc39/proposal-nullish-coalescing
+   * - https://babeljs.io/docs/en/babel-plugin-proposal-nullish-coalescing-operator
+   */
+  actions.setBabelPlugin({
+    name: "@babel/plugin-proposal-nullish-coalescing-operator",
+    options: {
+      loose: false
+    }
+  });
+
+  /*
+   * Allows to use the "Optional Chaining" proposal through the experimental `?.` operator in combination with the
+   * "Nullish Coalescing" proposal operator.
+   *
+   * References:
+   * - https://github.com/tc39/proposal-optional-chaining
+   * - https://babeljs.io/docs/en/babel-plugin-proposal-optional-chaining
+   */
+  actions.setBabelPlugin({
+    name: "@babel/plugin-proposal-optional-chaining",
+    options: {
+      loose: false
+    }
+  });
+};
+
+module.exports = onCreateBabelConfig;

gatsby-node.js

@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) 2018-present Arctic Ice Studio <[email protected]>
+ * Copyright (C) 2018-present Sven Greb <[email protected]>
+ *
+ * Project:    Nord Docs
+ * Repository: https://github.com/arcticicestudio/nord-docs
+ * License:    MIT
+ */
+
+/**
+ * @file Implementation of Gatsby Node APIs.
+ * @author Arctic Ice Studio <[email protected]>
+ * @author Sven Greb <[email protected]>
+ * @see https://gatsbyjs.org/docs/node-apis
+ */
+
+exports.onCreateBabelConfig = require("./.gatsby/onCreateBabelConfig");