Firebase Stub refactoring

__mocks__/gatsby-plugin-firebase/database/dataSnapshot.js

@@ -0,0 +1,47 @@
+import Reference from './reference';
+
+class DataSnapshot {
+  #eventType = '';
+  #reference = null;
+  #value = undefined;
+
+  constructor(eventType, reference, value = undefined) {
+    if (!eventType) {
+      throw new Error('eventType must be provided.');
+    } else if (typeof eventType !== 'string') {
+      throw new Error('eventType should be a string.');
+    }
+
+    this.#eventType = eventType;
+
+    if (!reference) {
+      throw new Error('reference must be provided.');
+    } else if (!(reference instanceof Reference)) {
+      throw new Error('reference must be an instance of the Reference class.');
+    }
+
+    this.#reference = reference;
+
+    this.#value = value;
+  }
+
+  get eventType() {
+    return this.#eventType;
+  }
+
+  get value() {
+    return this.#value;
+  }
+
+  val() {
+    if (this.eventType === 'value') {
+      return typeof this.value !== 'undefined'
+        ? this.value
+        : this.#reference.getData();
+    }
+
+    return undefined;
+  }
+}
+
+export default DataSnapshot;