js: snapshot now returns cloned objects/arrays of this.value

lib/modules/database/snapshot.js

@@ -29,16 +29,14 @@ export default class Snapshot {
     this.childKeys = snapshot.childKeys || [];
   }
 
-  /*
-   * DEFAULT API METHODS
-   */
-
   /**
    * Extracts a JavaScript value from a DataSnapshot.
    * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#val
    * @returns {any}
    */
   val(): any {
+    // clone via JSON stringify/parse - prevent modification of this.value
+    if (isObject(this.value) || Array.isArray(this.value)) return JSON.parse(JSON.stringify(this.value));
     return this.value;
   }