snapshot.forEach() now correctly implemented - now allows returning true to break iteration as per web sdk docs.

+ misc commenting / flow types

lib/modules/database/snapshot.js

@@ -30,11 +30,22 @@ export default class Snapshot {
    * DEFAULT API METHODS
    */
 
-  val() {
+  /**
+   * Extracts a JavaScript value from a DataSnapshot.
+   * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#val
+   * @returns {any}
+   */
+  val(): any {
     return this.value;
   }
 
-  child(path: string) {
+  /**
+   * Gets another DataSnapshot for the location at the specified relative path.
+   * @param path
+   * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#forEach
+   * @returns {Snapshot}
+   */
+  child(path: string): Snapshot {
     const value = deepGet(this.value, path);
     const childRef = this.ref.child(path);
     return new Snapshot(childRef, {
@@ -48,12 +59,36 @@ export default class Snapshot {
     });
   }
 
-  exists() {
+  /**
+   * Returns true if this DataSnapshot contains any data.
+   * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#exists
+   * @returns {boolean}
+   */
+  exists(): Boolean {
     return this.value !== null;
   }
 
-  forEach(fn: (key: any) => any) {
-    return this.childKeys.forEach(key => fn(this.child(key)));
+  /**
+   * Enumerates the top-level children in the DataSnapshot.
+   * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#forEach
+   * @param action
+   */
+  forEach(action: (key: any) => any): Boolean {
+    if (!this.childKeys.length) return false;
+    let cancelled = false;
+
+    for (let i = 0, len = this.childKeys.length; i < len; i++) {
+      const key = this.childKeys[i];
+      const childSnapshot = this.child(key);
+      const returnValue = action(childSnapshot);
+
+      if (returnValue === true) {
+        cancelled = true;
+        break;
+      }
+    }
+
+    return cancelled;
   }
 
   getPriority() {