Add solution to Challenge (#19369)

Added a solution to "Iterate Through the Keys of an Object with a for...in Statement"'s "Get a Hint" documentation.

curriculum/challenges/english/02-javascript-algorithms-and-data-structures/basic-data-structures/-iterate-through-the-keys-of-an-object-with-a-for...in-statement.english.md

@@ -75,6 +75,35 @@ console.log(countOnline(users));
 <section id='solution'>
 
 ```js
-// solution required
+let users = {
+  Alan: {
+    age: 27,
+    online: false
+  },
+  Jeff: {
+    age: 32,
+    online: true
+  },
+  Sarah: {
+    age: 48,
+    online: false
+  },
+  Ryan: {
+    age: 19,
+    online: true
+  }
+};
+
+function countOnline(obj) {
+  let online = 0;
+  for(let user in obj){
+    if(obj[user].online == true) {
+      online += 1;
+    }
+  }
+  return online;
+}
+
+console.log(countOnline(users));
 ```
 </section>