Update Data-Types.md

alexpaul · web-flow · commit ed251dcd4d83 · 2020-10-17T06:28:54.000-04:00
diff --git a/Data-Types.md b/Data-Types.md
@@ -108,6 +108,20 @@ let newStringInstance = new String('Hello');
 console.log(newStringInstance instanceof String); // true 
 ```
 
+#### Using `typeof` check in a function 
+
+```javascript 
+function add(a, b) {
+  if ((typeof a === 'number') && (typeof b === 'number')) {
+    return a + b; 
+  }
+  return -1; 
+}
+
+console.log(add('alex', 5)); // -1
+console.log(add(10, 5)); // 15
+```
+
 
 ## Equality 
 
