add null undefined

Author: geekysrm

09-types-null-undefined.md

@@ -0,0 +1,21 @@
+
+## Null and Undefined
+
+-   Used to express 'nothing' in JavaScript.
+    
+-   If we declare a variable, and don't set anything to it, then it has value `undefined`.
+    
+    ```javascript
+      let dog;
+      console.log(dog); // undefined
+    
+    ```
+    
+    When we try to access a variable that is created but not defined/set a value, we get `undefined`.
+    
+-   **null:** value of nothing
+    
+    ```javascript
+      const somethingNull = null;
+      // we have explicitly set the value to be nothing with null
+    ```