Finish Undefined and Null

Signed-off-by: Michael Tang <michael.h.tang@gmail.com>

undefined-null.js

@@ -0,0 +1,26 @@
+//Undefined for variable
+let name
+
+name = 'Jen'
+
+if (name === undefined) {
+    console.log('Please provide a name')
+} else {
+    console.log(name)
+}
+
+// Undefined for function arguments
+// Undefined as function return default value
+let  square = function (num) {
+    console.log(num)
+}
+
+let result  = square()
+console.log(result)
+
+// Null as assigned value
+let age = 27
+
+age = null
+
+console.log(age)