Update undocumented.md

content/docs/undocumented.md

@@ -21,6 +21,29 @@ personal notes. Contributions are highly appreciated, especially:
 If you have questions about any of these, don't hesitate to reach
 out on [Discord](https://discord.gg/mkcbkRw).
 
+## isa
+
+`instanceof` is shortened to `isa` in imba:
+
+```imba
+[] isa Array
+```
+
+But we've added more functionality. If the right hand side is a string,
+it will check `typeof`:
+
+```imba
+let s = 'hello'
+typeof s is 'string' # true
+s isa 'string' # true
+```
+
+It also works similarly to `is`:
+
+```
+7 isa ('string' or 'number') # true
+```
+
 ## Awaits
 
 You can await desired changes to observable properties with `imba.awaits`,