closes basarat#497 🌹

Author: basarat

docs/types/typeGuard.md

@@ -202,3 +202,15 @@ if (foo.bar) {
  });
 }
 ```
+
+### null and undefined with `strictNullChecks`
+
+TypeScript is smart enough to rule out `null` and `undefined` with a `== null` / `!= null` checks. E.g.
+
+```ts
+function foo(a?: number | null) {
+ if (a == null) return;
+
+ // a is number now.
+}
+```