added whole content

content/Typescript/Type guards.md

@@ -0,0 +1,20 @@
+Useful for type checking at compile and run time:
+
+```typescript
+function isFish(pet: Fish | Bird): pet is Fish {
+  return (pet as Fish).swim !== undefined;
+}
+```
+
+Usage:
+
+```typescript
+const pet = getSmallPet();
+ 
+if (isFish(pet)) {
+  pet.swim();
+} else {
+  pet.fly();
+}
+```
+