Sometimes, you must check whether a value is of a certain type. For instance, if you have a value of a class Base, you might want to check if it is of a certain subclass Derived. In JavaScript you would write this with an instanceof check. Since TypeScript is an extension of JavaScript, you can also use instanceof in TypeScript. In other typed languages, like C#, you must then add a type cast, which tells the compiler that a value is of a type, different from what the compiler analyzed. You can also add type casts in two different ways. The old syntax for type casts uses < and >, the new syntax uses the as keyword. You can see them both in the next example:

class Base { 
  a: string; 
} 
class Derived extends Base { 
  b: number; 
} 
const foo: Base; 
if (foo instanceof Derived) { 
  (<Derived> foo).b; 
  (foo as Derived).b; 
} 

When you use a type guard, you say to the compiler: trust me, this value will...