Guards and Assertions

We often refine more general types into more specific types (i.e. downcasting). There are 2 approaches for this in TypeScript: type assertions and type guards.

TypeScript generally infers upcasts automatically.

Type assertions

We use type assertions to explicitly change the compile-time type of a value. We do this when we know something the compiler doesn't.

The types any and unknown can be asserted as any type. Other types can be asserted too, but TypeScript will prevent the assertion if the types are incompatible (e.g. 4 as string).

Type guards

We use type guards to refine the compile-time type of a value by reasoning about its runtime value. We can use built-in expressions like typeof, standard library functions like Array.isArray, or even write our own.