Learning TypeScript

Chapter 15. Type Operations

Conditionals, maps

With great power over types

comes great confusion

TypeScript gives us awesome levels of power to define types in the type system. Even the logical modifiers from Chapter 10, “Generics” pale in comparison to the capabilities of the type operations in this chapter. Once you’ve completed this chapter, you’ll be able to mix, match, and modify types based on other types—giving you powerful ways to represent types in the type system.

Warning

Most of these fancy types are techniques you generally don’t want to use very frequently. You’ll want to understand them for the cases where they are useful, but beware: they can be difficult to read through when overused. Have fun!

Mapped Types

TypeScript provides syntax for creating a new type based on the properties of another type: in other words, mapping from one type to another. A mapped type in TypeScript is a type that takes in another type and performs some operation on each property of that type.

Mapped types create a new type by creating a new property under each key in a set of keys. They use a syntax similar to index signatures, but instead of using a static key type with : like [i: string], they use a computed type from the other type with in like [K in OriginalType]:

type NewType = {
    [K in OriginalType]: NewProperty;
};

One common use case for mapped types is to create an object whose keys are each of the string literals in an existing union type. This AnimalCounts type creates a new ...