Hands-On Functional Programming with TypeScript

Pattern matching

Pattern matching allows you to match a value (or an object) against some patterns to select a branch of the code. In functional languages, pattern matching can be used to match on standard primitive values such as strings. TypeScript allows us to implement pattern matching using literal types and control flow analysis.

For example, we can define three types, named Circle, Square, and Rectangle. We can then define a new type, named Shape, which is the union of the Circle, Square, and Rectangle types:

const enum ShapeKind {    circle = "circle",    square = "square",    rectangle = "rectangle",}type Circle = { kind: ShapeKind.circle, radius: number };type Square = { kind: ShapeKind.square, size: number };type Rectangle = { kind: ShapeKind.rectangle, ...