Discriminated unions in Typescript

Discriminated unions use a property to discriminate between union members, so that Typescript knows what type you're dealing with it a certain time.

When every type in a union contains a common property with literal types, TypeScript considers that to be a discriminated union, and can narrow out the members of the union.

interface Circle {
  kind: "circle";
  radius: number;
}

interface Square {
  kind: "square";
  sideLength: number;
}

type Shape = Circle | Square;

When using a type guard style check on the discriminant property (e.g. kind), Typescript will narrow the object into that type.

if (shape.kind === 'circle') {
    console.log(shape.radius) // Works fine
    console.log(shape.sideLength) // Type error, as shape's type is Circle
}

[[20230403123124-union-intersection-extends-ts]] [[20210705102809-ts-narrowing]]

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