TypeScript - Union Type Definition and Usage

What is Union Type?

A Union Type is a combination of two or more types.

For example, if there are multiple data types,

interface Square {
  kind: 'square'
  size: number
}
interface Rectangle {
  kind: 'rectangle'
  width: number
  height: number
}
interface Circle {
  kind: 'circle'
  radius: number
}

The Union Type is defined as follows:|

type Shape = Square | Rectangle | Circle

Type Inference for Union Types

In TypeScript, the Type Inference to deduce each type. When checking TypeScript, use the TypeScript Playground You can check it out online.

const getArea = (shape: Shape) => {
  if (shape.kind === "square") {
    // If you use the properties of Rectangle
    // and Circle here, a type error occurs.
    return shape.size * shape.size;
  }
  // The rest is ommited
}