in operator

The in operator checks whether a property exists on an object. A check like "swim" in animal lets TypeScript narrow a union type; if the property exists only on one member, the object is narrowed to that member's type inside the block.

interface Fish { swim(): void }
interface Bird { fly(): void }

function move(animal: Fish | Bird) {
  if ("swim" in animal) {
    animal.swim();  // Fish
  } else {
    animal.fly();  // Bird
  }
}

The in operator is ideal for distinguishing objects that have different fields rather than a common one. If each member has a unique property, checking it with in narrows the type. Discriminated unions use a common kind field, but in is more flexible.

interface Square { size: number }
interface Circle { radius: number }

function area(shape: Square | Circle): number {
  if ("radius" in shape) {
    return Math.PI * shape.radius ** 2;  // Circle
  }
  return shape.size ** 2;  // Square
}