Extra Properties

Most of the time, when you pass an object to a function in TypeScript, it's:

• Okay to have more properties than those defined in the function's parameter type
• Not okay to have missing properties

However, when you pass an object literal directly to a function, TypeScript performs what's called "excess property checking". Which means it also will not allow extra properties.

For example, say we have this type:

type Spaceship = {
  name: string;
  speed: number;
};

and we make an object with one extra property:

const falcon = {
  name: "Millennium Falcon",
  speed: 75,
  weapons: 4,
};

We can pass this object to a function that expects a Spaceship:

function pilot(ship: Spaceship) {
  console.log(`Piloting ${ship.name} at ${ship.speed} light-years per hour`);
}

// this is fine
pilot(falcon);

But interestingly, if we pass in the same object literal (no variable assignment), TypeScript will throw an error:

// Error: Object literal may only specify known properties, and 'weapons' does not exist in type 'Spaceship'.
pilot({ name: "Millennium Falcon", speed: 75, weapons: 4 });

It's also worth noting that many of these kinds of rules are configurable in the tsconfig.json file, which we'll cover later. We'll mostly refer to default behavior in this course.