Interfaces describe shapes

Most data in a real app is objects: a user, an order, a smith. An interface lists the properties such an object must have, and the type of each one.

Once you've named the shape, you can use it everywhere: as a variable type, a parameter, a return type. Forget a property or misspell one, and TypeScript points at it.

interface Smith {
  name: string;
  level: number;
}

const brunn: Smith = { name: "Brunn", level: 7 };

function promote(s: Smith): Smith {
  return { ...s, level: s.level + 1 };
}

• Interfaces exist only at compile time: no JavaScript is generated for them.
• Every property listed is required unless you say otherwise.