What is an interface in TypeScript?

An interface in TypeScript is a contract that defines the structure (shape) of an object — the properties and methods it must have. It is a purely TypeScript construct that disappears after compilation. Syntax: interface User { name: string; age: number; email?: string; } (the ? marks optional properties). A class can implement an interface with implements: class Admin implements User { name = "Alice"; age = 30; }. Interfaces can extend other interfaces: interface Admin extends User { role: string; }. They can describe function signatures: interface Greeter { (name: string): string; } and index signatures: interface StringMap { [key: string]: string; }. Declaration merging: multiple interface declarations with the same name are automatically merged — useful for extending third-party library types. Interfaces enforce the "duck typing" philosophy of TypeScript — if an object has the required shape, it satisfies the interface regardless of how it was created.