Interfaces

Interfaces have zero runtime JS impact. There is a lot of power in TypeScript interfaces to declare the structure of variables.

The following two are equivalent declarations, the first uses an inline annotation, the second uses an interface:

// Sample A
declare var myPoint: { x: number; y: number; };

// Sample B
interface Point {
    x: number; y: number;
}
declare var myPoint: Point;

However the beauty of Sample B is that if someone authors a library that builds on the myPoint library to add new members, they can easily add to the existing declaration of myPoint:

// Lib a.d.ts
interface Point {
    x: number; y: number;
}
declare var myPoint: Point;

// Lib b.d.ts
interface Point {
    x: number; y: number; z: number;
}

// Your code
var myPoint.z; // Allowed!

This is because interfaces in TypeScript are open ended. This is a vital tenet of TypeScript that it allows you to mimic the extensibility of JavaScript using interfaces.