ClassType.ts

interface ClockConstructor {
	new (hour: number, minute: number);
}

interface ClockInterface {
	tick();
}

function createClock(ctor: ClockConstructor, hour: number, minute: number): ClockInterface {
	return new ctor(hour, minute);
}

class DigitalClock implements ClockInterface {
	constructor(h: number, m: number) {	}
	tick() {
		console.log("beep beep");
	}
}

class AnalogClock implements ClockInterface {
	constructor(h: number, m: number) { }
	tick() {
		console.log("tick tock");
	}
}

let digital = createClock(DigitalClock, 12, 17);
let analog  = createClock(AnalogClock, 7, 32);

I've taken this example of the doc (link bellow).
I didn't understand how this elements (functions and interfaces) relate to each other.

• Why createClock's first parameter is of the type ClockConstructor, but we pass one class that implements other interface's type, that's ClockInterface? I expected, at least, one type that implement ClockConstructor. How it works?

http://www.typescriptlang.org/docs/handbook/interfaces.html