You can make (c) optional by postfixing it with a question mark. This means it's still there but it is not required. If you do try and pass C it must be of type boolean. Therefore the param is optional however the type is not.

optionalParameter.ts

interface Thing {
	a: number;
	b: string;
	c?: boolean;
}

function process(x: Thing){
	return x.a;
}

var n = process({a: 10, b:'Beeby', c:'fail' });
n.toFixed(0.0);