@timsims Super Tipp! (Which of course I already knew...pfff! ;-) )

if (
unknownFoo.prop2 
&& typeof unknownFoo.prop2 === 'object' 
&& 'foo' in unknownFoo.prop2
) {
    console.log(unknownFoo.prop2.foo);
}

keyword in can do the trick

@Ispilledteaonthecarpet For a stateless method, like flash, this wouldn't make any difference. But usually you have bundled several members together (functions, private/inner functions, state variables). Imagine the good old car example with accelerate, brake and currentSpeed. All members of useCar would work with the same speed state. And if you use the useCar composable at different components each component has a separate useCar state.