getUserMedia Not working when inside a function

Why is this code not working:

videoCallButton.onclick = function()
{navigator.getUserMedia(mediaConstraints, handleUserMedia, handleUserMediaError);
};

but this one It is:

videoCallButton.onclick = navigator.getUserMedia(
mediaConstraints, handleUserMedia, handleUserMediaError
);

and the event onclick works everytime page is loaded, without clicking

 videoCallButton.onclick = alert("yes");