Why does the function init() call the function update(e) without reference to parameter 'e'--event in javascript?

The code as follows:

function update(e) {

getmousexy(e);

document.getElementById('showXY').value = 'x: ' + mousex + ' ' + 'y: ' + mousey

}

function init() {

document.onmousemove = update;

update();

}