无法理解处理mouseevent的代码

I'm relatively new to JavaScript and can't understand the code from the book Eloquent Javascript. It is used to fire mousemove not frequenter than 500ms, but I can't understand what is null here for.

let scheduled = null;
window.addEventListener("mousemove", event => {
    if (!scheduled) {//*if scheduled ==null
        setTimeout(() => {
           document.body.textContent =
           `Mouse at ${scheduled.pageX}, ${scheduled.pageY}`;
           scheduled = null;
        }, 500);
    }
    scheduled = event;
});

评论