Movable <span> window

Number.prototype.inRange = function(a, b, exclusive=0){
    if(a>b){c=b;b=a;a=c}
    switch (exclusive) {
        case 0: {return this >= a && this <= b;}
        case 1: {return this > a && this <= b;}
        case 2: {return this >= a && this < b;}
        case 3: {return this > a && this < b;}
    }
}
document.body.appendChild(box=document.createElement('span'));
box.innerText = 'hello';
box.style.border = '1px solid black';
box.style.position = 'absolute';
box.style.top = '0px';
box.style.left = '0px';
mouseDown = trueMouseDown = false;
mouseCoords = [];
function upd(e){
    if(!trueMouseDown){mouseCoords = [e.clientX-box.offsetLeft, e.clientY-box.offsetTop];};
    if(!mouseDown){return}
    box.style.left = e.clientX-mouseCoords[0]+'px';
    box.style.top = e.clientY-mouseCoords[1]+'px';
    if(box.offsetLeft+box.offsetWidth >= innerWidth){
        box.style.left = innerWidth-box.offsetWidth-1+'px';
    }
    if(box.offsetTop+box.offsetHeight >= innerHeight){
        box.style.top = innerHeight-box.offsetHeight-1+'px';
    }
    if(box.offsetLeft<=0){box.style.left='0px';}
    if(box.offsetTop<=0){box.style.top='0px';}
}
document.addEventListener('mousedown', function(e){
    mouseCoords = [e.clientX-box.offsetLeft, e.clientY-box.offsetTop];
    mouseDown = mouseCoords[0].inRange(0, +box.offsetWidth)&&mouseCoords[1].inRange(0, +box.offsetHeight);
    trueMouseDown = true;
    upd(e)
});
document.addEventListener('mouseup', e=>{mouseDown = trueMouseDown = false;upd(e)});
document.addEventListener('mousemove', e=>upd(e));