Почему не работает drag and drop?

window.onload = function () {
let field = document.getElementById('field')  
let activeElement;
let offX = 0;
let offY = 0;
let mDown = function(e) {   
    console.log(e.target);
    activeElement = e.target;    
    offY = e.offsetY;
    offX = e.offsetX;
};
let mUp = function(e) {
    activeElement = null;
};
let mMove = function(e) {
    if(activeElement) {        
        activeElement.style.top = (e.clientY - offY) + 'px';
        activeElement.style.left = (e.clientX - offX) + 'px';

    }

};

field.addEventListener('mousedown', mDown);
field.addEventListener('mouseup', mUp);
field.addEventListener('mousemove',mMove)
}