MouseEvent.prototype.buttons Polyfill

mouseeventbuttens.js

if (!MouseEvent.prototype.hasOwnProperty('buttons')) {
    let mousedown = 0;

    addEventListener('mousedown', e => mousedown = e.button || 1);
    addEventListener('mouseup', () => mousedown = 0);

    Object.defineProperty(MouseEvent.prototype, 'buttons', {
        get () {
            return mousedown;
        }
    });
}