myNum = 5;

this.box.addEventListener("click", clickHandler);

function clickHandler(e) {
    console.log(e.currentTarget.myNum);
}

<button id="elem" data-mynum="3"> hello</button>

var elem = document.getElementById('elem');
elem.addEventListener("click", clickHandler);

function clickHandler(e) {
    console.log(e.currentTarget.dataset.mynum);
}