function createArrow() {
var arrowDown = {
    parent: document.querySelector('.landingItem'),
    div: document.createElement('div'),
    initDiv: function() {
        this.div.classList.add('arrow-down');
        this.div.style.borderLeftWidth = parseInt(document.documentElement.clientWidth)/2 + 'px';
        this.div.style.borderRightWidth = parseInt(document.documentElement.clientWidth)/2 + 'px';
        this.parent.appendChild(this.div);
    },
    checkDiv: function () {
        if(this.parent.querySelector('.arrow-down')) {
            this.parent.removeChild(this.parent.querySelector('.arrow-down'));
        }
        this.initDiv();
    }

};

arrowDown.checkDiv();

.arrow-down {
  margin-top: -2px;
  border-top: 60px solid #501a70;
  border-left: 160px solid transparent;
  border-right: 160px solid transparent;
}