Uncaught TypeError: this.getXY is not a function

class Sortable {
    constructor(el) {
        //var self = this;
        this.element = el
        this.items = document.querySelectorAll(this.element.dataset.sortable)
        this.itemWidth = this.items[0].getBoundingClientRect().width
        this.itemHeight = this.items[0].offsetHeight
        this.cols = Math.floor(this.element.offsetWidth / this.itemWidth)
        this.onStart()
    }

    onStart() {
        Array.from(this.items).forEach((item) => {
            item.style.position = "absolute";
            item.style.top = "0px";
            item.style.left = "0px";
            const position = item.dataset.position;
            const x = this.getXY(position).x // OK
            const y = this.getXY(position).y // OK
            item.style.transform = `translate3D(${x}px, ${y}px, 0)`
        })

        this.element.style.height = (this.itemHeight * Math.ceil(this.items.length / this.cols)) + "px"

        interact(this.element.dataset.sortable, {context: this.element}).draggable({inertia: false, manualStart: false, onmove: this.onMove})
    }

    onMove(e) {
        const p = this.getXY(e.target.dataset.position) // is not a function
        const x = p.x + e.clientX - e.clientX0
        const y = p.y + e.clientY - e.clientY0
        e.target.style.transform = `translate3D(${x}px, ${y}px, 0)`
    }

    getXY(position) {
        const x = this.itemWidth * (position % this.cols)
        const y = this.itemHeight * Math.floor(position / this.cols)

        return {
            x: x,
            y: y
        }
    }

}

window.addEventListener('load', () => new Sortable(document.querySelector('#sort')))