properties-ce.js

class MyCarousel extends HTMLElement {  
    static get observedAttributes() {
        return ['index'];
    }
    // Called anytime the 'index' attribute is changed
    attributeChangedCallback(attrName, oldVal, newVal) {
        this[attrName] = newVal;
    }
    // Takes an index value
    set index(idx) {
        // First check if it is numeric
        const numericIndex = parseInt(idx, 10);
        if (isNaN(numericIndex)) {
            return;   
        }
        // Update the internal state
        this._index = numericIndex;
        /* Perform the associated DOM operations */
        moveCarousel();
    }
    get index() {
        return this._index;
    }
}