Intersection observer that checks if a .buy-now button is in view

intersectionobserver.js

/*
 * function to add observer
 */
function inViewport(selector, callback, options = {}) {
    const elems = document.querySelectorAll( selector );
    function observerCallback(entries, observer) {
        entries.forEach(entry => callback(entry))
    };
    const observer = new IntersectionObserver( observerCallback , options );
    elems.forEach( (i) => {
       if (i) { observer.observe(i); }
    })
}

inViewport('.buy-now', 
    entry => {
      // returns true or false
      if (entry.isIntersecting) {
          document.querySelector( 'body' ).classList.add( 'buy-now-in-view' );
      } else {
          document.querySelector( 'body' ).classList.remove( 'buy-now-in-view' );
          
      }
    },
    { root: null } // use browser viewport
);