code: function inViewport(el, options = { tolerance: 0 }) {
      if (!el) {
          throw new Error('You should specify the element you want to test');
      }
  
      if (typeof el === 'string') {
          el = document.querySelector(el);
      }
  
      const elRect = el.getBoundingClientRect();
  
      return (
          // Check bottom boundary
          elRect.bottom - options.tolerance > 0 &&
  
          // Check right boundary
          elRect.right - options.tolerance > 0 &&
  
          // Check left boundary
          elRect.left + options.tolerance < (window.innerWidth ||
          document.documentElement.clientWidth) &&
  
          // Check top boundary
          elRect.top + options.tolerance < (window.innerHeight ||
          document.documentElement.clientHeight)
      );
  }
docs: Checks an element's position in respect to the viewport
  and determines wether it's inside the viewport.
  
  @param {node} element The DOM node you want to check
  @return {boolean} A boolean value that indicates wether is on or off the viewport.