Using a mutation observer to watch for attribute changes

MutationObserver-attributes.js

(function() {

  // Select the node to observe
  var targetNode = document.querySelector('.sqs-add-to-cart-button');
  // The class we'll watch for
  var className = 'cart-added';

  // If the targetNode exists on our page
  if(targetNode){

    // Observe attributes, specifically the class attribute
    // We don't care about descendant nodes
    var config = {
      attributes: true,
      attributeFilter: ['class'],
      subtree: false
    };

    // When a mutation is observed
    var callback = function(mutationsList) {
      for(var mutation of mutationsList) {
        // If the classList contains the className we want, go to the cart page
        if(mutation.target.classList.contains(className)){
          window.location.href = '/cart';
        }
      }
    };

    // Create an new observer
    var observer = new MutationObserver(callback);

    // Start observing
    observer.observe(targetNode, config); 
  }

})();