Example of onstepback custom event

event-onstepback.js

var stepperElement = document.querySelector('.mdl-stepper');
// Get the MaterialStepper instance of element.
var Stepper = stepperElement.MaterialStepper;
// Select any step other than the first (for obvious reasons).
var step = element.querySelector('.mdl-step:nth-child(2)');

step.addEventListener('onstepback', function (event) {
  // When the step (this) action button/link with [data-stepper-back] attribute is clicked
  // you can call the back() method or do anything else.
  Stepper.back();
});