Example of onstepskip event

event-onstepskip.js

var element = document.querySelector('.mdl-stepper');
// Get the MaterialStepper instance of element.
var Stepper = element.MaterialStepper;
// Select any optional step. 
var optionalStep = element.querySelector('.mdl-step.mdl-step--optional');

optionalStep.addEventListener('onstepskip', function (event) {
  // When the step (this) action button/link with [data-stepper-skip] attribute is clicked
  // you can call the skip() method or do anything else.
  var skipped = Stepper.skip(); 
  
  if (!skipped) {
    alert('This step cannot be skipped');
  }
});