Example of onstepnext event

event-onstepnext.js

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

step.addEventListener('onstepnext', function (event) {
  
  // When the step (this) action button/link with [data-stepper-next] attribute is clicked
  // you can do some condition to advances and error or do anything else.
  if (true) {
    // Your conditionally stepper advances.
    Stepper.next();
  } else {
    myStepper.error('Alert message here');
  }  
});