SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/configure-multipage-forms

MultiPageFormSubscribe.js

var formContainer = $('[data-sf-role="form-container"]')[0];
var formElement = $(formContainer);

formElement.on("form-page-changed", function (e, index, previousIndex) {
  // Your code goes here
});