code: function _nextStep() {
      this._direction = 'forward';
  
      if (typeof this._currentStepNumber !== 'undefined') {
          for (var i = 0, len = this._introItems.length; i < len; i++) {
              var item = this._introItems[i];
              if (item.step === this._currentStepNumber) {
                  this._currentStep = i - 1;
                  this._currentStepNumber = undefined;
              }
          }
      }
  
      if (typeof this._currentStep === 'undefined') {
          this._currentStep = 0;
      } else {
          ++this._currentStep;
      }
  
      if (typeof this._introBeforeChangeCallback !== 'undefined') {
          var continueStep = this._introBeforeChangeCallback.call(this);
      }
  
      // if `onbeforechange` returned `false`, stop displaying the element
      if (continueStep === false) {
          --this._currentStep;
          return false;
      }
  
      if (this._introItems.length <= this._currentStep) {
          //end of the intro
          //check if any callback is defined
          if (typeof this._introCompleteCallback === 'function') {
              this._introCompleteCallback.call(this);
          }
          _exitIntro.call(this, this._targetElement);
          return;
      }
  
      var nextStep = this._introItems[this._currentStep];
      _showElement.call(this, nextStep);
  }
docs: Go to next step on intro
  
  @api private
  @method _nextStep