jQuery event when an element enters the user's view? How to run this script when an element enters the user's field of view?

Question

JavaScript

$('.skill-per').each(function(){
        var $this = $(this);
        var per = $this.attr('per');
        $this.css("width",per+'%');
        $({animatedValue: 0}).animate({animatedValue: per},{
          duration: 1000,
          step: function(){
            $this.attr('per', Math.floor(this.animatedValue) + '%');
          },
          complete: function(){
            $this.attr('per', Math.floor(this.animatedValue) + '%');
          }

        });

      });