Jquery animate not working with variables

Question

I am trying to animate a small circle inside a box, using Jquery animate left function. It is working fine when I pass harcoded values. Like shown below, 474 is the resultant if I subtract circle width from box width.

    
        $(document).ready(function(){
             $("#start_a").toggle(function(){
                if ($("#start_a").html() === 'Roll'){
                    $("#start_a").html("Stop"); 
                    function start(){
                        $("#circle").animate({left: "+=474"},"slow",function(){
                            $("#circle").animate({left: "-=474"},"slow",function(){
                                start();
                            });
                        }); 
                    }
                    start();
                }
             },    
             function(){
                if ($("#start_a").html() === 'Stop'){
                    $("#start_a").html("Roll"); 
                    $("#circle").stop();
                    $("#circle").css("left", "initial");
                }
             });
        });

To make it dynamic I am trying to do the same animation by passing variables as shown below. (Only pasting new Jquery).

$(document).ready(function(){
  
           var roll = $("#box").width() - $("#circle").width();
                 var effect = {};
                 var reverse = {};
                 effect['left'] = roll;
                 reverse['left'] = -roll;
                 $("#start_a").toggle(function(){
                    if ($("#start_a").html() === 'Roll'){
                        $("#start_a").html("Stop"); 
                        function start(){
                            $("#circle").animate(effect,"slow",function(){
                                $("#circle").animate(reverse,"slow",function(){
                                    start();
                                });
                            }); 
                        }
                        start();
                    }
                 },    
                 function(){
                    if ($("#start_a").html() === 'Stop'){
                        $("#start_a").html("Roll"); 
                        $("#circle").stop();
                        $("#circle").css("left", "initial");
                    }
                 });
            });

Circle is not animating as expected,it is going beyond the box border. Can someone tell me what might be the issue? Thanks in advance!

Below is my HTML/CSS

<div id="box">
            <div id="circle"></div>           
        </div>
        <div id="box2">
            <button id="start_a" >Roll</button>
<!--            <button id="stop_a">Stop</button>-->
        </div>

<style>    
        #circle {
	    width: 20px;
	    height: 20px;
	    background: black;
	    border-radius: 50px;
            left: 0px;
            position: absolute;
        } 
        
        #box{
            border: 3px solid green;
            background-color: white;
            margin-left: 400px;
            margin-top: 100px;
            width: 494px;
            min-height: 300px;
            position: relative;
        }
        #box2{
            /*border: 3px solid red;*/
            margin-left: 400px;
            /*margin-top: 100px;*/
            width: 494px;
            min-height: 50px;
            position: relative;
        }
        #start_a{
            margin-left: 200px;
            top: 0px;
            width: 100px;
            height: 50px;
            text-align: center;
            position: relative;
        }
        #stop_a{
            /*margin-left: 720px;*/
            /*top: 0px;*/
            width: 100px;
            height: 50px;
            text-align: center;
            position: relative;
        }
        </style>

Could you give us the corresponding HTML / CSS. A JSFIDDLE would be helpful as well. — Matt Greenberg
– Matt Greenberg, Commented Dec 4, 2015 at 7:17

P. Frank · Accepted Answer · 2015-12-04 08:56:09Z

2

Ok please try:

$(document).ready(function(){
  
  var roll = $("#box").width() - $("#circle").width();
  var effect = {};
  var reverse = {};
  effect['left'] = roll;
  reverse['left'] = 0;
  $("#start_a").click(function(){
    if ($("#start_a").html() === 'Roll'){
      $("#start_a").html("Stop"); 
      start();
    }else{
      $("#start_a").html("Roll"); 
      $("#circle").stop();
      $("#circle").css("left", "initial"); 
    }
  })   

  function start(){
    $("#circle").animate(effect,"slow",function(){
      $("#circle").animate(reverse,"slow",function(){
        start();
      });
    }); 
  }
});

#circle {
	    width: 20px;
	    height: 20px;
	    background: black;
	    border-radius: 50px;
            left: 0px;
            position: absolute;
        } 
        
        #box{
            border: 3px solid green;
            background-color: white;
            margin-left: 100px;
            margin-top: 100px;
            width: 494px;
            min-height: 300px;
            position: relative;
        }
        #box2{
            /*border: 3px solid red;*/
            margin-left: 400px;
            /*margin-top: 100px;*/
            width: 494px;
            min-height: 50px;
            position: relative;
        }
        #start_a{
            margin-left: 0;
            top: 0px;
            width: 100px;
            height: 50px;
            text-align: center;
            position: relative;
        }
        #stop_a{
            /*margin-left: 720px;*/
            /*top: 0px;*/
            width: 100px;
            height: 50px;
            text-align: center;
            position: relative;
        }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="box">
  <div id="circle"></div>           
</div>
<div id="box2">
  <button id="start_a" >Roll</button>
</div>

edited Dec 4, 2015 at 8:56

answered Dec 4, 2015 at 8:09

P. Frank

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4 Comments

sri Over a year ago

@Frank - I just tried your code. It is giving same result as mine. Can you please recheck.

P. Frank Over a year ago

ok i have update my answer. Your reverse array width must be the initial position. When you add -roll the position is -474 and no 0. Please try now

sri Over a year ago

This is working. But please help me understand, why -474 while hard coding and why 0 while using widths.

P. Frank Over a year ago

Because with hard coding your element posA is 474px and you precises after posB -474px. so posA - posB (474 - 474 = 0) but with your code you precises posA is 474px and posB is (-posA), so your element go to 474px to -474px. Please is work for you solved it, thanks.

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