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Editorial Team
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Asked: 2026-06-14T21:58:14+00:00

I’m trying to set a background-image in a div to change every second or

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I’m trying to set a background-image in a div to change every second or two with the setTimer(); I set it up and it’s saying that I have 

  'Invalid left-hand side in assignment'

Not sure what I’m doing that’s wrong. Can someone help out please?

  <html><body>
  <div class="box">
   <div id="b1"></div>
  </div>
  <input id = 'spin' type="button" value="Spin" onclick='changeIcons()' />
  <script>
  var arr = new Array();
  arr[0] = '1.png';
  arr[1] = '2.png';
  arr[2] = '3.png';
  var first = document.getElementById('b1');

  function changeIcons(){
   for(var spinIcons= 0; spinIcons < 10; spinIcons++){
   arr.sort(function() {return 0.5 - Math.random()});
   var v1 = setTimeout(function(){first.style.background-image= 'url(arr[0])'},   1000);
}
  }
 </script> 
 </body>
 </html>

THIS FINALLY ENDED UP WORKING, IT HAS TIMING ISSUES AND i NEED TO MAKE A FRAMED PLACE HOLDER FOR THE BACKGROUNDIMAGES BUT IT’S WORKING:

     <div class="first"></div>
     <script>
     for(var i=0; i<11; i++){
        $("div.first").slideUp(300).delay(100).fadeIn(400);
     }
     </script>

THANKS TO T.J.CROWDER FOR SETTING ME STRAIGHT!!!

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1 Answer

  1. Editorial Team

    There are two problems.

    The one causing the error you mention is here:

    //                                        here ------v
var v1 = setTimeout(function(){first.style.background-image= 'url(arr[0])'},   1000);

    That makes the engine think you’re trying to assign to the result of an expression (background - image), which is invalid on the left-hand side of an assignment. In JavaScript, property names can’t have dashes in them. You can use the camelCase version backgroundImage instead.

    Separately, you’re always setting the background image to the string 'url(arr[0])', not putting the value from arr[0] in the string. You need to adjust the quotes:

    So:

    var v1 = setTimeout(function(){first.style.backgroundImage= "url(" + arr[0] + ")"},   1000);

    Some other notes:

    • You’re setting up the setTimeout in a loop, so you’re scheduling this to happen 10 times. But those 10 times will all happen one right after another, one second later, and they’ll all do exactly the same thing: Set the background image to whatever’s in arr[0] at that time. The fact you’re changing the order of the array 10 times previously makes no difference, because the function doesn’t receive a copy of the value of arr[0] as of when you schedule it, it receives an enduring reference to the array, which it uses when it runs. More: Closures are not complicated

    • Sorting the array and using index 0 seems a fairly inefficient way to pick from the array randomly.

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