how to set tabindex using javascript

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Javascript

HTML

how can I set tabindex property for an HTML input item using javascript
I tried the following

function onPageLoad()
{
  if(condition)
  {
    document.getElementById("fieldName").tabindex =12;
    alert(document.getElementById("fieldName").tabindex);
  }
}

well , it alerts '12' and that's good , but the tabindex property doesn't work for this element , while it works for other elements that are set statically ...

thanks in advance ...