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Editorial Team
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Asked: 2026-05-15T19:53:57+00:00

Is it true that there are no guarantees across major browsers that the following

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Is it true that there are no guarantees across major browsers that the following script tags will always execute both sequentially AND in order of declaration? i.e. should I assume that the following code will not always yield x == ‘ab’ in alert?

<head>
    <script type="text/javascript">
      //tag A
      var x = x || ''; x += 'a';
    </script>
    <script type="text/javascript">
      //tag B
      var x = x || ''; x += 'b';
    </script>
</head>
<body>
    <script type="text/javascript">
       alert('x='+x);
    <script>
</body>

… and it’s possible that x will instead be one of the following:

  1. ‘ba’ – if tag B executes before A
  2. ‘a’ or ‘b’ – race condition where A and B execute in parallel (Though seems like this thread clearly says that browsers only allocate a single thread of JS)
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1 Answer

  1. Editorial Team

    The execution order of these non-dynamically added script tags should be purely sequentially in every browser:

    Snippet from this link:

    JavaScript statements that appear
    between <script> and </script> tags
    are executed in order of appearance;
    when more than one script appears in a
    file, the scripts are executed in the
    order in which they appear.

    However, things could change as soon as you’re:

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