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Editorial Team
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Asked: 2026-06-04T05:18:14+00:00

Not sure if this is the correct place to post, But looking at the

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Not sure if this is the correct place to post, But looking at the HTML5 boilerplate I have…

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.7.2/jquery-ui.js"></script>
<script>window.jQuery || document.write('<script src="_includes/js/libs/jquery-1.6.2.min.js"><\/script>')</script>

Now I’ve read the second script is a fall back for if the first is down, but how?
I always thought || was an OR operator and to me it just looks like it says when the page loads write an old jQuery lib to the page?

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1 Answer

  1. Editorial Team

    The second half of the || operator is only evaluated if the left hand side is “falsey”. This is known as “short circuit evaluation” *

    So when window.jQuery is undefined, it does the document.write().

    * true || x == true, regardless of x’s value, so a short circuit || operator given true for its left hand operand will skip evaluating the right hand of the expression since it doesn’t change the result.

    Conversely false && x == false.

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