CODEWITHSUNDEEP
jqueryhtml
Liam
Liam

Reputation: 9855

jQuery document.write?

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?

Upvotes: 0

Views: 330

Answers (2)

Alnitak
Alnitak

Reputation: 339816

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.

Upvotes: 4

Rory McCrossan
Rory McCrossan

Reputation: 337560

Because javascript is a falsy language, if jQuery is not available window.jQuery will return undefined which equates to false.

Therefore, the second part of the statement will be executed, which writes the <script> tag to the document containing the local backup version of jQuery.

Upvotes: 1

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