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Editorial Team
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Asked: 2026-05-11T22:12:57+00:00

I was looking at the jQuery UI code, and I found that every file

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I was looking at the jQuery UI code, and I found that every file starts with a construct like this:

jQuery.ui || (function($) {

My question is: why is there a semicolon before jQuery, and why is the logical OR being done?

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  1. Editorial Team

    why is there a semicolon before jQuery?

    The semi-colon is there to ensure safe file concatenation. (libraries and library components are frequently packed into a single file)

    why is the logical OR being done?

    The self-invoking anonymous function on the right hand-side will only run if the left-hand-side of the statement evaluates to a falsey value. So if jQuery.ui already exists on the page then the function will not run. It only runs when jQuery.ui does not yet exist.

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