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copied!<p>The <code>callback && function() {callback.call(...)};</code> is doing two things:</p> <ul> <li>testing whether a <code>callback</code> has been defined and</li> <li>if it has, calling it in the correct context</li> </ul> <p>JavaScript has a feature called <em>"short-circuiting logical expressions"</em>. If a part of a logical expression cannot change the overall result, it is not going to be evaluated.</p> <p>If <code>callback</code> is <code>undefined</code>, then it evaluates to false in the context of a logical expression (it's <em>"falsy"</em>). So the expression becomes the equivalent of <code>false && ...</code>, in which case it does not matter anymore what <code>...</code> actually is, the result will always be false <sup>†1</sup>. JavaScript is not looking at the <code>...</code> in this case.</p> <p>If <code>callback</code> is <em>not</em> <code>undefined</code> (but a function), the first part of the logical expression evaluates to true (it's <em>"truthy"</em>). At this point JavaScript evaluates the <code>...</code>. The overall result happens to be a function expression. The function expression results in an anonymous function, which is then passed to jQuery's <code>animate()</code>.</p> <p>The <code>callback.call()</code> executes <code>callback</code> <em>and</em> determines the meaning of <code>this</code> within <code>callback</code>. So <code>callback.call(this)</code> makes sure that <code>this</code> refers to the same object as in the outer function.</p> <hr> <p><sup>†1</sup> <em>To be absolutely correct: The overall result of the expression will not be <code>false</code>, it will be <code>undefined</code>.</em></p>

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