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copied!<p>Although jQuery objects act like arrays, they are actually only <em>array-like</em> objects. An array-like object is an object using numeric keys and having a <code>length</code> property - that is the minimum needed for compatibility with the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#Methods" rel="noreferrer">native array methods</a>.</p> <p>Because jQuery objects are only array-like and not actual <code>Array</code> objects, native array operations (like <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf" rel="noreferrer"><code>indexOf</code></a> or <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reverse" rel="noreferrer"><code>reverse</code></a>) cannot be called directly. You can use <code>Array.prototype</code> though, or extend jQuery's functionality.</p> <pre><code>$('div').reverse(); // TypeError: $("div").reverse is not a function // we can use Array.prototype though Array.prototype.reverse.apply($('div')); // or we can extend jQuery very easily $.fn.reverse = Array.prototype.reverse; $('div').reverse(); // now it works! </code></pre> <p>You are correct in your assumption that Firebug does not include any special-casing for formatting jQuery objects. A quick search reveals <a href="http://www.mail-archive.com/firebug@googlegroups.com/msg01548.html" rel="noreferrer">a relevant post</a> on the Firebug mailing list. Assuming the information is still correct (the post is from January) Firebug will format an object as an array if it has a <strong>finite length</strong> and a <strong><code>splice</code> method</strong>.</p> <p>JQuery fulfils both of these criteria, but <a href="https://github.com/jquery/jquery/blob/fef476def30f4b1f3cfa902fff11bb761ab6ecfd/src/core.js#L273" rel="noreferrer">their implementation of <code>splice</code></a> is nothing more than a direct copy of the native <code>Array</code> method. It is undocumented, which means it's either only for internal use, or perhaps added solely for the purpose of tricking Firebug into formatting jQuery objects nicely.</p>

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