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Half of people say one, half the other. The only way to find things out is to measure them, so here we go.
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<h1><a href="/questions/7262116/jquery-selectors-efficiency" class="question-hyperlink">jQuery Selectors, efficiency</a></h1>
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<p>I have been reading more lately about the efficiency of the different selector engines. I know that jQuery uses the Sizzle engine <a href="http://net.tutsplus.com/tutorials/javascript-ajax/14-helpful-jquery-tricks-notes-and-best-practices/" rel="nofollow">this blog post</a> about some jQuery stuff mentioned that the Sizzle engine will break apart your selector into an array then parse left to right.</p>
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<p>It then, from right to left, begins deciphering each item with regular expressions. What this also means is that the right-most part of your selector should be as specific as possible — for instance, an id or tag name. </p>
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<p>My question is whether it is more efficient to run a selector with <strong>just</strong> the ID specified or the tag name as well:</p>
<pre><code>var div = $('#someId');
//OR
var div = $('div#someId');
</code></pre>
<p>Since I write my CSS in the <code>div#someId</code> form I tend to do my selectors the same way, am I causing Sizzle to perform extra work (assuming <em>QuerySelectorAll</em> is unavailable)?</p>
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<div class="user-info"><div class="user-action-time">asked <span title="2011-08-31 19:08:35Z" class="relativetime">5 mins ago</span></div><div class="user-gravatar32"><a href="/users/725851/chad"><div class=""><img src="http://www.gravatar.com/avatar/6eb2d58a613015ccde3f2002ace4e08e?s=32&d=identicon&r=PG" alt=""></div></a></div><div class="user-details"><a href="/users/725851/chad">Chad</a><br><span class="reputation-score" title="reputation score">1,734</span><span title="1 silver badge"><span class="badge2"></span><span class="badgecount">1</span></span><span title="8 bronze badges"><span class="badge3"></span><span class="badgecount">8</span></span></div></div>
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<td class="comment-text"><div><span class="comment-copy">Great question! I was thinking about asking a similar question myself.</span> – <a href="/users/238260/rice-flour-cookies" title="3666 reputation" class="comment-user">Rice Flour Cookies</a> <span class="comment-date"><span title="2011-08-31 19:10:31Z">3 mins ago</span></span></div></td>
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<td class="comment-text"><div><span class="comment-copy">For performance questions the best answer is usually to try it yourself. jsperf.com can help you benchmarking javascript snippets.</span> – <a href="/users/272031/eduardocereto" title="1046 reputation" class="comment-user">eduardocereto</a> <span class="comment-date"><span title="2011-08-31 19:10:42Z">3 mins ago</span></span></div></td>
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<h2>4 Answers</h2>
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<div class="post-text"><p>It appears that using the tag names, ie:</p>
<pre><code>var div = $('div#someId');
</code></pre>
<p>should perform faster, according to the following Study:</p>
<p><a href="http://www.componenthouse.com/article-19" rel="nofollow"><strong>jQuery Selector Performance Testing</strong></a></p>
<p><em>(Performance can vary based on browser and what it supports)</em></p>
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<div class="user-info"><div class="user-action-time">answered <span title="2011-08-31 19:11:25Z" class="relativetime">2 mins ago</span></div><div class="user-gravatar32"><a href="/users/557445/rionmonster"><div class=""><img src="http://www.gravatar.com/avatar/93a0d4a722b2fae0bf159c9283fe1aff?s=32&d=identicon&r=PG" alt=""></div></a></div><div class="user-details"><a href="/users/557445/rionmonster">Rionmonster</a><br><span class="reputation-score" title="reputation score">7,323</span><span title="11 silver badges"><span class="badge2"></span><span class="badgecount">11</span></span><span title="27 bronze badges"><span class="badge3"></span><span class="badgecount">27</span></span></div></div>
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<td class="comment-text"><div><span class="comment-copy">Article is from 2007, little outdated.</span> – <a href="/users/14104/epascarello" title="8186 reputation" class="comment-user">epascarello</a> <span class="comment-date"><span title="2011-08-31 19:13:21Z">34 secs ago</span></span></div></td>
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<div class="post-text"><p>jQuery and Sizzle optimize the <code>#id</code> selector to <code>document.getElementById(id)</code>. I think they aren't able to optimize it like this with the <code>tag#id</code>.</p>
<p>The first is faster.</p>
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<div class="user-info"><div class="user-action-time">answered <span title="2011-08-31 19:11:10Z" class="relativetime">2 mins ago</span></div><div class="user-gravatar32"><a href="/users/576875/arnaud576875"><div class=""><img src="http://www.gravatar.com/avatar/9bc38525ee93e607328ff0374481162b?s=32&d=identicon&r=PG" alt=""></div></a></div><div class="user-details"><a href="/users/576875/arnaud576875">arnaud576875</a><br><span class="reputation-score" title="reputation score">9,117</span><span title="5 silver badges"><span class="badge2"></span><span class="badgecount">5</span></span><span title="25 bronze badges"><span class="badge3"></span><span class="badgecount">25</span></span></div></div>
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<div class="post-text"><p>Tag names should definately be used with class selectors</p>
<pre><code>var div = $('div.myclass') //faster than the statement below
</code></pre>
<p>versus</p>
<pre><code>var div = $('.myclass') //slower
</code></pre>
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<div class="user-info"><div class="user-action-time">answered <span title="2011-08-31 19:11:27Z" class="relativetime">2 mins ago</span></div><div class="user-gravatar32"><a href="/users/254528/john-hartsock"><div class=""><img src="http://www.gravatar.com/avatar/74e7ac83b5de18dc1ab6b2cd6bb67cb5?s=32&d=identicon&r=PG" alt=""></div></a></div><div class="user-details"><a href="/users/254528/john-hartsock">John Hartsock</a><br><span class="reputation-score" title="reputation score 14902">14.9k</span><span title="1 gold badge"><span class="badge1"></span><span class="badgecount">1</span></span><span title="11 silver badges"><span class="badge2"></span><span class="badgecount">11</span></span><span title="40 bronze badges"><span class="badge3"></span><span class="badgecount">40</span></span></div></div>
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<div class="post-text"><p>If you are using jQuery, you can assume a browser with <code>getElementById</code>. <code>$('#someId')</code> can be converted to <code>document.getElementById('someId')</code>. <code>$('div#someId')</code> won't be, so it will be faster to lose the tag name.</p>
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