我从谷歌的AdWords网站上获取数据，该网站有多个元素，相同的id个元素.
Could you please explain why the following 3 queries doesn't result with the same answer (2)?
HTML:
<div>
<span id="a">1</span>
<span id="a">2</span>
<span>3</span>
</div>
JS:
$(function() {
var w = $("div");
console.log($("#a").length); // 1 - Why?
console.log($("body #a").length); // 2
console.log($("#a", w).length); // 2
});
根据W3C规范，具有2个具有相同ID的元素不是有效的html.
When your CSS selector only has an ID selector (and is not used on a specific context), jQuery uses the native document.getElementById method, which returns only the first element with that ID.
However, in the other two instances, jQuery relies on the Sizzle selector engine (or querySelectorAll, if available), which apparently selects both elements. Results may vary on a per browser basis.
但是，在同一个页面上不应该有两个具有相同ID的元素.如果您的CSS需要两个元素，请使用一个类来代替.
如果绝对必须按重复ID Select ，请使用属性 Select 器:
$('[id="a"]');
Take a look at the fiddle: http://jsfiddle.net/P2j3f/2/
Note:如果可能，您应该使用标记 Select 器来限定该 Select 器，如下所示:
$('span[id="a"]');