How do I select class within a class or a tag using Javascript getElementsByClass() function?

In jQuery it would simply be

$("a.class").randommethod();

or

$(".class1 .class2").randommethod();

How do I achieve the same effect using pure Javascript? (I'm editing a open-source software for personal use and it does开发者_如何学编程n't use jQuery)

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You can't do it using getElementsByClassName. You would need to use something more like querySelectorAll, which accepts CSS style selectors.

var result = document.querySelectorAll('.class1 .class2');

You should note that neither method has full cross-browser support. That's why people use javascript libraries. If you don't need to support older browsers, then querySelectorAll is a good choice.

Another option is to use the Sizzle selector engine that jQuery uses.

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You'll need to write your own selector engine, or use an established one such as sizzle (http://sizzlejs.com/), which is used by jQuery.