javascript - How is the jQuery selector $('#foo a') evaluated?

Question

As a example of jQuery code (https://coderwall.com/p/7uchvg), I read that the expression $('#foo a'); behaves like this:

Find every a in the page and then filter a inside #foo.

And it does not look efficient.

Is that correct? And if yes, how should we do that in a better way?

Answer 1

That is correct - Sizzle (jQuery's selector engine) behaves the same way as CSS selectors. CSS and Sizzle selectors are evaluated right-to-left, and so #foo a will find all a nodes, then filter those by nodes that descend from #foo.

You improve this by ensuring that your leaf selectors have a high specificity, usually by giving them a class or ID.