let's say I have a markup like this:

<div id="foo">
  ...
  <span id="moo">
    ...
  </span>
  ...
</div>

and I want to s开发者_开发技巧elect #moo.

why $('#foo').find('span') works, but $('span', $('#foo')); doesn't ?

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You can use any one these [starting from the fastest]

$("#moo") > $("#foo #moo") > $("div#foo span#moo") > $("#foo span") > $("#foo > #moo")

Take a look

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Actually, $('#id', this); would select #id at any descendant level, not just the immediate child. Try this instead:

$(this).children('#id');

or

$("#foo > #moo")

or

$("#foo > span")

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You can use find option to select an element inside another. For example, to find an element with id txtName in a particular div, you can use like

var name = $('#div1').find('#txtName').val();

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Why not just use:

$("#foo span")

or

$("#foo > span")

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$('span', $('#foo')); works fine on my machine ;)

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Have a look here -- to query a sub-element of an element:

$(document.getElementById('parentid')).find('div#' + divID + ' span.child');

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....but $('span', $('#foo')); doesn't work?

This method is called as providing selector context.

In this you provide a second argument to the jQuery selector. It can be any css object string just like you would pass for direct selecting or a jQuery element.

eg.

$("span",".cont1").css("background", '#F00');

The above line will select all spans within the container having the class named cont1.

DEMO

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both seem to be working.

see fiddle: http://jsfiddle.net/maniator/PSxkS/