Best way to find out if element is a descendant of another

I am in the process of implementing jQuery, and taking out Prototype libraries in my codebase, and I am wondering if you could give me the best way to implement this functionality in jQuery. I am familiar with the jQuery ancestor>descendant syntax, but just want to check if an element is a descendant by true of false, like the code below: can someone give me the most efficient jQuery solution for this ?

<div id="australopithecus">
  <div id="homo-herectus">
    <div id="homo-sapiens"></div>
  </div>
</div>

$('homo-sapiens').descendantOf('australopithecus');
// -> true

$('homo-herectus').descendantOf('homo-sapiens');
// -> false

In jQuery 1.6, you can use the following code generically, e.g. targetElt and parentElt can both be DOM elements or jQuery-wrapped objects, as well as selectors:

$(targetElt).closest(parentElt).length > 0

Some of the other answers require you to refer to elements by their IDs, which isn't useful if all you have is a DOM element without an ID. Also, if you want to make sure that the targetElt is a strict descendant of parentElt (in other words, you don't want to count parentElt as its own descendant), make sure to add a targetElt != parentElt check before your call to .closest(), or use .parents().find() as Jonathan Sampson suggests.

JQuery

With jQuery >=1.4 (2010) you can use the very fast function jQuery.contains()

This static method works with DOM elements, not with jQuery elements and returns true or false.

jQuery.contains( container, descendant )

Example: To check if a element is in the document you could do this:

jQuery.contains( document.body, myElement )

Native DOM

There is also a native DOM method Node.contains() that all browsers since ie5+ supports. So you can do it without jQuery:

document.body.contains( myElement )

I would think you could take advantage of CSS style selection here, with returned length..

$('#australopithecus #homo-sapiens').length // Should be 1
$('#homo-sapiens #homo-herectus').length // Should be 0

Not exactly true/false, but checking 0/1 as a boolean should work. :)

Alternately, you could do something like $('#parent').find('#child') and check the length there.

How about

$("#homo-herectus").parents().is("#australopithecus");

You can use the is() function like so:

alert($('#homo-sapiens').is('#australopithecus *'));
// -> true

alert($('#homo-herectus').is('#homo-sapiens *'));
// -> false

$.fn.descendantOf = function(element) {
    element = $(element)[0];
    var current = this;
    var body    = document.body;
    while (current && current != element && current != document.body) {
        current = $(current).parent()[0];
    }
    if (typeof(current) == "undefined" || typeof(current) == "null") {
        return false;
    } else if (current == element) {
        return true;
    } else if (current == document.body) {
        return false;
    }
}

Example:

<div id="foo">
    <div id="bar">
        <div id="baz"></div>
    </div>
</div>

And:

$('#foo').descendantOf('#bar');  // false
$('#foo').descendantOf('#foo');  // false
$('#foo').descendantOf(document.body);  // true
$('#bar').descendantOf('#foo');  // true
$('#baz').descendantOf('#foo');  // true

You could attempt to .find() it in the Elements .children()

$("#lucy").find("#homo-erectus").length;

Or the opposite direction:

$("#homo-erectus").parents().find("#lucy").length;

Best method that I found is using Dan G. Switzer, II's method found here: http://blog.pengoworks.com/index.cfm/2008/9/24/Using-jQuery-to-determine-if-an-element-is-a-child-of-another-element

jQuery.fn.isChildOf = function(b){ 
    return (this.parents(b).length > 0); 
};

Then you would just use the plugin as:

$('homo-sapiens').isChildOf('australopithecus');
// -> true

$('homo-herectus').isChildOf('homo-sapiens');
// -> false

An alternative to closest() that uses (almost) the same traverse principle and doesn't include the element itself: child.parentsUntil(ancestor).last().parent().is(ancestor).

var child = $('#homo-sapiens');
var ancestor = $('#australopithecus');

console.log(child.parentsUntil(ancestor).last().parent().is(ancestor)); // true

function descendantOf(parentId, childId) {
   return ( $('#'+parentId+' > #'+childId).length === 1 );
}

That should work.

As was pointed out in the comment below, if you don't want it to be just direct descendants:

function descendantOf(parentId, childId) {
   return ( $('#'+childId, $('#'+parentId)).length === 1 );
}

Supposing to rewrite your initial statement in:

$('#homo-sapiens').descendantOf('#australopithecus');

try to plugin:

(function($) {
    $.fn.descendantOf = function(parentId) {
        return this.closest(parentId).length != 0;
    }
})(jQuery)