how do I write a `is_a_last_of_b(a, b)` to check whether `a` is the last child of `b`

a = $('div:first');
b开发者_如何学JAVA = $('div');

b may contain multiple DOM elements.

But a only contains 1 element.

How do I know if a is the last element of b?

I tried a == $('div:last'),but it's wrong after my test.

And if a is an element of b, how do I get the sibling of a within b?

Clarify

the problem is how I can write a is_a_last_of_b(a, b) to check whether a is the last child of b.

And if a is a direct child of b,how to write next_sibling(a) to return the next sibling of a(only one)?

Can try this

var elements = $("div");
var lastElement = elements.filter(":last");
var firstElement = elements.filter(":first");

For sibling check this out jquery siblings

Maybe you need last-selector and siblings?

Ok, to compare jquery objects, you can look at some other questions on SO like this one: jQuery object equality

Essentially, in your case where you only want to check for one item you could do this:

var a = $("div:first"),
    b = $("div");

if (b[b.length - 1] === a[0]) {}

// or

if (b.last()[0] = a[0]) {}

Then, to get the next sibling of an element:

b = b.next();