Develop Reference

.eq() out of order

I'm trying to select the 3rd element of a JQuery object, by using eq() method. But for some reason the 2nd and 3rd selections pop out in changed order:
var selection = $("[name=input0], [name=input1], [name=input2], [name=input3]");
selection.eq(1); //turns out to be input2!!
What could be the reasons for this behavior? Can I trust acessing it by index in my script?

According to:
https://api.jquery.com/multiple-selector/
The order of the DOM elements in the returned jQuery object may not be identical, as they will be in document order.

With the help of the answers and comments and a bit of reading on the documentation.
Turns out that jQuery selects elements in the order they appear in the DOM (HTML), so:
Using a direct array access to a jQuery selection will work if you use that order, but you can only trust it if you are also responsible for the HTML, and you would need to remember this if you are ever going to change the layout.
Bottom line: not the best way to select a specific element.

Finding the number of options in multiple selects that have the same IDs

Firstly, I do know that it is against best practice for multiple elements to have the same ID in a single page. However in this case I need for there to be two selects with the same id.
I have seen some success using this method:
$('#undo_redo_to option').length;
$('#undo_redo_to:eq(0) option').length;
*$('#undo_redo_to:eq(1) option').length;*
However, the code enclosed in *'s does not give me the proper length.
Please see the following pen, where I have created my selects and did my debugging.
LINK TO CODE
Thanks!

The problem is that jQuery uses the native JS functions first, so when you do a $('#this_id') you are effectively calling document.getElementById('this_id'). it's not just against best practice, it actually won't work... you could loop through your selects and check the id:
$('select option').each(function(){
if($(this).closest('select').attr('id')=='my_id'){
//do something
}
});

Assuming you've inherited the HTML and cannot change the ids to classes:
Note that eq() is a jQuery selector. If you change it to nth-child(2), you'll be using a CSS selector. That gives you what you need:
$('#undo_redo_to:nth-child(2) option').length;
http://codepen.io/anon/pen/WwERGW?editors=1011

You can do it with native javascript, like this:
document.querySelectorAll("[id=undo_redo_to]")
You can do it with jquery like this:
$("[id=undo_redo_to]")
This is the property selector, as the #-type selector will stop searching when the first element having that id was found.
So you can do this using plain javascript or jquery, but don't do it, because id should be unique and if it is not unique, then the HTML is invalid and it is bad for SEO.

Using filter() to filter jquery based on parents

I see on jquery documentation that I can use .parent() to filter my matched elements based on the parent. But in the process, the final result I get is the set of parent elements, not the original set of elements. So I see that I can use filter to achieve what I want. But I found so few documentation about how to use filter to filter based on the parent.
For example, my html is:
<div id="social">
Facebook<br/>
Twitter<br/>
</div>
<div id="topsites">
Facebook<br/>
Stack Overflow<br/>
</div>
I want to get a set of elements, which consist of <a> tag that has facebook in it's href attribute, but within the social div parents.
I suspect the code will be something like this:
$('a[href*="facebook"]').filter( ... ).click(function() {
});
But I have absolutely no idea what to put on the filter. "parent#social" ?
Another way to put it is to use filter function.
$('a[href*="facebook"]').filter(function(index) {
return ...
}
.click(function() {
});
I also don't know what code to put on the ... . Is it something like this.parent.id == "social" ? If possible, I prefer the first form, but if the solution can only be achieved by using the second form (filter function) then it's okay. Thank you very much.

.parent() doesn't filter, it traverses. It takes a jQuery object, that lists an array (of size [0, n) ), and generates a new jQuery object with each element's parent.
Getting a jQuery object with the list you're looking for is much simpler though...
CSS Selectors, which jQuery is based upon (with various extensions) are heirarchical by nature. That means selecting specific children of some parent(s) is quite trivial. a CSS selector to pick the element you want is:
#social a[href*="facebook"]
and if you use this inside a jQuery constructor, you'll get you object:
$('#social a[href*="facebook"]')

I want to get a set of elements, which consist of <a> tag that has facebook in it's href attribute, but within the social div parents.
No need of filter here,
$('#social a[href*="facebook"]')
will do.

How to access elements from a certain form and certain type with jquery

so I have a page with multiple forms and I would like to be able to access input elements of type="datetime" within a particular form. I know I can do $('input[type="datetime"]') but that would give me all input tags on the page. I also cannot use the "form" attribute because not all browser use it (Sigh IE). Worse scenario for me is to do something like:
$(document.forms["..."].elements).each(function() {
if (this.type="datetime") {.....}
});
but I am sure this can be done in jQuery with one selector. Can someone tell me how do this with one selector?

Add id to your form and then select DOM inside of that form as below.
$('#form input[type="datetime"]')

Without seeing some HTML this is just a shot in the dark. But if you give your forms an id you can do:
$("#yourFormId input[type='datetime']");
If you do not have ids, but you know the number, then this might do it:
$("form:eq(4) input[type='datetime']");

There are multiple ways to do it
Solution 1.
use descendant selector
ex:
$('#yourform input[type="datetime"]') //or
$('.yourform input[type="datetime"]') //or
$('form:eq(3) input[type="datetime"]')
Solution 2:
Use context based look up
Ex:
$('input[type="datetime"]', yourform)

jQuery each() rewritten?

I've written a plugin for some divs. textbox is the class of divs and deactivateTextBox() is the plugin.
I need to deactivate all text boxes on the page. Are the following two pieces of code both doing this? And is the second one more efficient?
Code:
$(".textbox").each(function(){
$(this).deactivateTextBox();
});
$(".textbox").deactivateTextBox();

Yes, they both do the same thing.
Yes, the second should be more efficient as the former gets all elements, passes the raw DOM element to the loop, and then re-wraps the element with the jQuery object.