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How can I use setAttribute on an input without an ID or class attribute?

I have a search input tag that is being added by a jQuery plug-in:
<input type="search" />
Note that this does not have an ID, CLASS, or NAME. I need the search input tag to look like this:
<input type="search" name="myname" />
A simple solution is for me to update the jQuery plug-in. However, I do not want to do this as it will cause challenges when I upgrade this plug-in in the future.
This JavaScript works properly and adds the name attribute:
$(document).ready(function() {
document.getElementsByTagName("input")[0].setAttribute("name", "myname");
});
The problem is that the "[0]" in this function relies on the search input being the first input field in the form. I do not think this solution is sustainable.
There are other inputs in the form. This is the only one with the type attribute equal to "search." Is there a way to identify it by this attribute? Or, is there another solution you propose?
Thank you for your time!

You can use the document.querySelector:
document.querySelector("input[type='search']")
Below is an example (you can inspect the output to see name attribute):
document.querySelector("input[type=search]").setAttribute("name", "myname");
<input type="search" value="foo" />
<input type="bar" value="bar" />

You can target a selection by anything. So, the selector input[type="search"]' will work.
If you want to apply this to all input's of type search, this is good enough, and you get all of them in here:
$('input[type="search"]')
This works without jQuery too:
document.querySelectorAll('input[type="search"]')
A more targeted approach would be
document.querySelectorAll('div.filter input[type="search"]')

The for-attribute on labels in jQuery forms

Regarding forms with labeled fields, this (out-dated) jQuery Mobile documentation states:
Be sure to pair them properly with label elements via the for attribute.
The for attribute still exists in the latest version, but the current documentation does not even mention it anymore. What is it used for? Some sort of form validation?

It's standard HTML (not jQm-specific) used to link text with a corresponding input element.
MDN Docs for <label>
Example:
<label for="your_name">Your Name:</label>
<input id="your_name" value="John Smith" />
When I tap on the label, the input text will gain focus. The for attribute should be set to the ID of the input element.

What is document.f.q.focus?

What is document.f.q.focus?
Is this a java script code or not
whether I can use document.f.id.value?
what is the difference between this and document.getElementbyID()

The HTML looks like this:
<form name="f">
<input name="q" />
</form>
In such a case, document.f refers to the form, and .q refers to the input element of that form. .focus() places the focus on that input.
It's worth noting that such code is unnecessary now that HTML5 is around:
<input name="q" autofocus />

It needs a form to make it work, Try this:
<form name="f">
<input name="q" value="test" type="text"/>
</form>
javascript:
document.f.q.focus();
document.f.q.value = 1;
Here is DEMO

The id attribute inside a html is meant to be unique.name can be an array(file[]) and with html5 should be used only on form elements.
html5 removed the support of the name atrribute on most elements except form elements.
id and name are 2 different things.
document refers to the whole html inside a page.
to get an element by it's id you need to call document.getElementById(id);
to get an element by it's name (considering html5) so inside a form
you call document.forms[0].name. form[0] refers to the first form inside the document
In your case the form has also a name so appart from html5 the code is correct.
form is called f,input is called q. thats why document.f.q returns the input field.
if you want to add an id to your input field then you have to add an id:
<input name="q" id="q">
to get the element:
document.getElementById('q');
to return the content:
document.getElementById('q').value;
And focus(); is a native function that points the focus to the choosen element.
In your case when you load the page you will see the blinking pointer inside the searchflied.

Setting the value of multiple inputs with the same id using jQuery?

Considering the following HTML:
<form id="upvoteForm" method="post" action="/post/upvote">
<input type="text" name="post_id" id="post_id"/>
</form>
<form id="downvoteForm" method="post" action="/post/downvote">
<input type="text" name="post_id" id="post_id"/>
</form>
<input type="hidden" id="_postid" value="1"/>
I'm trying to set the two input fields with the name post_id to to value from _postid using this JavaScript and jQuery:
$(document).ready(function() {
$('#post_id').val($('#_postid').val());
});
However, as you can see in this jsFiddle, it's only setting the value of the first one. How do I set the value of both of them? I thought the selector would end up grabbing both.
Now, I realize you might be wondering why I have two forms on this page. The basic reason is I have button inputs that I've styled the way I want but then I use the onclick to call the submit of the appropriate form here. I am ultimately going to be leveraging AJAX here, but that's coming later.

id is always unique. you cannot select 2 elements with same id. select by name
$(document).ready(function() {
$('input[name=post_id]').val($('#_postid').val());
});

Having two HTML elements with the same ID is illegal and will cause undefined behavior such as what you're experiencing. Using the same name is valid, however. Therefore you could use a selector like $('form > input[name=post_id]'), which would look for an input inside of a form with the name attribute set to post_id.

Setting a label's for attribute to a DOM element, not an ID in jQuery

Short Question:
How do you link a label element to an input element without using the input element's id using jQuery and javascript?
Long Question:
I am using jQuery to clone a form with possibly more than one instance of the form being available for the user to fill in.
A label's 'for' attribute is supposed to be set to the 'id' attribute of the input element that it is for. This works when the input element has a unique id.
Because I am cloning the same input element there will be multiple input elements with the same id in the document. Therefore I'm avoiding having id attributes for input elements but I'd still like to focus on the input element when the label is clicked. I also want to avoid generating random ids for fields or setting onclick events on labels.
Edit #1
Example mark up (note no ids)
<form>
<label>First Name:</label><input type='text' name='FirstName' /><br/>
<label>Last Name:</label><input type='text' name='LastName' /><br/>
</form>
Example cloning code:
var newForm = $('form').clone();
$(newForm).find('label').each(function(){
var inputElement = $(this).next('input');
// I'd love to set the label's for attribute to an element
$(this).attr('for', inputElement);
});
$(document).append(newForm);
Edit #2
There currently are three options:
Set onclick events for labels to focus on the input field they're for. Criteria for deciding which labels are for which inputs can be the next input element or something else
Embed the input fields in the label fields (might not be possible due to designer's choices)
Generate random ids while cloning each form

Well it would be nice to see the markup, but if i can assume that the markup will look somewhat like this
<form name="f1">
<label>this is my label</label>
<input />
<label>this is my other label</label>
<input />
</form>
<form name="f2">
<label>this is my label</label>
<input />
<label>this is my other label</label>
<input />
</form>
then you could do something like this
$('form label').live('click',function(){
$(this).next('input').focus();
});
you will need to use live or delegate since you're cloning the forms on the fly i'm assuming.

The simplest solution is to move the <input> tags inside the <label> tags and forgo the for attribute altogether. Per the HTML spec, <input> tags without for attributes are implicitly associated with their contents.
Try this:
<form>
<label>First Name: <input type='text' name='FirstName' /></label><br/>
<label>Last Name: <input type='text' name='LastName' /></label><br/>
</form>
(See: http://www.w3.org/TR/html401/interact/forms.html#h-17.9.1)

You shouldn't have multiple identical ids in the page. It defeats the purpose of the id attribute and is against the W3C spec.
Regardless, jQuery's $(this) could help you in this situation. Say you gave all your the "focusable" class. Then you could do:
$('.focusable').focus( function(){
$(this).doSomething();
});

This is really an HTML question. A label can be associated wtih a form control either by its for attribute having the same value as the associated control's id attribute, or by having the control as a child of the label, e.g.
<form ...>
<label for="nameField">Name:<input id="nameField" name="nameField" ... ></label>
<label>email:<input name="emailField" ... ></label>
</form>
I suppose in jQuery you need something like:
var labelAndInput = $('<label>text<input ... ></label>');
or whatever. Note that older versions of IE (and maybe more recent ones too) the label will not be associated with the control without the for attribute (or htmlFor property), there is no other way.