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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"]')

Default value in an input tag at a particular position

I don't know if this is possible or not and hence here's the part. Suppose I have an input tag like the following.
<input id="expiry" name="expiry" type="text" placeholder="MM/YY">
Now what I want is that the user should already see the '/' part, i.e when he types 1212, the textbox should become 12/12 automatically, can it be done if yes , how. Thanks in advance.

For an HTML/CSS-only approach, you could use two inputs, and style them with no borders on the inside edges:
<input type="text" maxlength="2">
<span> / </span>
<input type="text" maxlength="2">
With the input and span elements set to display: inline-block; and some styling of borders, this is a strong, semantic approach with no required Javascript.

Recently I found a plugin that looks nice and I think it does what you're after, so I'm sharing it with you: cleave.js

<input id="expiry" name="expiry" type="text" placeholder="MM/YY">
<script type="text/javascript">
$("#expiry").bind('keyup mouseup', function () {
if($('#expiry').val().length ==2){
$('#expiry').val($('#expiry').val()+'/');
}
});
</script>
check the fiddle

try to use jquery mask plugin :
$(document).ready(function(){
$('#expiry').mask('00/00');
});

Auto suggest when write a character in input field using JavaScript

I have an input field. When I click on this input field and write something then it will show all the suggested words that I want.
Like: If I write 'c' then it will show suggested words that i declared 'C#','Code','C++'
HTML:
<input type="text" class="input_field" />
How can I do it using JavaScript? Thank you.

Please refer for the auto complete http://jqueryui.com/autocomplete/

You may try following. It is my favourite at least.
Twitter Typeahead JS

how to only allow text in parsely.js validation

I am using the nifty parsley js As you can see in the documentation
there is only data-parsley-type="alphanum" which allows numbers and letters. I am trying to create fields that ONLY allow letters.
Anyone know how to do this?

You could use a pattern:
data-parsley-pattern="^[a-zA-Z]+$"

You can use such input validator, in order to obtain an only letter field.
<input type="text" class="form-control" required data-parsley-pattern="^[a-zA-Z ]+$" placeholder="Type something" />

javascript input focus

<form id="commentform" method="post" action="wp-comments-post.php">
<input type="text" aria-required="true" tabindex="1" size="22" value=""
id="author" name="author">
</form>
I set default value "visitor" to the input box. When focus is in text box or mouose enters it, I want to hide "visitor" string and want to show it when it loses focus or mose moves out.

Try using the HTML5 placeholder attribute:
<input type="text" aria-required="true" tabindex="1" size="22"
placeholder="visitor" id="author" name="author">
While browser support is not 100% there yet, this will give you a standard way to achieve what you're trying to achieve, without going through unnecessary hoops.
Another thing you can try is to overlay the input element over some text and make it transparent/translucent when not in focus and opaque when in focus/filled.
As of today, Tumblr's login page uses this trick:
<div class="input_wrapper" id="">
<label for="user_password">Password</label>
<input type="password" id="user_password" name="user[password]" data-validation-type="password" value="">
</div>
Through CSS magic this becomes:

Looks like you are using WordPress, so you have the jQuery library on your site.
You can use my jQuery plugin to achieve this.
Example
jQuery
$('#author').inputLabel({
customLabel: 'Visitor'
});
In this case, I had to specify the label myself, but the plugin works without this by finding the relevant label element to the input, which should be present for accessibility.
jsFiddle.

If you are up to HTML 5 yet then try this:
<script type="text/javascript">
var prompt="visitor";
var txt=document.getElementById("author");
txt.onfocus=function(){txt.value='';}
txt.onblur=function(){
if(txt.value==''){
txt.value=prompt;
}
}
</script>

Ates Goral's answer looks very interesting. please try it first shot. this is an alternative if you do not want to sweat..:)
i would suggest using a watermark plugin. there are many available.
have used this plugin before. worked fine. gives you nice control.
the plugin requires jQuery

Though I too would use jQuery or CSS and a pseudo-class (:focus)....
Here's an easy JS solution that does exactly what you're after. Again, I wouldn't recommend this approach for more than one or two input fields.
<input type="text" value="Visitor" onFocus="this.value='';" onBlur="this.value='Visitor';" id="author"/>