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How do I make an html text input readonly but be able to update with a script?

I have these inputs that take the values of a from a in my table when I click on a row. I want to make it so that the user cannot change the input themselves but want to bring values into them when a user clicks a table row. I will be passing these inputs in as a form. I know that when the input is like this:
that it will not be updated. Is there any other way to do it with an input. Is there a different type of tag I can use that can be passed through a form?

Rather than a read-only <input>, I'd go with a combination of a display element and a hidden form element. Something like:
<div id="my-display">This is a value</div>
<input id="my-input" name="my-input" type="hidden" />
And in the code update both:
$('#my-display').text(yourValue);
$('#my-input').val(yourValue);
You can style the display to the user however you like and don't have to worry about whether or not it "de-activates" the form input.
If you really want it to be an inactive input, you can use the same approach:
<input class="my-input" type="text" disabled />
<input class="my-input" type="hidden" name="my-input" />
Which may even save you a line of code here, since both can now use .val():
$('.my-input').val(yourValue);

Try disabled keyword as here
<div id="my-display">This is a value</div>
<input id="my-input" name="my-input" type="text" disabled/>
You can change the value by javascript as below:
document.querySelector('#my-input').value = 'the value you want to enter by javascript';

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.

jQuery: having radio buttons and a textarea update hidden form fields

Really new to using jQuery and trying to find an example I need.
1) if I have, say, 5 radio buttons to choose an item, how do I pass the selected item to a hidden form field?
2) same question for a textarea. How do I pass the text written to a hidden form field and make sure it's escaped safely for a form submission?
Thanks for any help.

You can just bind to the change event:
<input type="hidden" id="myradiovalue" />
<input type="radio" name="myradio" value="0" />
<input type="radio" name="myradio" value="1" />
$('input[name=myradio]').change(function() {
$('#myradiovalue').val($(this).val());
});
And almost the same for textarea:
<input type="hidden" id="mytextarevalue" />
<textarea id="mytextareavalue"></textarea>
$('textarea').change(function() {
$('#mytextareavalue').val($(this).val());
});

For both <input type="radio"> and <textarea>, you will want to use jQuery change() method. If you want to sanitize the input before it is inserted into a <input type="hidden"> then you will need to use some regex or a library that does it for you, like jQuery Validation Plugin. Keep in mind that any sanitation/validation you do with javascript/jQuery will need to be double-checked server-side after the form is submitted.
But I don't know why you are copying data from one form input to another, can't you just use the form input as it is? What is the point of having the data in both a <textarea> and a <input type="hidden">?

Passing forms between html pages

I've got an assignment to pass data between 2 .htm pages, in a manner which the source gets copied to the destination.
sourcePage.htm contains a form. (it contains more controls this is just a sample)
<form id="myform" action="destPage.htm" method="get" >
<input type="text" name="user" />
<input type="submit" name="submit" value="Submit" />
</form>
and destPage.htm is blank.
Using JavaScript I am required to parse the data from the url, that part isn't the problem
, the problem is that I am also required that destPage would be an exact duplicate of sourcePage.
My question is, if there's a way to pass the form as an object or some way to pass the control types and their properties along side the data.

You specified in the answer of ek_ny, that you want to dynamically build the form, based on it's input.
You can do this, in fact, with the JavaScript DOM:
var i = document.createElement('input');
i.setAttribute('type', "text");
i.setAttribute('name', "user");
var f = document.createElement('form');
f.setAttribute('action', "destpage.html");
// etc.
f.appendChild(i);
document.getElementById('container').appendChild(f);
The form will be added as a child in the <div id="container"> container.
Now you can use hidden input elements, which give, for instance, the specifics of the form:
<form>
<input type="hidden" name="x_type" value="input-text" />
<input name="x" type="text" />
<input type="hidden" name="y_type" value="select:[...]" />
<select name="y">
...
</select>
</form>

As far as I know, you won't be able to do a post between two pages. At least when I've attempted that you get an error-- it really doesn't make sense to have a post from one static page to the other (right?). What you can do is serialize the data you want to pass, put it on the url string to the next page and then deserialize that data and populate the controls on the destination page. If the html between the two pages is identical, then it should be pretty straightforward, if not it will be a little tricker. If you used jQuery it would be pretty easy, because you could serialize an entire form. If you need to come up with a generic solution (and you should, because it will help you learn) that's one thing, if you need to just get it working for this assignment and there are only a couple of form fields, you'll just need to encode the values you want to pass and pass them on a URL string with a get request.

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.