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What's the best way to send an input's data to a hidden input?

I have a form on a Website, and then another form inside of a Bootstrap modal.
The main form has certain fields e.g "Neck, Chest, Waist" while the form inside of the modal has only one e-mail field.
I'm planning to add some "hidden" inputs into the secondary form named "chest, waist" etc and I would like the main form field's value to be passed into the secondary form's hidden inputs as that's the one which is actually going to be submitted.
Is it possible without javascript? If not, I'd prefer some jQuery solution as it must be something pretty minor but I just can't figure it out.

Just copy the values of the form to the form with hidden inputs when it is submitted. Copying on keyup is unnecessary.
<form id="form1">
Input
<input type="text" id="input"/>
<button type="submit">Submit</button>
</form>
<form id="form2">
<input type="text" id="input2"/>
</form>
<script>
document.getElementById("form1").addEventListener("submit", (e)=>{
e.preventDefault();
document.getElementById("input2").value = document.getElementById("input").value;
document.getElementById("form2").submit();
});
</script>

How to enable HTML elements on submit

I have a checkbox and a few input elements related to this checkbox as shown below
<input name="balanceFeaturesOn" id="balanceFeaturesOn" type="checkbox" disabled="disabled" checked="" />Control
<input name="IntervalDays26566521" type="Text" onclick="" onchange="" value=" 31 ">
<input name="IntervalHours26566521" type="Text" onclick="" onchange="" value=" 12 ">
For some reasons, I will have to keep my checkbox always disabled.
On the submit of above form (Say that the checkbox and inputs are inside a form), in the server code, I want to grab the text inputs based on if the checkbox was checked/unchecked. However since the checkbox is disabled, the request parameter does not contain the balanceFeaturesOn property.
So when I execute the below line:
String[] balanceFeatArr = request.getParameterValues("balanceFeaturesOn");
I am not getting any value...
So my question is how do I be able to get the value of the checkbox while still keeping it disabled on the UI?

Try the following code,
In the form use javascript function to submit the form,
<input type='submit' onclick='javascript:submitMe()' />
Javascript function,
function submitMe(){
$('#balanceFeaturesOn').removeAttr('disabled');
$('#formId').submit(); //Replace with the actual id of the form
}
Make sure you have included jquery library in your code.

Use Hidden Fields.
Hidden fields are similar to text fields, with one very important difference!
The difference is that the hidden field does not show on the page. Therefore the visitor can't type anything into a hidden field, which leads to the purpose of the field:
To submit information that is not entered by the visitor.
http://www.echoecho.com/htmlforms07.htm

populate a hidden field on hitting submit

I have a html form that has a hidden field. Upon hitting submit, I want the hidden field to be populated with the value entered in another field before it gets to form validation.Is there a way I can do that in Javascript?
HTML code:
<form>
Email: <input type="text" name="email"><br>
<input type="hidden" name="retype-email">
<input type="submit" value="Submit">
</form>
JSfiddle: http://jsfiddle.net/x1r8sunh/

Given the HTML you posted, this should work:
$('form').on('submit', function(e){
$('[name="retype-email"]').val($('[name="email"]').val());
});
Example Here
Pure JS option:
document.forms[0].addEventListener('submit', function(){
document.querySelector('[name="retype-email"]').value = document.querySelector('[name="email"]').value;
});
Example Here
You will probably want to use ids or classes to select the elements. The above JS will probably conflict with other elements - change accordingly.

Why is Symphony CMS not saving the empty input value as such when submitting form via jQuery?

I have the following HTML form, which allows a user to optionally save a custom label for their product.
<form action="http://domain.com/members/systems" method="post" class="mod-SystemLabel-EditForm">
<label class="mod-SystemLabel-EditLabel" for="label-623">Customer label</label>
<input type="text" value="sdff sdf sd" name="fields[customer-label]" class="mod-SystemLabel-EditInput" id="label-623">
Clear
Cancel
<input type="submit" value="Save" name="action[system-edit-label]">
<input type="hidden" value="623" name="id">
</form>
If I manually clear my text input and submit my form, Symphony CMS records the empty value as expected.
If I use jQuery to trigger the form submission as below, Symphony CMS leaves (or re-saves?) the current value as it was.
$('.mod-SystemLabel-OtherButton-clear').click(function(e) {
e.preventDefault();
$(this).siblings('.mod-SystemLabel-EditInput').val('');
//alert($(this).closest('form').serialize());
$(this).closest('form').submit();
});
If I uncomment the commented line, the alert contains:
fields%5Bcustomer-label%5D=&id=623
This serialization is the same as what is produced when I backspace the input myself, so it looks like the actual form submission should be the same as a manual input clearing and click of the submit button.
The Symphony field is not set to be required and does not have any validation rules.
Why is the end result different and how can I get the empty value to be saved, overwriting the previous product label?

The form’s submit input’s name is not passed when the form is submitted via JavaScript, and Symphony CMS uses this to trigger the appropriate event.
To get the event name passed along with the submit input, trigger a “click”.
$('.mod-SystemLabel-OtherButton-clear').click(function(e) {
e.preventDefault();
$(this).siblings('.mod-SystemLabel-EditInput').val('');
$(this).siblings('input[type=submit]').trigger('click');
});

How do I detect how a form was submitted via JavaScript?

I have a form with multiple submit buttons and I'm listening for the 'submit' event via JavaScript. I want to know which submit button or form field (if the user pressed 'Enter/Return') triggered the submit event. Is there a way to get the HTML element that the user clicked on or pressed 'Enter/Return' in?
Update since people aren't understanding me:
This is via JavaScript before the form is submitted. No server-side detection allowed. I also need to handle the form being submitted via the user pressing Enter or Return.
Code
<form action="" method="POST">
<input type="text" name="first_name">
<input type="text" name="item">
<input type="submit" value="Add item">
<input type="submit" value="Submit">
</form>
Clicking 'Add Item' or pressing Return/Enter inside name="item" will add another form field.
Final Note
As far as I can tell, there isn't a way to detect which form field triggered a form submission. If you need to prevent submitting a form that has multiple buttons and/or from Enter/Return, you'll need to use <input type="button"> and bind event handlers to the form fields you want to stop form submission from.

If you have multiple submit buttons, the way you can tell is by giving each of them a unique name attribute, like this:
<input type="submit" name="submit1" value="Submit 1"/>
<input type="submit" name="submit2" value="Submit 2"/>
<input type="submit" name="submit3" value="Submit 3"/>
The one that is focused is sent along with the form submit, so if you clicked the one with a name of "submit2", that would come through in the form POST (or GET). If enter is hit, the first button in the source (in this case submit1) is considered the default and is sent along. You could set it to display:none to use as a dummy for detecting whether enter was pressed vs actually clicking a submit button.
EDIT:
In response to your comments, to capture the enter key getting pressed in certain elements you can do this with jQuery.
Note, you'll need to give first_name and add_item id attributes, and turn add_item into a type="button" instead of type="submit".
HTML:
<form action="" method="POST">
<input type="text" name="first_name"/>
<input type="text" id="item" name="item"/>
<input type="button" id="add_item" value="Add item"/>
<input type="submit" value="Submit"/>
</form>
JS:
$("#item").keydown(function(event){
if(event.keyCode == 13) {
addFields();
event.preventDefault();
event.stopPropagation();
}
});
$("#add_item").click(function(event) {
addFields();
});

You could set the onclick event on each element you are interested and call a javascript function with a different parameter for each element clicked.
From that function you send the idendifier of the button to the server side as a parameter

Just put a different name on each submit button, whichever one was clicked will be submitted (i.e. its name/value pair) with the form. Forms have worked like this since the begining of (WWW) time.
If the form is sumitted by enter or other keypress, no the first submit button name/value pair will be submitted.
Edit
Re-reading your question, you may want to determine how the form was submitted before it is sent. A click listener on the form can remember the last submit button clicked, but in Firefox, pressing enter in an input dispatches a fake click on the first submit button so you can't detect it.
I think you can't do it reliably other than using the basic method suggested above or Jordan's hidden submit button. If you say why you need to do this, perhaps more help can be provided.

here's an option if you don't mind using jQuery:
example: http://jsfiddle.net/U4Tpw/
use something like
$('form').submit(function() {
// identify the form by getting the id attribute
handleWhichForm($(this).attr('id'));
});