Develop Reference

Copy value from one text field to another when button clicked

I am trying to get one field to copy over to another field when a div is clicked on, and the code that I have currently is not working. It's showing '0' in field1, even though field2 is set to 1 by default.
$(document).on('click', '#button', function() {
$('#textfield1').val === "document.getElementById('#textfield2').value";

Try with:
$(document).on('click', '#button', function() {
$('#textfield1').val($('#textfield2').val())
});

You're using an odd mix of JS and jQuery here.
Your main issue is that val() is a method, not a property. Therefore your code should look something like this:
$(document).on('click', '#button', function() {
$('#textfield1').val($('#textfield2').val());
});
I'd strongly suggest you familiarise yourself with the jQuery documentation, specifically val() in this case.

It's showing '0' in field1, even though field2 is set to 1 by default.
You were assigning a string to $('#textfield1').val method which is why your code was not having any effect on textfield1's value.
Make it
$(document).on('click', '#button', function() {
$('#textfield1').val( $('#textfield2').val()); //use jquery val method
}

Generally speaking, JQuery offers only functions, and not properties (as #Craicerjack stated), hence remove that === and pass the new value as an argument, as follows:
$('#textfield1').val("yourText");
Also, you're passing a CSS selector rather than just an element ID to the Document.prototype.getElementById() function. Remove that # qualifier!
Moreover, you shoudln't be using a stringified JavaScript expression as a value, otherwise you'll get that exact JS expression as the input value. Rather, don't put those quotes around the expression, so that the interpreter will be evaluating it. Below is some working code.
$('#textfield1').val(document.getElementById('textfield2').value);
However, as #Rory McCrossan pointed out, you're using an odd mix of plain DOM and JQuery, and that makes no sense. It would be more consistent to also read the value of the other text field using JQuery, as follows:
$('#textfield1').val($('#textfield2').val());
Alternatively, you may do not need JQuery and opt for the standard DOM interfaces like in the example below:
document.getElementById('textfield1').value = document.getElementById('textfield2').value;

What does $($(this)) mean?

I saw some code around the web that uses the following statement
if ($($(this)).hasClass("footer_default")) {
$('#abc')
.appendTo($(this))
.toolbar({position: "fixed"});
}
What is the use of $($(this)) and why is that necessary here?

Yes, $($(this)) is the same as $(this), the jQuery() or $() function is wonderfully idempotent. There is no reason for that particular construction (double wrapping of this), however, something I use as a shortcut for grabbing the first element only from a group, which involves similar double wrapping, is
$($('selector')[0])
Which amounts to, grab every element that matches selector, (which returns a jQuery object), then use [0] to grab the first one on the list (which returns a DOM object), then wrap it in $() again to turn it back into a jQuery object, which this time only contains a single element instead of a collection. It is roughly equivalent to
document.querySelectorAll('selector')[0];, which is pretty much
document.querySelector('selector');

You can wrap $ as many times as you want, it won't change anything.
If foo is a DOM element, $(foo) will return the corresponding jQuery object.
If foo is a jQuery object, $(foo) will return the same object.
That's why $($(this)) will return exactly the same as $(this).

There is no specific need for double-wrapping and $($(this)) is exactly the same as $(this).
That said, I once found this double-wrapping in one file in my project, committed by another developer. Tracking the changes through revision, turned out that it started as $($(this).find('selector').first()) - that is, the result of some selector was wrapped to create a new object. Then for whatever reasons, the selector was removed and only the double-wrapping of this remained. Needless to say, on the next commit it was changed to $(this).

As explained before me, $($(this)) and $(this) are absolutely identical. jQuery returns the same jQuery object if you try to wrap it more than once.
Additionally, for performance considerations it is a good practice to reuse jQuery objects - it is quite expensive to create jQuery objects, especially the ones with complex selectors. Example:
var $this = $(this);
if ($this.hasClass("footer_default")) {
$('#abc')
.appendTo($this)
.toolbar({position: "fixed"});
}
Just google for 'jQuery best practices' - it will take a 30 min for you to learn these basics and you will use jQuery way more effectively.

There is no meainig of doing that.
The following code return the same:
console.log($($(this)).hasClass("footer_default"))
console.log($(this).hasClass("footer_default"))
a boolean value depenging on if the selected element has or not the class footer_default:
.hasClass( className )Returns: Boolean
Demo: http://jsfiddle.net/IrvinDominin/aSzFn/

$(this) and $($(this)) both return jquery object.
There is no difference between these two.

JavaScript JQuery String Var to show in a field

I'm working with JavaScript JQuery, and when i try to show the content of the vars in a field, it doesn't work.
There is my code:
function editEvolution(pos, nature, desc, di) {
$('#diE').val(di);
$('#natureE').val(nature);
$('#descE').val(desc);
$('#BtnAddEditEvo').attr('value', "Update");
$('#BtnAddEditEvo').attr('onclick', "doEditEvolution("+pos+")");
}
Thank's in advance for help.
the value of fields $('#diE'), $('#natureE') and $('#descE') doesn't change if the vars nature, desc, are strings but it works if it is a number

The problem is that you are mixing the worst from both the native JS, inline JS and jQuery worlds. You NEVER EVER put code inside a string, period. If you do it, you are doing it wrong.
So.. how to do it properly?
$('#BtnAddEditEvo').val('Update').on('click', function() {
doEditEvolution(pos);
});
In case you call editEvolution multiple times on the same object, add .off('click') before the .on(...) call to unbind previous handlers.

First do not use attr to set onclick...
I will explain what is wrong
$('#BtnAddEditEvo').attr('onclick', "doEditEvolution("+pos+")");
if pos="foo", it will render as
<div onclick="doEditEvolution(foo);"></div>
See the problem? It is looking for a variable foo. You would need to add quotes.
$('#BtnAddEditEvo').attr('onclick', "doEditEvolution('"+pos+"')");
^ ^
Why does a number work? Because numbers do not need quotes. It makes a valid call. It renders as:
<div onclick="doEditEvolution(3);"></div>
What you need to do is use jQuery the right way and use events.
$('#BtnAddEditEvo').on('click', function() { doEditEvolution(pos); });
and you should set value with .val()

How do I set the value of an input-element via jQuery, when I only know it's "name"-attribute

I need to reset some text-boxes which only have an unique name (not an unique id).
I tried it with jQuery, but my code seems to do nothing:
$('input[name=customergroupname]').value="";

Try this:
$('input[name="customergroupname"]').val("");

Getting and setting value of form elements wrapped in a jQuery object is being done with use jQuery val function:
$('input[name="customergroupname"]').val("");
.value can only be used for DOM elements, like this:
$('input[name="customergroupname"]').eq(0).value ="";
$(...) // This is a jQuery object.
$(...)[n] // This is a DOM object in the nth place in the set.
$(...).eq(n) // This is a DOM object in the nth place in the set.

Working demo http://jsfiddle.net/vN74v/2/
Code
$('#hullk').click(function(){ // in example when you click the button
$('input[name="customergroupname"]').val("");
});
​

According to DOM level 2 specifications, you can access:
document.forms["foo"].elements["customergroupname"].value = '';
If formName and fieldName are constants rather than variables you can use literal syntax:
document.forms.foo.elements.customergroupname.value = '';

try:
$('input[name="customergroupname"]').value="";
I've wrapped customergroupname in quotes.
Very true, I mixed the quotes up. Edited now.

Weird Behaviour in jQuery's html method

Any good reason why $("p").html(0) makes all paragraphs empty as opposed to contain the character '0'?
Instead of assuming I found a bug in jQuery, it's probably a misunderstanding on my part.

jQuery only accepts a string as an argument for the val parameter of the html() method. If you pass a number like you are it will call the html() method override that sets the contents of the element but the value of the argument will end up being null or an empty string.
Try this:
$("p").html((0).toString())
Relevant documentation

I guess that at some point, it checks if (newContent == false), and doesn't continue with adding any content? I tried looking at the source, but got a bit lost...
I also guess that this would not be counted as a bug, since the function calls for a string, and if "0" is passed (as a string), it works as expected.
A workaround would be to do this:
var myNum = 0;
$('p').html('' + myNum);

The code performing the html call was within someone else's plugin and rather than modify it, making upgrading it tedious, I just wrote the following tiny plugin that modifies the html method to do as spoon16 recommended.
(function($) {
var oldHtml = $.fn.html;
$.fn.html = function (content) {
oldHtml.apply(this, [content.toString()]);
}
})(jQuery);
It's a little bit of a hack, but it's working for me and doesn't require me to modify the Plugin I'm using.
I just thought someone else might like to see this.

Try using text() instead html().

I geuss you missed part of how jQuery works,
$('p')
returns all paragraphs and the html( val ) function:
Set the html contents of every matched element. This property is not available on XML documents (although it will work for XHTML documents).
http://docs.jquery.com/Attributes/html#val
So if you just want to set the contents for the first p use
$("P").eq(0).html( 'something' );
or to get the html:
$("P").eq(0).html();
http://docs.jquery.com/Core/eq#position
more on jQuery selectors here:
http://docs.jquery.com/Selectors