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What triggers the onsubmit event? How to configure that?

Naively you might assume that the event is triggered that when the (a?) input button of type "submit" is clicked.
In fact, as far as I'm able to ascertain, every button within a form triggers the event.
Is there some way to set a button to not trigger the event?

every button within a form triggers the event
No. Only submit buttons do. (As well as a few other cases such as when the Enter key is pressed in a text input).
Note that <button> elements are type="submit" by default and should be type="button" if you want a non-submit button for handing JS from.
You can also attach a JavaScript click event listener to a button and call preventDefault on the event object.

Button's default type is submit. If you want buttons that don't submit, you have to change their type to button.

How does preventDefault and type submit work with forms?

So I tried a couple of things when I using JS and try submitting inputs, and there are odd things I didn't understand the logic.
There is a difference between <input tpye="submit/> inside and outside of <form> tag. If it's inside the form tag I need to use preventDefault() function, but if it's outside form tag I am not required to do that if write simple js vanilla code.
Why is that?
What the difference between onsubmit and onclick ? especially in a form tag. Because If I use onsubmit the js code doesn't really work.
If I use preventDefault(event) it's preventing from the form to be sended into a server, and instead it doing the calculations with the browser only ?
Thanks !

By default, if you have an input of type "submit" in a form then clicking that input (or hitting enter after filling in ANY inputs in the form) will send a post or get request to the server, redirecting the current page to the server's response.
This was the original way to submit form data to a server, without using javascript. If you want to prevent this from happening, you can either replace the submit input with a plain button (<button onclick="doSomething()">Submit</button>), or prevent the default submission event: (form.onsubmit = event => event.preventDefault()).
The difference between onsubmit and onclick is that onsubmit only fires when a submission event is emitted from the form. To emit a submit event, the form needs an <input type="submit"> to be clicked, or for the user to trigger a submission by hitting enter.
Another way to prevent this default behavior is to return false in the submission handler.
onclick only gets fired when an element is clicked. Because events in javascript propagate up to parents, this will also trigger any onclick handlers on all parent elements.
If you want to completely ignore the default form submission behavior, then you can define a button with an onclick handler to handle your custom submission logic.

If you have a type="submit" input (Note, The default value for the type attribute of button elements is "submit") inside a form, the default action on click is to submit that form. To prevent it from submitting, place the button outside the </form>. This works because the button doesn't refer to any form to submit. You can instead, add a listener and call event.preventDefault(). This works because you are telling the browser NOT to take the default action (submitting the form)
onsubmit is used mostly on <form> elements. It triggers right before the form is submitted regardless of how it is submitted. onclick can be emitted from just about any element. It occurs when you click on an element. This could be on an <input>, a <button>. or even an entire <form>
Don't use this.

Form inputs with onchange() callbacks update on submit

I have a form with some input fields and all those fields have onchange triggered callbacks (which can be different depending on the field).
My problem is that when the form is submitted the focus is still on the last modified field and the onchange callback is not called for this field, which can lead to submit wrong values.
I am looking for a way to force the triggering on the onchange event for the last field modified but I can't determine which element it is from my submit function.
How can I achieve this ?

Try this
$focused_element = $(':focus');
if($focused_element.is('input')){
$focused_element.trigger('change');
}

Submit form when contents of MVC3 Html.TextBoxFor changes

This should be fairly easy but I've tried a few things with no luck.
I have a series of Html.TextBoxFor fields on a page, each inside their own Ajax.BeginRouteForm. Next to each box I have a submit button, and this, when clicked, performs the Ajax update as desired.
I'd like to automate this so that when the user changes a value in a field (the onchange event) the form is submitted the same way it currently using using the submit button.
I tried using the htmlattributes to assign a JavaScript function to the onchange event (as shown below) and this causes the form to submit, but it redirects the page instead of working in the ajax fashion (as opposed to clicking the submit button which works correctly).
#(Html.TextBoxFor(model => answer.Value, new { onchange = "document.forms[" + answer.AnswerID + "].submit()" }));
(fortunately my answer.AnswerID is numeric and matches up with the numeric position of the appropriate form in the forms collection; I was referencing them by name but Razor (or something) was htmlencoding my JavaScript code...)
My only guess is that I'm breaking something by attaching code directly to the onchange event, but I'm at a loss as to the "right" way to hook into that event chain.

If you're willing to use JQuery, it's very simple to do:
http://net.tutsplus.com/tutorials/javascript-ajax/submit-a-form-without-page-refresh-using-jquery/

Calling submit() on a form will ignore any submit event handlers, as seen here. You can either
call the event handler directly, or
call click() on the submit button for the form.
The former works best if you use onsubmit and return false instead of using the event argument to the callback, because otherwise you need to pass another messy object or something.

How can I exclude HTML form radio button values from being submitted?

I need to submit just one input field value to a cgi script via a web form.
I've added a couple of extra form controls (a check box and radio buttons) which manipulate the input value depending on the states selected.
When the form is submitted, the extra form field values are submitted as well which breaks the cgi script (which I don't have access to). I removed the 'name' attribute from the check boxes so they are not submitted but cannot do this for the radio buttons as it breaks their grouping.
How can I prevent radio button values from being submitted?

You can add a disabled attribute to them in the submit handler, this will prevent them from being serialized, either by jQuery or a normal <form> submission. For example:
$("#myForm").submit(function() {
$(this).find(":radio, :checkbox").attr("disabled", true);
});
Or you can .serialize() only the elements you want, for example:
$.post("myPage.cgi", $("#myForm input[type=text]").serialize());

Make them "unsuccessful". There are several ways to achieve this:
http://www.w3.org/TR/html401/interact/forms.html#h-17.13.2

It is also possible to have two different forms: one that has visible form elements and one that has a hidden input that represents the end result to be submitted. You can either attach onchange handlers to your visible form elements so that they call some JavaScript to update the invisible field, or you can run a function as part of the onsubmit handler to set the invisible value directly before it is submitted.
Here's a jsFiddle demonstrating the second approach (the onsubmit handler): http://jsfiddle.net/gtU4J/