Develop Reference

What is the outcome of having an onclick action that submits a form on a submit button?

Out of curiosity, I was wondering if there is an expected output when you have something like this:
<input type="submit" onclick="document.formname.submit()">
I was working with a web page that someone created a few years ago and they had something similar to the above, for whatever reason. I was testing the page on the other end of the submit button, which adds things to a mySQL database. I found that it was occasionally adding everything twice, and was doing this more frequently the more items there are in the form.
Now I wonder if there is a way this is expected to work? I mostly just want to make sure that my code on the other side of the submit button is working properly. Why would this sometimes execute everything twice, and sometimes only once?

if this tag is within a <form> then submits the values in the form to the server side and it depends on action attribute of the tag

Submit form in background and use information in background

Basically my user inputs code. I can't check whether this code is valid or not(algorithm is a secret). Therefore I send(with form submit) this code to website that checks it (as $_POST variable). I have managed to do that. But how do I:
-Not display this webpage (user must stay on my webpage!)
-Get part of html from the website, that should not show and use it to fill variables (I know exactly which part of html I am looking for)
Can anyone point in the right direction with this problem..

jQuery has a simple function to add an event handler for submition and a function for posts, if you use the preventDefault function for the submit event, the form's action doesn't get performed, i.e. the side doesn't get reloaded. the html code you want to receive has to be fetched on server side, and transmitted back to the client
$('form').submit(function(e){
e.preventDefault();
$.post("example.com/somefile.php", sentData, function(receivedData){
useTheResponseData(receivedData);
});
});

Replace text on page on form submit

I have a form to let people submit news articles to my site (a company intranet). Sometimes the form submission takes a few seconds to resolve due to some actions I have in place on the relevant model save method. I wanted to replace the text on the form page with a message saying "Sending your article. This may take a few seconds, please do not refresh the page." as soon as someone hits submit. I've seen these on a number of websites when buying things online.
My first attempt at doing this was to add an onClick event to the form button. Submitting then successfully replaced the text but did not submit the form. I had a look at this answer on binding two events to one submit button but it doesn't seem to address my need as it looks PHP-specific. I'm certain javascript is the right tool for the job but I can't think of how to do this other that binding to clicking the submit button. Does anyone know the correct way to do this?

JavaScript is indeed the right way to do so. If you are using jQuery you can go ahead and use something like:
$('form#your-form-id').submit(function(){
$(this).hide().after('Loading, please wait...');
});
Where #your-form-id would be the id of the form. This function is hiding the form content and showing that text, you could do anything instead actually.

Change element value on external website

I have such a problem - I want to change value of an element on some external website.
Namely: I have webcam http interface which is password protected and there is a page with motion detection checkbox and "Apply" button (form submit). I want to create simple program with some sort of delayed toggling of motion detection (so I can launch this program and have some time to leave the building before motion detection starts). So I want to change checkbox state and write this change to system. I tried something like this, but that doesn't work:
jQuery.get("http://admin:password#192.168.0.1:12345/motion-page.asp",
function(data){
$('input[name="checkbox1"]').prop('checked', false);
// and there "simulate" clicking on Apply button - submit the form -- don't know how ...
}
);
Can anybody help me with this, please?

I would backtrack from the page that shows when you submit the camera form. See if the form itself is submitting the "turn camera on" variable as GET or POST. If you already know this, then all you would have to do is access the same URL as the form from the camera (assuming it's HTTP accessible on a network like this) and submit that same set of variables.
If you don't want to open a browser to do this, you could write yourself a custom application that submits it for you, but either way you have to open something to make the submission, as a script has to wait [X] amount of time before making the request. The fastest way will be through a browser.
I am not sure you need jquery for this (I never use jquery hardly at all). What I would do on the scripting side, since merely accessing this script means you want to activate the timer most likely, would be to create a timer object in javascript, and then make a single function that either accesses the URL of the camera form submission with the GET string parameters (that's easiest if it's doable via GET, because you wont have to build a form), or, if it's POST, have the function build a form and submit the form via POST to the same URL.
Google how to create a timer in javascript, and google how to automatically submit a form. Doing the code for you would be a waste of my time if you can figure it out on your own. If not, come back and we'll see what we can do :)
Good luck.

Why not after hitting the submit button, or after checking the box, have javascript actually run a timer? Look into the timer functions in js or jquery if that's more your thing. Not sure if you need it written to disk or whatever... since you're not giving much info, but whatever data you're wanting recorded could be captured when the box is checked and can be submitted along with the form whenever the timer runs out.
Submitting a form in jquery is simple:
http://api.jquery.com/submit/
:)

What HTML Form Action to use when no data is actually being sent?

I am working on learning JQuery and creating a simple HTML / JS calculator. I used a standard HTML form to allow the user to enter the data they want calculated and when the user clicks submit my JS / JQuery calculates and spits out the answer.
My question is what would be the semantically correct way to deal with the HTML form action being that Im not actually posting any data? I dont want to leave it default because when I click my to trigger an event it changes the URL and I dont want to use POST because Im not posting anything. Any help is appreciated!

I would replace the submit button with a normal button, and prevent the form being "submitted" at all. then use javascript to do the calculations on button click. This way the form never gets submitted, and you don't need a method or action at all.

If you really want to do a request at all, you probably just want to do a GET...check the list of HTTP request methods here to see if another one would better fit your needs.
If you are doing everything with javascript, though, you shouldn't be submitting anything at all. Try changing the submit button into a link (or just a regular button) and bind your calculator logic to its click event.

Don't specify any action(Default is GET). Use an html button which would call the js function on the click event. That would do the work on client side

You don't actually need to put input elements inside a form. Since you don't intend to submit the form, I would just omit it entirely.