html form with multiple submitbuttons

It is not possible to check the button clicked through the onsubmit event. Instead move the call to verifyData() to the onclick handler of each button. Use return in the onclick call to cancel submission if false is returned by verifyData()

<script language="javascript" type="text/javascript">
    function verifyData(button) {
        // validate
        switch (button.value) {
            case "submit1":
                // do somehting
                break;
            case "submit2":
                // do somehting
                break;

            // ...
        };

        // submit the form
        return true;
    }
</script>

<form method="post">
    <input type="submit" value="submit1" onclick="return verifyData(this);">
    <input type="submit" value="submit2" onclick="return verifyData(this);">
</form>

How about putting an onclick event handler on both buttons which will set a variable to say which button was clicked? like so:

<script language="javascript" type="text/javascript">
    function verifyData(formdata) {
        alert(btnClicked);
        // Here I want to know from which submit button this function is triggered

        // I can't use type button instead of submit

    }

    var btnClicked = 0;


    function setSubmit(which) {
        btnClicked = which;  return true;
    }
</script>

<form onsubmit="verifyData(this);" method="post">
    <input type="submit" value="submit1" onclick="return setSubmit(1);">
    <input type="submit" value="submit2" onclick="return setSubmit(2);">
</form>

Are you allowed to use the jQuery library?

If you can using this you can easily bind to each submit button based on an id.

For example:

<form id="form1" method="post">
    <input type="submit" value="submit1" id="submit1">
    <input type="submit" value="submit2" id="submit2" >
</form>

<script type="text/javascript">
    $("#submit1").click(function(e)
    {
     // Do stuff when 1 is clicked.
     $("#form1").submit();
    });
    $("#submit2").click(function(e)
    {
     // Do stuff when 2 is clicked.
     $("#form1").submit();
    });    
</script>

you could also have the buttons as a type of button to avoid any issues, but you should be able to simply return false; to stop the button of type submit from... submitting

Here is how I would do it... Firstly I would use jQuery so you must include that in your document like this:

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>

It would also mean your HTML can be simplified to:

<form method="post">
    <input type="submit" value="submit1"/>
    <input type="submit" value="submit2"/>
</form>

Then you can use jQuery:

<script>
    // When the document is ready
    $(function(){
        // Action performed when a submit button in the form is clicked
        $("form[type='submit']").click(function(e){

            // Get the value attribute
            var val = $(this).val(),
            validation_has_passed = false;

            // If it is submit1
            if(val == "submit1") {
                // Validate submit 1
                validation_has_passed = true;

            // If it is submit2
            } else if(val == "submit2") {
                // Validate submit 2
                validation_has_passed = true;

            }

            // If all validation is OK submit the form
            if(validation_has_passed === true) {
                $("form").submit();
            }

            // Ensure pressing these buttons doesn't submit the form
            e.preventDefault();

        });
    });
</script>