Testing if value is a function

Question

I need to test whether the value of a form's onsubmit is a function. The format is typically onsubmit="return valid();". Is there a way to tell if this is a function, and if it's callable? Using typeof just returns that it's a string, which doesn't help me much.

EDIT: Of course, I understand that "return valid();" is a string. I've replaced it down to "valid();", and even "valid()". I want to know if either of those is a function.

EDIT: Here's some code, which may help explain my problem:

$("a.button").parents("form").submit(function() {
    var submit_function = $("a.button").parents("form").attr("onsubmit");
    if ( submit_function && typeof( submit_function.replace(/return /,"") ) == 'function' ) {
        return eval(submit_function.replace(/return /,""));
    } else {
        alert("onSubmit is not a function.\n\nIs the script included?"); return false;
    }
} );

EDIT 2: Here's the new code. It seems that I still have to use an eval, because calling form.submit() doesn't fire existing onsubmits.

var formObj = $("a.button").parents("form");
formObj.submit(function() {
    if ( formObj[0].onsubmit && typeof( formObj.onsubmit ) == 'function' ) {
        return eval(formObj.attr("onsubmit").replace(/return /,""));
    } else {
        alert("onSubmit is not a function.\n\nIs the script included?");
        return false;
    }
} );

Suggestions on possibly how to do this better?

Answer 1

I'm replacing a submit button with an anchor link. Since calling form.submit() does not activate onsubmit's, I'm finding it, and eval()ing it myself. But I'd like to check if the function exists before just eval()ing what's there. – gms8994

<script type="text/javascript">
function onsubmitHandler() {
    alert('running onsubmit handler');
    return true;
}
function testOnsubmitAndSubmit(f) {
    if (typeof f.onsubmit === 'function') {
        // onsubmit is executable, test the return value
        if (f.onsubmit()) {
            // onsubmit returns true, submit the form
            f.submit();
        }
    }
}
</script>

<form name="theForm" onsubmit="return onsubmitHandler();">
<a href="#" onclick="
    testOnsubmitAndSubmit(document.forms['theForm']);
    return false;
"></a>
</form>

EDIT : missing parameter f in function testOnsubmitAndSubmit

The above should work regardless of whether you assign the onsubmit HTML attribute or assign it in JavaScript:

document.forms['theForm'].onsubmit = onsubmitHandler;

Answer 2

Try

if (this.onsubmit instanceof Function) {
    // do stuff;
}