Testing if value is a function

What browser are you using?

alert(typeof document.getElementById('myform').onsubmit);

This gives me "function" in IE7 and FireFox.

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;

You could simply use the typeof operator along with a ternary operator for short:

onsubmit="return typeof valid =='function' ? valid() : true;"

If it is a function we call it and return it's return value, otherwise just return true

Edit:

I'm not quite sure what you really want to do, but I'll try to explain what might be happening.

When you declare your onsubmit code within your html, it gets turned into a function and thus its callable from the JavaScript "world". That means that those two methods are equivalent:

HTML: <form onsubmit="return valid();" />
JavaScript: myForm.onsubmit = function() { return valid(); };

These two will be both functions and both will be callable. You can test any of those using the typeof operator which should yeld the same result: "function".

Now if you assign a string to the "onsubmit" property via JavaScript, it will remain a string, hence not callable. Notice that if you apply the typeof operator against it, you'll get "string" instead of "function".

I hope this might clarify a few things. Then again, if you want to know if such property (or any identifier for the matter) is a function and callable, the typeof operator should do the trick. Although I'm not sure if it works properly across multiple frames.

Cheers

Try

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