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

// This should be a function, because in certain JavaScript engines (V8, for
// example, try block kills many optimizations).
function isFunction(func) {
    // For some reason, function constructor doesn't accept anonymous functions.
    // Also, this check finds callable objects that aren't function (such as,
    // regular expressions in old WebKit versions), as according to EcmaScript
    // specification, any callable object should have typeof set to function.
    if (typeof func === 'function')
        return true

    // If the function isn't a string, it's probably good idea to return false,
    // as eval cannot process values that aren't strings.
    if (typeof func !== 'string')
        return false

    // So, the value is a string. Try creating a function, in order to detect
    // syntax error.
    try {
        // Create a function with string func, in order to detect whatever it's
        // an actual function. Unlike examples with eval, it should be actually
        // safe to use with any string (provided you don't call returned value).
        Function(func)
        return true
    }
    catch (e) {
        // While usually only SyntaxError could be thrown (unless somebody
        // modified definition of something used in this function, like
        // SyntaxError or Function, it's better to prepare for unexpected.
        if (!(e instanceof SyntaxError)) {
            throw e
        }

        return false
    }
}

Answer 2

What browser are you using?

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

This gives me "function" in IE7 and FireFox.

Answer 3

You can always use one of the typeOf functions on JavaScript blogs such as Chris West's. Using a definition such as the following for the typeOf() function would work:

function typeOf(o){return {}.toString.call(o).slice(8,-1)}

This function (which is declared in the global namespace, can be used like this:

alert("onsubmit is a " + typeOf(elem.onsubmit));

If it is a function, "Function" will be returned. If it is a string, "String" will be returned. Other possible values are shown here.