Testing if value is a function

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;

Try

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

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

What browser are you using?

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

This gives me "function" in IE7 and FireFox.

Make sure you are calling typeof on the actual function, not a string literal:

function x() { 
    console.log("hi"); 
}

typeof "x"; // returns "string"

typeof x; // returns "function"

You can try modifying this technique to suit your needs:

 function isFunction() {
   var functionName = window.prompt('Function name: ');
   var isDefined = eval('(typeof ' + functionName + '==\'function\');');
   if (isDefined)
     eval(functionName + '();');
   else
     alert('Function ' + functionName + ' does not exist');
 }
 function anotherFunction() {
   alert('message from another function.');
 }

If it's a string, you could assume / hope it's always of the form

return SomeFunction(arguments);

parse for the function name, and then see if that function is defined using

if (window[functionName]) { 
    // do stuff
}

form.onsubmit will always be a function when defined as an attribute of HTML the form element. It's some sort of anonymous function attached to an HTML element, which has the this pointer bound to that FORM element and also has a parameter named event which will contain data about the submit event.

Under these circumstances I don't understand how you got a string as a result of a typeof operation. You should give more details, better some code.

Edit (as a response to your second edit):

I believe the handler attached to the HTML attribute will execute regardless of the above code. Further more, you could try to stop it somehow, but, it appears that FF 3, IE 8, Chrome 2 and Opera 9 are executing the HTML attribute handler in the first place and then the one attached (I didn't tested with jQuery though, but with addEventListener and attachEvent). So... what are you trying to accomplish exactly?

By the way, your code isn't working because your regular expression will extract the string "valid();", which is definitely not a function.

Isn't typeof xxx === 'function' the best and the fastest?

I made an bench in wich you can try it out, compared to instanceof and _underscore

1. Its just seems to be faster than instanceof (using chrome)
2. It won't trow an error if the variable is not defined

Here a bench: https://jsbench.me/qnkf076cqb/1

Checking the call method on the value seems to be a good enough test. e.g., val.call && val()

> a = () => {}
[Function: a]
> function b() {}
undefined
> c = function(){}
  [Function: c]
> d = 2
2
> e = []
  []
> f = {}
{}
> a.call
  [Function: call]
> b.call
  [Function: call]
> c.call
  [Function: call]
> d.call
undefined
> e.call
undefined
> f.call
undefined

Note: Except when it's a class.

Well, "return valid();" is a string, so that's correct.

If you want to check if it has a function attached instead, you could try this:

formId.onsubmit = function (){ /* */ }

if(typeof formId.onsubmit == "function"){
  alert("it's a function!");
}

  if ( window.onsubmit ) {
     //
  } else {
     alert("Function does not exist.");
  }

I think the source of confusion is the distinction between a node's attribute and the corresponding property.

You're using:

$("a.button").parents("form").attr("onsubmit")

You're directly reading the onsubmit attribute's value (which must be a string). Instead, you should access the onsubmit property of the node:

$("a.button").parents("form").prop("onsubmit")

Here's a quick test:

<form id="form1" action="foo1.htm" onsubmit="return valid()"></form>
<script>
window.onload = function () {
    var form1 = document.getElementById("form1");

    function log(s) {
        document.write("<div>" + s + "</div>");
    }

    function info(v) {
        return "(" + typeof v + ") " + v;
    }

    log("form1 onsubmit property: " + info(form1.onsubmit));
    log("form1 onsubmit attribute: " + info(form1.getAttribute("onsubmit")));
};
</script> 

This yields:

form1 onsubmit property: (function) function onsubmit(event) { return valid(); }
form1 onsubmit attribute: (string) return valid()

Beware that es6 class is also a function but not callable

class C {}

typeof C === "function"  // true
C instanceof Function    // true

C()       // error
C.call()  // error
new C()   // okay
new C     // okay

A simple check like this will let you know if it exists/defined:

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