How to tell if a Javascript function is defined

Question

How do you tell if a function in Javascript is defined?

I want to do something like

function something_cool(text, callback){
    alert(text);
    if( callback != null ){ callback(); };
}

but that gets me a 'callback is not a function' error when callback is not defined.

Answer 1

typeof(callback) == "function"

Answer 2

try

if (typeof(callback) == 'function')

Answer 3

typeof(callback) == "function"

Answer 4

try:

if (!(typeof(callback)=='undefined')) {...}

Answer 5

typeof is an operator, not a function, it does not require brackets/braces (although they also don't hurt anything).

if (typeof yourFunction === 'function') { ... }

Answer 6

function something_cool(text, callback){
    alert(text);
    if(typeof(callback)=='function'){ 
        callback(); 
    };
}

Answer 7

if (callback && typeof(callback) == "function)

Note that callback (by itself) evaluates to false if it is undefined, null, 0, or false. Comparing to null is overly specific.

Answer 8

All of the current answers use a "magic" string - this does not:

function isFunction(possibleFunction) {
  return (typeof(possibleFunction) == typeof(Function));
}

Personally, I try to reduce the number of strings hanging around in my code...

---

Also, while I am aware that typeof is an operator and not a function, there is little harm in using syntax that makes it appear as the latter.

Answer 9

if ('function' === typeof callback) ...

Answer 10

self[callback] also works, but only on FF, this if what i'm using:

function isfn(x) { if (eval("typeof "+x+" == 'function'")) { return true; } else { return false; } }

Answer 11

Those methods to tell if a function is implemented also fail if variable is not defined so we are using something more powerful that supports receiving an string:

function isFunctionDefined(functionName) {
    if(eval("typeof(" + functionName + ") == typeof(Function)")) {
        return true;
    }
}

if (isFunctionDefined('myFunction')) {
    myFunction(foo);
}