How to tell if a JavaScript function is defined

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

I want to do something like this

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

But it gets me a

callback is not a function

error when callback is not defined.

typeof callback === "function"

All of the current answers use a literal string, which I prefer to not have in my code if possible - this does not (and provides valuable semantic meaning, to boot):

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.

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.

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);
}

Try:

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