How to check if a function is already defined?

Like so:

if (typeof myFunc != 'undefined') {
    // Assign myFunc
}

Don't just test it against undefined, which is not a constant and can be reassigned.

Javascript functions that check if a function exists.

With jQuery.isFunction() you may test a parameter to check if it is (a) defined and (b) is of type "function." Since you asked for jQuery, this function will tickle your fancy.

jQuery.isFunction(YourFunction)

If you wish not to use jQuery for whatever reason, here's a barebones function based on code from Idealog that will check if the variable is of type function.

function isFunction(fn){
    return typeof fn === 'function'
}

Sometimes you already know it's a function and for the sake of optimization find no reason to recheck it's type, in this case, here's function that simply checks if the variable [possibly a function] is defined

function isDefined(foo){
    return typeof(foo) !== 'undefined'
}

How to use these functions

Using jQuery:

function foo(){}
if(jQuery.isFunction(foo)) alert('This is a function');

With either of the non-jQuery Javascript functions provided above. Depending on the context of usage, these functions may, or may not be reliable. See more below

function foo(){}
if(isFunction(foo)) alert('is of type function');
if(isDefined(foo)) alert('if this is a function, it is defined');

Check both undefined and using jQuery.isFunction

if (typeof myfunc !== 'undefined' && $.isFunction(myfunc)) {
    //do something
}

Source

Is jQuery.isFunction() superior?

According to Kyle Florence jQuery.isFunction() it could superior in some situations. Particularly useful in some edge cases when using jQuery methods, see his explanation.

In certain situations in some browsers, things are incorrectly returned as the "function" type, or things that are in fact functions are returned as another type. There are several test cases you can see here: https://github.com/jquery/jque...

One example:

var obj = document.createElement("object");

// Firefox says this is a function typeof obj; // => "function"

Keep in mind these are mostly edge cases, but the reason $.isFunction was made was simply to be positive about something being a function (which can be quite important for the jQuery library itself, maybe not so much for your code).

Thanks patrick dw for pointing out Kyles Article. (Patrick DW deleted his account)

From jQuery.com

Note: As of jQuery 1.3, functions provided by the browser like alert() and DOM element methods like getAttribute() are not guaranteed to be detected as functions in browsers such as Internet Explorer.

if (typeof(functionName) == 'function') {
}

.