Check if a Javascript Function Exists or Is Defined

To check if a Javascript function exists before calling it, try this:

if(typeof yourFunctionName == 'function') {
yourFunctionName();
}

I was surprised how hard it was to google the right answer to this...but I found it at CodingForums.

Technorati Tags: javascript, function defined, function exists, function existence

Comments

[andho]

how about if the function name is in a variable like this:
var funcName = 'myFunc';

then i want to check if the function stored in funcName exists:
if (typeof eval(funcName) == 'function') {

this gives an error in:
myFunc is not defined

in firebug plugin for firefox

[steve]

Of course eval() cannot evaluate this variable, because it might be undefined. :)
But it works in this way:

if (eval("typeof " + funcName + " == 'function'")) {
}

[Søren Lund]

I solved the 'function name is in a variable'-problem with the code below:

if (typeof funcName == 'string' &&
eval('typeof ' + funcName) == 'function') {

eval(funcName+'()');

You have to eval the typeof and not typeof the eval.

[Ketan Solanki]

Soren,

if typeof funcName == 'string'
then eval('typeof ' + funcName) == 'function')
will the same as eval('string')
and you will get the error saying 'string' is undefined

[jon]

exactly what i needed, works beautifully.

tiny caveat when trying to test the existence of a function off of a jquery selector. have to select first...

if(typeof $("#datePicker").datepicker == 'function')

[db0]

Is there a way to check the same thing through php? For example I want to check if the scrptalicious blind function exists before hiding an html element.

[Ehsun]

Thanks! very fruitful post

[ken neville]

How often are you calling functions that don't exist? Sounds a little overdefensive to me :)

Instead, try:

myFunction && myFunction.call && myFunction() || otherFunction();

This eliminates an unneeded "if()" closure.

[Thanks!]

Short and to the point, and it worked perfectly! Much appreciated!

[Joe]

Why not just use...
if(document['funcName']) // call function

[Lachlan]

First piece of code I tried, and it worked perfectly. Many thanks.

[Tavish]

Ken,

That will still call otherFunction() if myFunction() doesn't return truthy.

However, I like the elegance. Is there a way to check for existence only in this way; regardless of the return value for the function?

[Big Dirty]

Thanks man, if I was a chick I'd send a pic of my titties for thanks!

[GreySkull]

Hey Dude !!!

Thanx a lot for this ... was lookin all over the internet for this solution !!!

[Phillip]

Looks like a pretty simplistic solution. Glad I found this post, as I've been having a rough time trying to figure out how to do this recently. Many thanks!

[Deanit]

Thanks. Very good solution!

[rajesh sharma]

If you just need to check if a function exists: you can just use if(window.function_name) [if the function is not contained in a class] OR if(className.function_name) [if the function is supposed to be in a class],
Remember not to include the "()" while checking :)

[Matt Densley]

IE seems to return "object" sometimes, even for functions.

[Adr]

How about using try{..}catch(e){..} it may be more suitable in many situations.

[TeMc]

@jon:

Not true, a special jQuery function is pretty much always introduced using $.fn

so instead of $("#bla").myFunction() use $.fn.myFunction() to test if it's a function