How to tell if a JavaScript function is defined

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.

Using optional chaining with function calls you could do the following:

function something_cool(text, callback) {
    alert(text);
    callback?.();
}

If callback is a function, it will be executed.

If callback is null or undefined, no error is thrown and nothing happens.

However, if callback is something else e.g. a string or number, a TypeError will still be thrown.

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')

I might do

try{
    callback();
}catch(e){};

I know there's an accepted answer, but no one suggested this. I'm not really sure if this fits the description of idiomatic, but it works for all cases.

In newer JavaScript engines a finally can be used instead.

New to JavaScript I am not sure if the behaviour has changed but the solution given by Jason Bunting (6 years ago) won't work if possibleFunction is not defined.

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

This will throw a ReferenceError: possibleFunction is not defined error as the engine tries to resolve the symbol possibleFunction (as mentioned in the comments to Jason's answer)

To avoid this behaviour you can only pass the name of the function you want to check if it exists. So

var possibleFunction = possibleFunction || {};
if (!isFunction(possibleFunction)) return false;

This sets a variable to be either the function you want to check or the empty object if it is not defined and so avoids the issues mentioned above.

typeof(callback) == "function"

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

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

Try:

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

Try this:

callback instanceof Function

If you use http://underscorejs.org, you have: http://underscorejs.org/#isFunction

_.isFunction(callback);

If you look at the source of the library @Venkat Sudheer Reddy Aedama mentioned, underscorejs, you can see this:

_.isFunction = function(obj) {
  return typeof obj == 'function' || false;
};

This is just my HINT, HINT answer :>

I was looking for how to check if a jQuery function was defined and I didn't find it easily.

Perhaps might need it ;)

if(typeof jQuery.fn.datepicker !== "undefined")

If the callback() you are calling not just for one time in a function, you could initialize the argument for reuse:

callback = (typeof callback === "function") ? callback : function(){};

For example:

function something_cool(text, callback) {
    // Initialize arguments
    callback = (typeof callback === "function") ? callback : function(){};

    alert(text);

    if (text==='waitAnotherAJAX') {
        anotherAJAX(callback);
    } else {
        callback();
    }
}

The limitation is that it will always execute the callback argument although it's undefined.

Most if not all previous answers have side effects to invoke the function

here best practice

you have function

function myFunction() {
        var x=1;
    }

//direct way
        if( (typeof window.myFunction)=='function')
            alert('myFunction is function')
        else
            alert('myFunction is not defined');

//byString
        var strFunctionName='myFunction'
        if( (typeof window[strFunctionName])=='function')
            alert(s+' is function');
        else
            alert(s+' is not defined');

This is what worked for me it always better to check whether 'undefined' at first

if(typeof ShowDialogOpen != 'undefined' && typeof ShowDialogOpen==='function'){
  console.log('function available');
}
else{
  console.log('function NOT available');
}

For global functions you can use this one instead of eval suggested in one of the answers.

var global = (function (){
    return this;
})();

if (typeof(global.f) != "function")
    global.f = function f1_shim (){
        // commonly used by polyfill libs
    };

You can use global.f instanceof Function as well, but afaik. the value of the Function will be different in different frames, so it will work only with a single frame application properly. That's why we usually use typeof instead. Note that in some environments there can be anomalies with typeof f too, e.g. by MSIE 6-8 some of the functions for example alert had "object" type.

By local functions you can use the one in the accepted answer. You can test whether the function is local or global too.

if (typeof(f) == "function")
    if (global.f === f)
        console.log("f is a global function");
    else
        console.log("f is a local function");

To answer the question, the example code is working for me without error in latest browers, so I am not sure what was the problem with it:

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

Note: I would use callback !== undefined instead of callback != null, but they do almost the same.

If you wish to redefine functions, it is best to use function variables, which are defined in their order of occurrence, since functions are defined globally, no matter where they occur.

Example of creating a new function that calls a previous function of the same name:

A=function() {...} // first definition
...
if (typeof A==='function')
   oldA=A;
A=function() {...oldA()...} // new definition

This worked for me

if( cb && typeof( eval( cb ) ) === "function" ){
    eval( cb + "()" );
}

I would rather suggest following function:

function isFunction(name) {
    return eval(`typeof ${name} === typeof Function`);
}

One-line solution:

function something_cool(text, callback){
    callback && callback();
}