[javascript] How to check if function exists in JavaScript?

My code is

function getID( swfID ){
     if(navigator.appName.indexOf("Microsoft") != -1){
          me = window[swfID];
     }else{
          me = document[swfID];
     }
}

function js_to_as( str ){
     me.onChange(str);
}

However, sometimes my onChange does not load. Firebug errors with

me.onChange is not a function

I want to degrade gracefully because this is not the most important feature in my program. typeof gives the same error.

Any suggestions on how to make sure that it exists and then only execute onChange?

(None of the methods below except try catch one work)

This question is related to javascript

The answer is


I would suggest using:

function hasMethod(subject, methodName) {
  return subject != null && typeof subject[methodName] == "function";
}

The first check subject != null filters out nullish values (null and undefined) which don't have any properties. Without this check subject[methodName] could throw an error:

TypeError: (undefined|null) has no properties

Checking for only a truthy value isn't enough, since 0 and "" are both falsy but do have properties.

After validating that subject is not nullish you can safely access the property and check if it matches typeof subject[methodName] == "function".


Applying this to your code you can now do:

if (hasMethod(me, "onChange")) {
  me.onChange(str);
}

For me the easiest way :

function func_exists(fname)
{
  return (typeof window[fname] === 'function');
}

Didn't see this suggested: me.onChange && me.onChange(str);

Basically if me.onChange is undefined (which it will be if it hasn't been initiated) then it won't execute the latter part. If me.onChange is a function, it will execute me.onChange(str).

You can even go further and do:

me && me.onChange && me.onChange(str);

in case me is async as well.


If you're checking for a function that is a jQuery plugin, you need to use $.fn.myfunction

if (typeof $.fn.mask === 'function') {
    $('.zip').mask('00000');
}

I like using this method:

function isFunction(functionToCheck) {
  var getType = {};
  return functionToCheck && getType.toString.call(functionToCheck) === '[object Function]';
}

Usage:

if ( isFunction(me.onChange) ) {
    me.onChange(str); // call the function with params
}

Put double exclamation mark i.e !! before the function name that you want to check. If it exists, it will return true.

function abc(){
}
!!window.abc; // return true
!!window.abcd; // return false

If you're using eval to convert a string to function, and you want to check if this eval'd method exists, you'll want to use typeof and your function string inside an eval:

var functionString = "nonexsitantFunction"
eval("typeof " + functionString) // returns "undefined" or "function"

Don't reverse this and try a typeof on eval. If you do a ReferenceError will be thrown:

var functionString = "nonexsitantFunction"
typeof(eval(functionString)) // returns ReferenceError: [function] is not defined

With no conditions

me.onChange=function(){};

function getID( swfID ){
     if(navigator.appName.indexOf("Microsoft") != -1){
          me = window[swfID];
     }else{
          me = document[swfID];
     }
}

function js_to_as( str ){
     me.onChange(str);
}

I would suspect that me is not getting correctly assigned onload.

Moving the get_ID call into the onclick event should take care of it.

Obviously you can further trap as previously mentioned:

function js_to_as( str) {
  var me = get_ID('jsExample');
  if (me && me.onChange) {
    me.onChange(str);
  }
}

In a few words: catch the exception.

I am really surprised nobody answered or commented about Exception Catch on this post yet.

Detail: Here goes an example where I try to match a function which is prefixed by mask_ and suffixed by the form field "name". When JavaScript does not find the function, it should throw an ReferenceError which you can handle as you wish on the catch section.

_x000D_
_x000D_
function inputMask(input) {_x000D_
  try {_x000D_
    let maskedInput = eval("mask_"+input.name);_x000D_
_x000D_
    if(typeof maskedInput === "undefined")_x000D_
        return input.value;_x000D_
    else_x000D_
        return eval("mask_"+input.name)(input);_x000D_
_x000D_
  } catch(e) {_x000D_
    if (e instanceof ReferenceError) {_x000D_
      return input.value;_x000D_
    }_x000D_
  }_x000D_
}
_x000D_
_x000D_
_x000D_


Here is a working and simple solution for checking existence of a function and triggering that function dynamically by another function;

Trigger function

function runDynamicFunction(functionname){ 

    if (typeof window[functionname] == "function") { //check availability

        window[functionname]("this is from the function it"); // run function and pass a parameter to it
    }
}

and you can now generate the function dynamically maybe using php like this

function runThis_func(my_Parameter){

    alert(my_Parameter +" triggerd");
}

now you can call the function using dynamically generated event

<?php

$name_frm_somware ="runThis_func";

echo "<input type='button' value='Button' onclick='runDynamicFunction(\"".$name_frm_somware."\");'>";

?>

the exact HTML code you need is

<input type="button" value="Button" onclick="runDynamicFunction('runThis_func');">

    function sum(nb1,nb2){

       return nb1+nb2;
    }

    try{

      if(sum() != undefined){/*test if the function is defined before call it*/

        sum(3,5);               /*once the function is exist you can call it */

      }

    }catch(e){

      console.log("function not defined");/*the function is not defined or does not exists*/
    }

And then there is this...

( document.exitPointerLock || Function )();

To illustrate the preceding answers, here a quick JSFiddle snippet :

_x000D_
_x000D_
function test () {_x000D_
console.log()_x000D_
_x000D_
}_x000D_
_x000D_
console.log(typeof test) // >> "function"_x000D_
_x000D_
// implicit test, in javascript if an entity exist it returns implcitly true unless the element value is false as :_x000D_
// var test = false_x000D_
if(test){ console.log(true)}_x000D_
else{console.log(false)}_x000D_
_x000D_
// test by the typeof method_x000D_
if( typeof test === "function"){ console.log(true)}_x000D_
else{console.log(false)}_x000D_
_x000D_
_x000D_
// confirm that the test is effective : _x000D_
// - entity with false value_x000D_
var test2 = false_x000D_
if(test2){ console.log(true)}_x000D_
else{console.log(false)}_x000D_
_x000D_
// confirm that the test is effective :_x000D_
// - typeof entity_x000D_
if( typeof test ==="foo"){ console.log(true)}_x000D_
else{console.log(false)}_x000D_
_x000D_
/* Expected :_x000D_
function_x000D_
true _x000D_
true _x000D_
false_x000D_
false_x000D_
*/
_x000D_
_x000D_
_x000D_


I had this problem.

if (obj && typeof obj === 'function') { ... }

kept throwing a reference error if obj happened to be undefined.

In the end I did the following:

if (typeof obj !== 'undefined' && typeof obj === 'function') { ... }

A colleague pointed out to me that checking if it's !== 'undefined' and then === 'function' is redundant of course.

Simpler:

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

Much cleaner and works great.


Modern Javascript to the rescue!

In 2021, this is solved* at the language level in Javascript (and Typescript too) with the new Optional Chaining syntax

me.onChange?.(str)

It's as simple as that - onChange only gets called if it exists.

If onChange doesn't exist, nothing happens, and the expression returns undefined.

So if onChange() would ordinarily return a value, you'd want to check value !== undefined before continuing.

An extra thing to note - if onChange does exist but is not a function, this will throw a TypeError. This is as you would expect, it's the same behaviour as calling any non-function as a function, just worth pointing out that Optional Chaining doesn't do any magic to make this go away.

* Optional Chaining is a stage 4 TC39 proposal, so isn't technically in the ECMAScript spec yet. But, stage 4 means it's finalised and essentially guaranteed to be included in the next version. You can use Optional Chaining today via Babel or Typescript with confidence it won't change.


This simple jQuery code should do the trick:

if (jQuery.isFunction(functionName)) {
    functionName();
}

I have tried the accepted answer; however:

console.log(typeof me.onChange);

returns 'undefined'. I've noticed that the specification states an event called 'onchange' instead of 'onChange' (notice the camelCase).

Changing the original accepted answer to the following worked for me:

if (typeof me.onchange === "function") { 
  // safe to use the function
}

The Underscore.js library defines it in the isFunction method as this (which comments suggest may cater for some browser bugs)

typeof obj == 'function' || false

http://underscorejs.org/docs/underscore.html#section-143


function js_to_as( str ){
     if (me && me.onChange)
         me.onChange(str);
}

I have also been looking for an elegant solution to this problem. After much reflection, I found this approach best.

const func = me.onChange || (str => {}); func(str);


//Simple function that will tell if the function is defined or not
function is_function(func) {
    return typeof window[func] !== 'undefined' && $.isFunction(window[func]);
}

//usage

if (is_function("myFunction") {
        alert("myFunction defined");
    } else {
        alert("myFunction not defined");
    }

function isFunction( o ) { return null !== o && "function" === typeof o && !!o.apply; }


I always check like this:

if(!myFunction){return false;}

just place it before any code that uses this function


Try typeof -- Look for 'undefined' to say it doesn't exist, 'function' for a function. JSFiddle for this code

function thisishere() {
    return false;
}
alert("thisishere() is a " + typeof thisishere);
alert("thisisnthere() is " + typeof thisisnthere);

Or as an if:

if (typeof thisishere === 'function') {
    // function exists
}

Or with a return value, on a single line:

var exists = (typeof thisishere === 'function') ? "Value if true" : "Value if false";
var exists = (typeof thisishere === 'function') // Returns true or false

// just pass your tested function name instead of myFunctionName
if ( $.isFunction($.fn.myFunctionName) ) {
    console.log( 'write your code here.' );
}

How about:

if('functionName' in Obj){
    //code
}

e.g.

var color1 = new String("green");
"length" in color1 // returns true
"indexOf" in color1 // returns true
"blablabla" in color1 // returns false

or as for your case:

if('onChange' in me){
    //code
}

See MDN docs.


This will verify if the function exists, if so it will be executed

me.onChange && me.onChange(str);

Thus the error TypeError: me.onChange is not a function is prevent.


Try this one:

Window.function_exists=function(function_name,scope){
//Setting default scope of none is provided
If(typeof scope === 'undefined') scope=window;
//Checking if function name is defined
If (typeof function_name === 'undefined') throw new 
Error('You have to provide an valid function name!');
//The type container
var fn= (typeof scope[function_name]);
//Function type
If(fn === 'function') return true;
//Function object type
if(fn.indexOf('function')!== false) return true; 
return false;
}

Be aware that I've write this with my cellphone Might contain some uppercase issues and/or other corrections needed like for example functions name

If you want a function like PHP to check if the var is set:

Window.isset=function (variable_con){
If(typeof variable_con !== 'undefined') return true;
return false;
}

I had the case where the name of the function varied according to a variable (var 'x' in this case) added to the functions name. This works:

if ( typeof window['afunction_'+x] === 'function' ) { window['afunction_'+x](); } 

function function_exists(function_name)
{
    return eval('typeof ' + function_name) === 'function';
}
alert(function_exists('test'));
alert(function_exists('function_exists'));

OR

function function_exists(func_name) {
  //  discuss at: http://phpjs.org/functions/function_exists/
  // original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // improved by: Steve Clay
  // improved by: Legaev Andrey
  // improved by: Brett Zamir (http://brett-zamir.me)
  //   example 1: function_exists('isFinite');
  //   returns 1: true

  if (typeof func_name === 'string') {
    func_name = this.window[func_name];
  }
  return typeof func_name === 'function';
}

I'll go 1 step further to make sure the property is indeed a function

function js_to_as( str ){
     if (me && me.onChange && typeof me.onChange === 'function') {
         me.onChange(str);
     }
}