Hướng dẫn check function exists 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)

animuson♦

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asked Jun 25, 2009 at 4:10

Alec SmartAlec Smart

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4

Try something like this:

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

or better yet (as per UpTheCreek upvoted comment)

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

j0k

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answered Jun 25, 2009 at 4:14

Andrew HareAndrew Hare

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14

I had this problem. if (obj && typeof obj === 'function') { ... } kept throwing a reference error if obj happened to be undefined, so in the end I did the following:

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

However, a colleague pointed out to me that checking if it's !== 'undefined' and then === 'function' is redundant, thus:

Simpler:

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

Much cleaner and works great.

ashleedawg

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answered Feb 6, 2013 at 23:27

Misha NasledovMisha Nasledov

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3

Modern JavaScript to the rescue!

me.onChange?.(str)

The Optional Chaining syntax (?.) solves this

• in JavaScript since ES2020
• in Typescript since version 3.7

In the example above, if a me.onChange property exists and is a function, it is called.

If no me.onChange property exists, nothing happens: the expression just returns undefined.

If a me.onChange property exists but is not a function, a TypeError will be thrown just like when you call any non-function as a function in JavaScript. Optional Chaining doesn't do any magic to make this go away.

answered Mar 3, 2020 at 15:57

7

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.

answered Feb 10, 2015 at 16:15

1

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

answered May 27, 2011 at 21:50

dhulihandhulihan

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2

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

answered Aug 14, 2013 at 20:32

Mat CarlsonMat Carlson

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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.

answered Jan 18, 2016 at 20:04

Samir AlajmovicSamir Alajmovic

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0

For me the easiest way :

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

answered Aug 27, 2018 at 12:21

Tu4n3rTu4n3r

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1

//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");
    }

answered Apr 23, 2015 at 13:14

3

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';
}

answered Oct 11, 2018 at 6:49

2

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

answered Dec 15, 2018 at 15:43

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

answered Jun 25, 2009 at 4:13

MiffTheFoxMiffTheFox

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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);
     }
}

answered Jun 25, 2009 at 8:04

AlexAlex

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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
}

answered Aug 13, 2015 at 17:08

David DouglasDavid Douglas

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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](); } 

answered Mar 18, 2016 at 14:02

0

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');
}

Evg

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answered Feb 15, 2017 at 15:24

Lucas BustamanteLucas Bustamante

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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

";

?>

the exact HTML code you need is

Brian C

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answered Mar 23, 2018 at 7:59

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.

function inputMask(input) {
  try {
    let maskedInput = eval("mask_"+input.name);

    if(typeof maskedInput === "undefined")
        return input.value;
    else
        return eval("mask_"+input.name)(input);

  } catch(e) {
    if (e instanceof ReferenceError) {
      return input.value;
    }
  }
}

marc_s

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answered Oct 24, 2019 at 18:24

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);
}

answered Jun 25, 2009 at 4:19

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);
  }
}

answered Jun 25, 2009 at 4:51

wombletonwombleton

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I always check like this:

if(!myFunction){return false;}

just place it before any code that uses this function

answered Sep 21, 2013 at 11:57

el Dudeel Dude

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This simple jQuery code should do the trick:

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

answered Nov 18, 2016 at 15:03

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
}

answered Mar 1, 2017 at 16:57

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);

answered Jul 27, 2019 at 9:29

JulianJulian

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2

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);
}

answered Nov 3, 2020 at 13:27

3limin4t0r3limin4t0r

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    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*/
    }

answered May 17, 2017 at 5:05

Ir CalifIr Calif

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And then there is this...

( document.exitPointerLock || Function )();

answered Dec 16, 2017 at 12:49

Master JamesMaster James

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4

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;
}

answered May 1, 2018 at 13:10

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

function test () {
console.log()

}

console.log(typeof test) // >> "function"

// implicit test, in javascript if an entity exist it returns implcitly true unless the element value is false as :
// var test = false
if(test){ console.log(true)}
else{console.log(false)}

// test by the typeof method
if( typeof test === "function"){ console.log(true)}
else{console.log(false)}


// confirm that the test is effective : 
// - entity with false value
var test2 = false
if(test2){ console.log(true)}
else{console.log(false)}

// confirm that the test is effective :
// - typeof entity
if( typeof test ==="foo"){ console.log(true)}
else{console.log(false)}

/* Expected :
function
true 
true 
false
false
*/

answered Jun 12, 2018 at 17:43

HoCo_HoCo_

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