check if a function exists with its name in a string?

Question

I create functions in Javascript dynamically. Sometimes I need to check if a certain function is actually already created.

I have the name of the function as a string. How can I check whether a function exists based on a given value in a string?

score 81 · Accepted Answer · edited Apr 10 '13 at 06:41

81

You can check whether it's defined in the global scope using;

if (typeof window[strOfFunction] === "function") {
    // celebrate
    //window[strOfFunction](); //To call the function dynamically!
}

edited Apr 10 '13 at 06:41

Vikas

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answered Dec 21 '11 at 15:16

Matt

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How would this work for a jQuery plugin? like $.fn.mask? – Joseph Astrahan Jun 03 '15 at 16:54
What about if I want to allow my function to pass `window` in string format too? `load.js("script.js", function() {}, "window.myCallbackFn");` – Fredrik Borggren Jun 03 '15 at 20:46
2

Doesn't work when you don't have a `window` object, like with node. – Vidar Sep 15 '18 at 00:01
Can you use `this` instead of `window`? Will that work in node and a browser? Or would `if (typeof (window || this)[strOfFunction] === "function") { /* ... */ }` be better form? – TamusJRoyce Oct 13 '18 at 22:53

score 8 · Answer 2 · answered Apr 15 '14 at 18:45

8

You can use eval:

if ( eval("typeof stringFunction === 'function'") ){ /*whatever*/ }

answered Apr 15 '14 at 18:45

Fredius

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The use of `eval` should never be taken lightly. One needs to understand the implications of using it. See [Eval is Evil](http://blogs.msdn.com/b/ericlippert/archive/2003/11/01/53329.aspx) and [Eval isn't evil, just misunderstood](http://www.nczonline.net/blog/2013/06/25/eval-isnt-evil-just-misunderstood/). – Jason Aller Apr 15 '14 at 19:07
2

You can use eval() for many things but you should really only use it if you have no other alternative. It is handy to be able to just slap a string together and get it to evaluate. if ( eval("typeof stringFunction === 'function'") ){ /*whatever*/ } IMHO should be eval("answer = typeof stringFunction === 'function'"); if ( answer ){ /*whatever*/ } Some funny stuff is happening with things like with, eval, this, and some muppet theory regarding the use of "use strict"; to help improve coding, fine if you are going to make it a compiled language but JS isn't... – Mark Giblin Jan 19 '15 at 11:39
1

Literally always some guy has to point out that "eval is evil". Using eval is fine if you know what you're doing. – Vidar Sep 14 '18 at 23:17
Yes, I love eval. Eval is the angel – Arash Sep 26 '20 at 12:11

score 0 · Answer 3 · answered Jun 12 '20 at 10:23

We're adding our 2 cents because the accepted answer is right, but benefits from a little clarification:

window["myFunctionNameHere"] is one simple way to solve the problem, but basically considering window as any global object accessible in the desired scope. Additionally, you must be sure to actually ASSIGN the function properly in such scope, of course.

In the case of window, for example, you'll first need

<script>
    $(document).ready(function () {
        window.myFunctionNameHere = function() {
            console.log('Hello world');
        }
    });
</script>

After this, the

typeof window["myFunctionNameHere"] === "function"

Will work as expected.

check if a function exists with its name in a string?

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