Correct way to check if JS function exists?

JavaScript
1

What is the correct way to check if a JS function exists?

I’m told that typeof works but isn’t a very good way to do it.

Also, this doesn’t work:


if( function_name ) {
function_name();
}
2

As far as I know, typeof is both good and standard. Did the person who said otherwise give a reason?

3

No, which is why I thought to check.

Considering ‘typeof’ works, but ‘if(function_name)’ doesn’t then typeof is good enough for me!

4

Do you have a test case where if(function_name) doesn’t work?


var yep = function() {}
if (yep) { console.log('yep') }
else { console.log('nope') }
5

If you don’t use typeof then you will not know if a function by that name exists - the name might be being used for something else. It might even be being used but contain a value that returns false.

var yep = {} // an object instead of a function
 if (yep) { console.log('yep') } // incorrect response
 else { console.log('nope') }

The safest way to test is:

if (typeof yep==='function')

That way if the name exists but isn’t a function your subsequent processing will still work correctly.

As an actual example of where just testing the name wouldn’t work - I have a calendar script where you can either supply an object containing a list of holiday dates or supply a function that will generate the object. By testing whether the value passed is a function or not the code can run the function to generate the object. Simply testing if the field exists wouldn’t work since both an object and a function would return true for that test.

6

Sure do.


if (yep) { console.log('yep') }
else { console.log('nope') }

This results in an error: “yep is not defined”

Of course, really this only happens because “yep” wasn’t declared with “var”. So instead of typeof, you could also do this.


// if yep exists, then re-declaring it won't do any harm
// if yep doesn't exist, then we declare it for the first time
var yep;

if (yep) { console.log('yep') }
else { console.log('nope') }
7

In strict JavaScript the code would just fail to run completely (since in strict JavaScript all variables MUST be defined) and so wouldn’t generate the error.