Tell if a Javascript function is defined by looking at self[name] - is this a good way?

Question

This is a follow up question to This Question.

I like (and understand) the solution there. However, in the code I am working in, another way to solve the same problem is used:

function exist(sFN) {
    if(self[sFN]) return true;
    return false;
}

It seems to work fine, although I don't understand how. Does it work? How? What are minuses of this approach? Should I switch to solution from the other question?

Answer 1

just use typeof.

typeof(foobar)  // -> undefined
typeof(alert)   // -> function

You can't, however, defined a function based on typeof, because you'd need to pass an identifier which might not exist. So if you define function isfun(sym) { return typeof(sym) }, and then tried calling isfun(inexistent), your code would throw.

The fun thing about typeof is that it's an operator, not a function. So you can use it to check a symbol that's not defined without throwing.

---

if you assume a function in the global scope (i.e., not within a closure), you can define a function to check it as follows:

function isfun(identifier) {
  return typeof(window[identifier]) == 'function';
}

Here you pass an string for the identifier, so:

isfun('alert');   // -> true
isfun('foobar');  // -> false

---

closure?

Here's an example of a function defined within a closure. Here, the printed value would be false, which is wrong.

(function closure() { 
  function enclosed() {}
  print(isfun('enclosed'))
})()

Answer 2

Try this:

function exist(sFN) {
 return (typeof sFN == 'function');
}

Answer 3

Your condition is checking the existence of the "sFN" property in the "self" object. Anything that isn't null, undefined, 0, and "" will evaluate to true.

As others have said, you can use typeof, or instanceof to see if it's actually a function.

Looking at your linked example, you should read up on the difference between ==/!= and ===/!== in javascript. Short answer: ("" == null) is true, ("" === null) is false.

Answer 4

You can't really wrap this in a method, but it's so simple there is really no need.

function bob()
{}

if( typeof bob == "function" )
    alert( "bob exists" );

if( typeof dave != "function" )
    alert( "dave doesn't" );

Answer 5

FYI: There is (or was) a nice pitfall for typeof.

FF2 returns 'function' for typeof(/pattern/). FF3, IE7, and Chrome all return 'object' for the same code. (I can't verify other browsers.)

Assuming everyone that used FF2 has upgraded, you're in the clear. But, that's probably a far-fetched assumption.

Answer 6

I read somewhere (here and here) that functions are properties of the window object, so you can do the following:

if (window.my_func_name) {
    my_func_name('tester!');
}

or for popups:

if (window.opener.my_func_name) {
    my_func_name('tester!');
}

A complete solution then:

function function_exists(func_name) {
    var eval_string;
    if (window.opener) {
        eval_string = 'window.opener.' + func_name;
    } else {
        eval_string = 'window.' + func_name;
    }
    return eval(eval_string + ' ? true : false');
}

Answer 7

Object.prototype.toString.apply(value) === '[object Function]'