Can you compare anonymous functions in javascript?

Question

I need to detect whether two anonymous functions are the same.

In the console, why does the following line return false?

(function() { alert("hello"); }) === (function() { alert("hello"); })

Is there a comparison operator or other method to identify that these are 'functionally' the same?

EDIT:

People are asking for the use of this.

It's for removing a function that has been previously pushed into an array of functions. See http://jsfiddle.net/w6s7J/ for a simplified test.

I'm 99% sure there is a better way to do whatever you are attempting to do — Jeff Shaver, Jan 24 '14 at 14:16
Would you count these as 'functionally' the same? `(function() { alert("hello"); }) === (function(doNothing) { alert("hello"); })`. I'm afraid there is no guaranteed way to compare. Even something like this `(function() { var a = 'hello'; alert(a); }) === (function() { var b = 'hello'; alert(b); })`. So it all depend on your definition and what you are prepared to check and accept. — Xotic750, Jan 24 '14 at 14:32
Comment on your fiddle: Doing it this way is not a good idea. First it's repetitive (you have to write the function body twice!) and second you create the problem of testing for equality. Why not just define the function once outside and then referring to the exact same function? Then you can do `func1 === func2` (http://jsfiddle.net/basilikum/w6s7J/1/). — basilikum, Jan 24 '14 at 14:32
Thanks for all the advice, I've learned a lot from this. I was trying to do it this way to avoid problems with scope when the callbacks run, but found out about using .call() to set the value of *this* inside callbacks. — Josh Harrison, Jan 25 '14 at 21:00
Required reading: https://en.wikipedia.org/wiki/First-class_function#Equality_of_functions — Bergi, Apr 03 '15 at 15:14

score 2 · Accepted Answer · answered Jan 24 '14 at 14:42

There is no real way to compare 2 different anonymous functions for 'functionality'.

You can check if they are the same object by using your example code.

var func = function () {
    alert('hello');
};

alert(func === func);

The above will work, as you are checking to see that both objects are the same.

The only other method for comparing is to compare them as a string.

var func1 = function () {
    alert("hello");
};

var func2 = function () {
    alert('hello');
};

alert(func1.toString() === func2.toString());

Oops!, they are functionally the same, the difference is the quotes used, So this returns false.

Comparing as string is actually pretty safe if the code is not actually typed twice. I had a function where I allowed registering callbacks to some action. I used object comparison to avoid duplication, but the noticed it didn't always work because some of the callbacks were anonymus functions. With string comparison it fixed my issue — LobsterMan, May 02 '23 at 14:54

score 1 · Answer 2 · edited Mar 23 '20 at 20:30

1

(function() { alert("hello"); }) === (function() { alert("hello"); })

Returns false because they are different objects in javascript.

(function() { alert("hello"); }).toString() === (function() { alert("hello"); }).toString()

Returns true because they are the same strings.

Also function objects have a name property which return the name of the function (but it does not work in IE):

var  a = function b() {}
alert(a.name);//write b

edited Mar 23 '20 at 20:30

infografnet

answered Jan 24 '14 at 14:19

Alex

But `(function(){ }).toString() === (function(){}).toSting() // false` because of space. – Ivan Black Aug 01 '14 at 08:59

2 Answers2