Get name as String from a Javascript function reference?

Question

I want to do the opposite of Get JavaScript function-object from its name as a string?

That is, given:

function foo()
{}

function bar(callback)
{
  var name = ???; // how to get "foo" from callback?
}

bar(foo);

How do I get the name of the function behind a reference?

@Alnitak. It should print foo. that's the function name. bat is a variable with a reference to the foo function. — gdoron
– gdoron, Commented May 16, 2012 at 18:06
what happens if the callback doesn't have a name? -- ie bar(function() {...}); — Spudley
– Spudley, Commented May 16, 2012 at 21:21
@Spudley: ideally, the code should throw an exception stating that anonymous functions are not supported. — Gili
– Gili, Commented May 17, 2012 at 1:03

Gili · Accepted Answer · 2012-12-19 22:15:12Z

31

If you can't use myFunction.name then you can:

// Add a new method available on all function values
Function.prototype.getName = function(){
  // Find zero or more non-paren chars after the function start
  return /function ([^(]*)/.exec( this+"" )[1];
};

Or for modern browsers that don't support the name property (do they exist?) add it directly:

if (Function.prototype.name === undefined){
  // Add a custom property to all function values
  // that actually invokes a method to get the value
  Object.defineProperty(Function.prototype,'name',{
    get:function(){
      return /function ([^(]*)/.exec( this+"" )[1];
    }
  });
}

edited Dec 19, 2012 at 22:15

Gili

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answered May 16, 2012 at 18:03

Phrogz

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5 Comments

gdoron Over a year ago

Ohhh, now I got it, the ( if for the exec match, not a part of the regex. cool. +1. BTW, why it doesn't alert foo)?

gdoron Over a year ago

Why it doesn't match foo){...?

Phrogz Over a year ago

@gdoron Because regex must match characters sequentially until it stops. The function in the regex starts it matching, and then it consumes zero or more anything-but-a-( characters (as many as possible). Once it has done that, it must stop; it can't "skip over" non-matches and continue on.

user1115652 Over a year ago

This relies on this referring to the function object. That's not always the case though! foo.bar() -> this === foo for example.

SoniCoder Over a year ago

modified a bit not to include spaces: /function ([^(\s]*)/

Community · Accepted Answer · 2019-07-30 12:32:13Z

24

var name = callback.name;

MDN:

The name property returns the name of a function, or an empty string for anonymous functions:

Live DEMO

edited Jul 30, 2019 at 12:32

CommunityBot

11 silver badge

answered May 16, 2012 at 18:01

gdoron

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1 Comment

CodeManX Over a year ago

[the name] property is not standard is no longer true, it's in ECMA 6.0 / 2015: ecma-international.org/ecma-262/6.0/#sec-setfunctionname / Also see 2ality.com/2015/09/function-names-es6.html

kojiro · Accepted Answer · 2012-05-16 18:09:38Z

6

function bar(callback){
    var name=callback.toString();
    var reg=/function ([^\(]*)/;
    return reg.exec(name)[1];
}

>>> function foo() { };
>>> bar(foo);
"foo"
>>> bar(function(){});
""

answered May 16, 2012 at 18:09

kojiro

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3 Comments

Phrogz Over a year ago

More elegant regex than mine; nice :) Are you certain that all browsers will place a space after the leading function?

kojiro Over a year ago

@Phrogz I'm confident enough not to change the above. If there is an edge case, it would only affect the anonymous function anyway, and bar could easily be adjusted to take that into account.

Phrogz Over a year ago

Having seen your code I recall looking at the spec long ago for the normalized string representation and IIRC the space is required.

osahyoun · Accepted Answer · 2012-05-16 18:02:59Z

2

var x = function fooBar(){};
console.log(x.name);
// "fooBar"

answered May 16, 2012 at 18:02

osahyoun

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Comments

JMax · Accepted Answer · 2012-07-18 15:47:08Z

1

try to access the .name property:

callback.name

edited Jul 18, 2012 at 15:47

JMax

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answered May 16, 2012 at 18:03

Tiago Peczenyj

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Comments

David Fooks · Accepted Answer · 2014-10-30 12:17:46Z

1

You can extract the object and function name with:

function getFunctionName()
{
    return (new Error()).stack.split('\n')[2].split(' ')[5];
}

For example:

function MyObject()
{
}

MyObject.prototype.hi = function hi()
{
    console.log(getFunctionName());
};

var myObject = new MyObject();
myObject.hi(); // outputs "MyObject.hi"

answered Oct 30, 2014 at 12:17

David Fooks

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3 Comments

Gili Over a year ago

This will probably work, but isn't it much more expensive to construct a stack-trace than to parse this.toString()?

David Fooks Over a year ago

Yeah, but I'm only using this for debug messages. The above answers would only return "hi" this returns "MyObject.hi".

Raisch Over a year ago

That is really nice for debugging. Thanks or sharing!

Pipe2290 · Accepted Answer · 2013-04-22 14:32:49Z

0

If you were looking for the function on an specific object event, this may help:

var a = document.form1
a.onsubmit.name

answered Apr 22, 2013 at 14:32

Pipe2290

11 bronze badge

Comments

JD Philippe · Accepted Answer · 2014-11-24 09:12:21Z

0

for me, with just a little modification (adding \ before parent), this work:

if (Function.prototype.name === undefined){
  // Add a custom property to all function values
  // that actually invokes a method to get the value
  Object.defineProperty(Function.prototype,'name',{
    get:function(){
      return /function ([^\(]*)/.exec( this+"" )[1];
    }
  });
}

answered Nov 24, 2014 at 9:12

JD Philippe

11 bronze badge

Comments

David Spector · Accepted Answer · 2024-09-01 14:12:45Z

-1

I think the answer is simpler than most of these. If the function is

function func() {}

then the name of the function as a string is func.toString() .

If the function is unnamed, then you would have to find the name of the variable or constant that stores its name (which depends on the program code).

answered Sep 1, 2024 at 14:12

David Spector

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