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, May 16 '12 at 18:06
what happens if the callback doesn't have a name? -- ie `bar(function() {...});` — Spudley, May 16 '12 at 21:21
@Spudley: ideally, the code should throw an exception stating that anonymous functions are not supported. — Gili, May 17 '12 at 01:03

score 31 · Accepted Answer · edited Dec 19 '12 at 22:15

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 '12 at 22:15

Gili

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answered May 16 '12 at 18:03

Phrogz

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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 May 16 '12 at 18:27
Why it doesn't match `foo){...`? – gdoron May 16 '12 at 19:54
1

@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. – Phrogz May 16 '12 at 19:57
This relies on `this` referring to the function object. That's not always the case though! foo.bar() -> `this` === foo for example. – Jul 26 '13 at 19:24
1

modified a bit not to include spaces: /function ([^(\s]*)/ – VuesomeDev Oct 02 '13 at 15:18

score 21 · Answer 2 · edited Jul 30 '19 at 12:32

21

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 '19 at 12:32

Community

1
1

answered May 16 '12 at 18:01

gdoron

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1

`[the name] property is not standard` is no longer true, it's in ECMA 6.0 / 2015: http://www.ecma-international.org/ecma-262/6.0/#sec-setfunctionname / Also see http://www.2ality.com/2015/09/function-names-es6.html – CodeManX Apr 08 '16 at 10:39

score 6 · Answer 3 · answered May 16 '12 at 18:09

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 '12 at 18:09

kojiro

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More elegant regex than mine; nice :) Are you certain that all browsers will place a space after the leading `function`? – Phrogz May 16 '12 at 18:13
@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. – kojiro May 16 '12 at 18:27
Having seen your code I recall looking at the spec long ago for the normalized string representation and IIRC the space is required. – Phrogz May 16 '12 at 18:40

score 2 · Answer 4 · answered May 16 '12 at 18:02

2

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

answered May 16 '12 at 18:02

osahyoun

5,173
2
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score 2 · Answer 5 · answered Oct 30 '14 at 12:17

2

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 '14 at 12:17

David Fooks

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10
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This will probably work, but isn't it much more expensive to construct a stack-trace than to parse `this.toString()`? – Gili Oct 30 '14 at 19:28
Yeah, but I'm only using this for debug messages. The above answers would only return "hi" this returns "MyObject.hi". – David Fooks Nov 07 '14 at 15:59
1

That is really nice for debugging. Thanks or sharing! – Raisch Feb 16 '17 at 13:56

score 1 · Answer 6 · edited Jul 18 '12 at 15:47

1

try to access the .name property:

callback.name

edited Jul 18 '12 at 15:47

JMax

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answered May 16 '12 at 18:03

Tiago Peczenyj

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score 0 · Answer 7 · answered Apr 22 '13 at 14:32

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 '13 at 14:32

Pipe2290

1
1

score 0 · Answer 8 · answered Nov 24 '14 at 09:12

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];
    }
  });
}

Get name as String from a Javascript function reference?

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