How to get the function name from within that function?

How can I access a function name from inside that function?

// parasitic inheritance
var ns.parent.child = function() {
  var parent = new ns.parent();
  parent.newFunc = function() {

  }
  return parent;
}

var ns.parent = function() {
  // at this point, i want to know who the child is that called the parent
  // ie
}

var obj = new ns.parent.child();

Answers:

Answer

In ES5, the best thing to do is:

function functionName(fun) {
  var ret = fun.toString();
  ret = ret.substr('function '.length);
  ret = ret.substr(0, ret.indexOf('('));
  return ret;
}

Using Function.caller is non-standard. Function.caller and arguments.callee are both forbidden in strict mode.

Edit: nus's regex based answer below achieves the same thing, but has better performance!

In ES6, you can just use myFunction.name.

Note: Beware that some JS minifiers might throw away function names, to compress better; you may need to tweak their settings to avoid that.

Answer

ES6 (inspired by sendy halim's answer below):

myFunction.name

Explanation on MDN. As of 2015 works in nodejs and all major browsers except IE.

Note: On bound functions this will give "bound <originalName>". You will have to strip the "bound " if you want to get the original name.


ES5 (inspired by Vlad's answer):

If you have a reference to the function, you can do:

function functionName( func )
{
    // Match:
    // - ^          the beginning of the string
    // - function   the word 'function'
    // - \s+        at least some white space
    // - ([\w\$]+)  capture one or more valid JavaScript identifier characters
    // - \s*        optionally followed by white space (in theory there won't be any here,
    //              so if performance is an issue this can be omitted[1]
    // - \(         followed by an opening brace
    //
    var result = /^function\s+([\w\$]+)\s*\(/.exec( func.toString() )

    return  result  ?  result[ 1 ]  :  '' // for an anonymous function there won't be a match
}
  • I have not run unit tests on this, or verified implementation differences, but in principle it should work, if not leave a comment.
  • Note: won't work on bound functions
  • Note: that caller and callee are considered deprecated.

[1] I include it here because it is legal and often enough syntax highlighting tools fail to take into account the white space between function name and parenthesis. On the other hand, I'm not aware of any implementation of .toString() that will include white space here, so that's why you can omit it.


As an answer to the original question, I would drop parasitic inheritance and go for some more traditional OOP design patterns. I wrote a TidBits.OoJs to comfortably write OOP code in JavaScript with a feature set mimicking C++ (not yet complete, but mostly).

I see from the comments that you would like to avoid passing information parent needs to it's constructor. I must admit that traditional design patterns won't save you from that one though, since it is generally a considered a good thing to make your dependencies obvious and enforced.

I would also suggest to steer away from anonymous functions. They only make debugging and profiling a PITA because everything just shows up as "anonymous function", and there is no benefit to them that I'm aware of.

Answer

what you're doing is assigning unnamed function to a variable. you probably need named function expression instead ( http://kangax.github.com/nfe/ ).

var x = function x() {
    console.log( arguments.callee.name );
}
x();

however I'm not sure how much cross-browser that is; there's an issue with IE6 that makes you function's name leak to the outer scope. also, arguments.callee is kind of deprecated and will result in error if you're using strict mode.

Answer

Any constructor exposes a property name, which is the function name. You access the constructor via an instance (using new) or a prototype:

function Person() {
  console.log(this.constructor.name); //Person
}

var p = new Person();
console.log(p.constructor.name); //Person

console.log(Person.prototype.constructor.name);  //Person
Answer

This might work for you:

function foo() { bar(); }

function bar() { console.log(bar.caller.name); }

running foo() will output "foo" or undefined if you call from an anonymous function.

It works with constructors too, in which case it would output the name of the calling constructor (eg "Foo").

More info here: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/Caller

They claim it's non-standard, but also that it's supported by all major browsers: Firefox, Safari, Chrome, Opera and IE.

Answer

It looks like the most stupid thing, that I wrote in my life, but it's funny :D

function getName(d){
  const error = new Error();
  const firefoxMatch = (error.stack.split('\n')[0 + d].match(/^.*([email protected])/) || [])[0];
  const chromeMatch = ((((error.stack.split('at ') || [])[1 + d] || '').match(/(^|\.| <| )(.*[^(<])( \()/) || [])[2] || '').split('.').pop();
  const safariMatch = error.stack.split('\n')[0 + d];

  // firefoxMatch ? console.log('firefoxMatch', firefoxMatch) : void 0;
  // chromeMatch ? console.log('chromeMatch', chromeMatch) : void 0;
  // safariMatch ? console.log('safariMatch', safariMatch) : void 0;

  return firefoxMatch || chromeMatch || safariMatch;
}

d - depth of stack. 0 - return this function name, 1 - parent, etc.;
[0 + d] - just for understanding - what happens;
firefoxMatch - works for safari, but I had really a little time for testing, because mac's owner had returned after smoking, and drove me away :'(

Testing:

function limbo(){
  for(let i = 0; i < 4; i++){
    console.log(getName(i));
  }
}
function lust(){
  limbo();
}
function gluttony(){
  lust();
}

gluttony();

Result:
Chrome:
enter image description here

Fitefox:
enter image description here

This solution was creating only just for fun! Dont use it for real projects. It not depends on ES specification, it depends only on browser realisation. After next chrome/firefox/safari update it may be broken.
More than that there is no error (ha) processing - if d will be more than stack length - you will get an error;
For other wrowsers error's messaage pattern - you will get an error;
It must work for ES6 classes (.split('.').pop()), but you sill can get an erorr;

Answer

You can't. Functions don't have names according to the standard (though mozilla has such an attribute) - they can only be assigned to variables with names.

Also your comment:

// access fully qualified name (ie "my.namespace.myFunc")

is inside the function my.namespace.myFunc.getFn

What you can do is return the constructor of an object created by new

So you could say

var obj = new my.namespace.myFunc();
console.info(obj.constructor); //my.namespace.myFunc
Answer

You could use this, for browsers that support Error.stack (not nearly all, probably)

function WriteSomeShitOut(){ 
  var a = new Error().stack.match(/at (.*?) /);
  console.log(a[1]);
} 
WriteSomeShitOut();

of course this is for the current function, but you get the idea.

happy drooling while you code

Answer

You can use name property to get the function name, unless you're using an anonymous function

For example:

var Person = function Person () {
  this.someMethod = function () {};
};

Person.prototype.getSomeMethodName = function () {
  return this.someMethod.name;
};

var p = new Person();
// will return "", because someMethod is assigned with anonymous function
console.log(p.getSomeMethodName());

now let's try with named function

var Person = function Person () {
  this.someMethod = function someMethod() {};
};

now you can use

// will return "someMethod"
p.getSomeMethodName()
Answer

You can use constructor name like:

{your_function}.prototype.constructor.name

this code simply return name of a method.

Answer

as part as ECMAScript 6 you can use Function.name method

function doSomething() {}

alert(doSomething.name); // alerts "doSomething"
Answer

You could use Function.name:

In most implementations of JavaScript, once you have your constructor's reference in scope, you can get its string name from its name property (e.g. Function.name, or Object.constructor.name

You could use Function.callee:

The native arguments.caller method has been deprecated, but most browsers support Function.caller, which will return the actual invoking object (its body of code): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/caller?redirectlocale=en-US&redirectslug=JavaScript%2FReference%2FGlobal_Objects%2FFunction%2Fcaller

You could create a source map:

If what you need is the literal function signature (the "name" of it) and not the object itself, you might have to resort to something a little more customized, like creating an array reference of the API string values you'll need to access frequently. You can map them together using Object.keys() and your array of strings, or look into Mozilla's source maps library on GitHub, for bigger projects: https://github.com/mozilla/source-map

Answer

I know this is a old question but lately I've been facing some similar issue while trying to decorate some React Component's methods, for debugging purposes. As people already said, arguments.caller and arguments.callee are forbidden in strict mode which is probably enabled by default in your React transpiling. You can either disable it, or I've been able to come up with another hack, because in React all class functions are named, you can actually do this:

Component.prototype.componentWillMount = function componentWillMount() {
    console.log('Callee name: ', this.__proto__.constructor.toString().substr(0,30));
...
}
Answer

This worked for me.

function AbstractDomainClass() {
    this.className = function() {
        if (!this.$className) {
            var className = this.constructor.toString();
            className = className.substr('function '.length);
            className = className.substr(0, className.indexOf('('));
            this.$className = className;
        }
        return this.$className;
    }
}

Test code:

var obj = new AbstractDomainClass();
expect(obj.className()).toBe('AbstractDomainClass');
Answer

I had a similar problem and I solved it as follows:

Function.prototype.myname = function() {
   return this.toString()
       .substr( 0, this.toString().indexOf( "(" ) )
       .replace( "function ", "" ); 
}

This code implements, in a more comfortable fashion, one response I already read here at the top of this discussion. Now I have a member function retrieving the name of any function object. Here's the full script ...

<script language="javascript" TYPE="text/javascript">

    Function.prototype.myname = function() { 
        return this.toString()
            .substr( 0, this.toString().indexOf( "(" ) )
            .replace("function ", "" ); 
    }
    function call_this( _fn ) { document.write( _fn.myname() ); }
    function _yeaaahhh() { /* do something */ }
    call_this( _yeaaahhh ); 

</script>
Answer

If I understood what you wanted to do, this is what I do inside a function constructor.

if (!(this instanceof arguments.callee)) {
    throw "ReferenceError: " + arguments.callee.name + " is not defined";
}
Answer

This will work in ES5, ES6, all browsers and strict mode functions.

Here's how it looks with a named function.

(function myName() {
  console.log(new Error().stack.split(/\r\n|\r|\n/g)[1].trim());
})();
at myName (<anonymous>:2:15)

Here's how it looks with an anonymous function.

(() => {
  console.log(new Error().stack.split(/\r\n|\r|\n/g)[1].trim());
})();
at <anonymous>:2:15
Answer

look here: http://www.tek-tips.com/viewthread.cfm?qid=1209619

arguments.callee.toString();

seems to be right for your needs.

Answer

you can use Error.stack to trace the function name and exact position of where you are in it.

See stacktrace.js

Answer

Easy way to get function name from within fuction you are running.

function x(){alert(this.name)};x()

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