Dynamic function name in javascript?

I have this:

this.f = function instance(){};

I would like to have this:

this.f = function ["instance:" + a](){};

This will basically do it at the most simple level:

"use strict";
var name = "foo";
var func = new Function(
     "return function " + name + "(){ alert('sweet!')}"
)();

//call it, to test it
func();

If you want to get more fancy, I have a written an article on "Dynamic function names in JavaScript".

You can use Object.defineProperty as noted in the MDN JavaScript Reference:

var myName = "myName";
var f = function () { return true; };
Object.defineProperty(f, 'name', {value: myName, writable: false});

In recent engines, you can do

function nameFunction(name, body) {
  return {[name](...args) {return body(...args)}}[name]
}



const x = nameFunction("wonderful function", (p) => p*2)
console.log(x(9)) // => 18
console.log(x.name) // => "wonderful function"

Update 2021: CherryDT's answer should be the easiest most straight forward way now, but it doesn't work consistently with different browsers for stack traces or Function.prototype.toString(), so if you need that you're stuck with this less convenient solution.

Old answer: Many suggestions here are suboptimal, by using eval, hacky solutions or wrappers. As of ES2015 names are inferred from the syntactic position for variables and properties.

So this will work just fine:

const name = 'myFn';
const fn = {[name]: function() {}}[name];
fn.name // 'myFn'

Resist the temptation to create named function factory methods as you wouldn't be able to pass the function from outside and retrofit it into the syntactic position to infer its name. Then it's already too late. If you really need that, you have to create a wrapper. Someone did that here, but that solution doesn't work for classes (which are also functions).

A much more in-depth answer with all the variants outlined has been written here: https://stackoverflow.com/a/9479081/633921

update

As others mentioned, this is not the fastest nor most recommended solution. Marcosc's solution below is the way to go.

You can use eval:

var code = "this.f = function " + instance + "() {...}";
eval(code);

What about

this.f = window["instance:" + a] = function(){};

The only drawback is that the function in its toSource method wouldn't indicate a name. That's usually only a problem for debuggers.

The syntax function[i](){} implies an object with property values that are functions, function[], indexed by the name, [i].
Thus
{"f:1":function(){}, "f:2":function(){}, "f:A":function(){}, ... } ["f:"+i].

{"f:1":function f1(){}, "f:2":function f2(){}, "f:A":function fA(){}} ["f:"+i] will preserve function name identification. See notes below regarding :.

So,

javascript: alert(
  new function(a){
    this.f={"instance:1":function(){}, "instance:A":function(){}} ["instance:"+a]
  }("A") . toSource()
);

displays ({f:(function () {})}) in FireFox.
(This is almost the same idea as this solution, only it uses a generic object and no longer directly populates the window object with the functions.)

This method explicitly populates the environment with instance:x.

javascript: alert(
  new function(a){
    this.f=eval("instance:"+a+"="+function(){})
  }("A") . toSource()
);
alert(eval("instance:A"));

displays

({f:(function () {})})

and

function () {
}

Though the property function f references an anonymous function and not instance:x, this method avoids several problems with this solution.

javascript: alert(
  new function(a){
    eval("this.f=function instance"+a+"(){}")
  }("A") . toSource()
);
alert(instanceA);    /* is undefined outside the object context */

displays only

({f:(function instanceA() {})})

• The embedded : makes the javascript function instance:a(){} invalid.
• Instead of a reference, the function's actual text definition is parsed and interpreted by eval.

The following is not necessarily problematic,

• The instanceA function is not directly available for use as instanceA()

and so is much more consistent with the original problem context.

Given these considerations,

this.f = {"instance:1": function instance1(){},
          "instance:2": function instance2(){},
          "instance:A": function instanceA(){},
          "instance:Z": function instanceZ(){}
         } [ "instance:" + a ]

maintains the global computing environment with the semantics and syntax of the OP example as much as possible.

The most voted answer has got already defined [String] function body. I was looking for the solution to rename already declared function's name and finally after an hour of struggling I've dealt with it. It:

• takes the alredy declared function
• parses it to [String] with .toString() method
• then overwrites the name (of named function) or appends the new one (when anonymous) between function and (
• then creates the new renamed function with new Function() constructor

function nameAppender(name,fun){
  const reg = /^(function)(?:\s*|\s+([A-Za-z0-9_$]+)\s*)(\()/;
  return (new Function(`return ${fun.toString().replace(reg,`$1 ${name}$3`)}`))();
}

//WORK FOR ALREADY NAMED FUNCTIONS:
function hello(name){
  console.log('hello ' + name);
}

//rename the 'hello' function
var greeting = nameAppender('Greeting', hello); 

console.log(greeting); //function Greeting(name){...}


//WORK FOR ANONYMOUS FUNCTIONS:
//give the name for the anonymous function
var count = nameAppender('Count',function(x,y){ 
  this.x = x;
  this.y = y;
  this.area = x*y;
}); 

console.log(count); //function Count(x,y){...}

For setting the name of an existing anonymous function:
(Based on @Marcosc's answer)

var anonymous = function() { return true; }

var name = 'someName';
var strFn = anonymous.toString().replace('function ', 'return function ' + name);
var fn = new Function(strFn)();

console.log(fn()); // —> true

Demo.

Note: Don't do it ;/

Dynamic methods of an object may be created using Object Literal Extensions provided by ECMAScript 2015 (ES6):

const postfixes = ['foo', 'bar'];

const mainObj = {};

const makeDynamic = (postfix) => {
  const newMethodName = 'instance: ' + postfix;
  const tempObj = {
    [newMethodName]() {
      console.log(`called method ${newMethodName}`);
    }
  }
  Object.assign(mainObj, tempObj);
  return mainObj[newMethodName]();
}

const processPostfixes = (postfixes) => { 
  for (const postfix of postfixes) {
    makeDynamic(postfix); 
  }
};

processPostfixes(postfixes);

console.log(mainObj);

The output of running the code above is:

"called method instance: foo"
"called method instance: bar"
Object {
  "instance: bar": [Function anonymous],
  "instance: foo": [Function anonymous]
}

the best way it is create object with list of dynamic functions like:

const USER = 'user';

const userModule = {
  [USER + 'Action'] : function () { ... }, 
  [USER + 'OnClickHandler'] : function () { ... }, 
  [USER + 'OnCreateHook'] : function () { ... }, 
}

The function's name property by default isn't writeable, but since it's configurable we can still use Object.defineProperty to change it. Since Object.defineProperty conveniently returns the object itself, we can write a function with a dynamic name like this:

const theName = 'foobar'

const fn = Object.defineProperty(function () { /* ... */ }, 'name', { value: theName })

console.log(fn.name) // Logs foobar

Of course this could be factored out into a helper function:

const nameFunction = (name, fn) => Object.defineProperty(fn, 'name', { value: name })

const fn = nameFunction('foobar', function () { /* ... */ })

console.log(fn.name) // Logs foobar

The above nameFunction function can also be used to rename an existing function, of course (here it's just renaming and returning the anonymous one).

There are two methods to achieve this, and they have their pros and cons.

---

name property definition

Defining immutable name property of a function.

Pros

• Every character is available for the name. (eg. () 全 {}/1/얏호/ :D #GO(@*#%! /*)

Cons

• The function's syntactic (“expressional”) name may not correspond with its name property value.

---

Function expression evaluation

Making a named function expression and evaluating it with Function constructor.

Pros

• The function's syntactic (“expressional”) name always corresponds with its name property value.

Cons

• Whitespaces (and etc.) are not available for the name.
• Expression-injectable (eg. For input (){}/1//, the expression is return function (){}/1//() {}, gives NaN instead of a function.).

---

const demoeval = expr => (new Function(`return ${expr}`))();

// `name` property definition
const method1 = func_name => {
	const anon_func = function() {};
	Object.defineProperty(anon_func, "name", {value: func_name, writable: false});
	return anon_func;
};

const test11 = method1("DEF_PROP"); // No whitespace
console.log("DEF_PROP?", test11.name); // "DEF_PROP"
console.log("DEF_PROP?", demoeval(test11.toString()).name); // ""

const test12 = method1("DEF PROP"); // Whitespace
console.log("DEF PROP?", test12.name); // "DEF PROP"
console.log("DEF PROP?", demoeval(test12.toString()).name); // ""

// Function expression evaluation
const method2 = func_name => demoeval(`function ${func_name}() {}`);

const test21 = method2("EVAL_EXPR"); // No whitespace
console.log("EVAL_EXPR?", test21.name); // "EVAL_EXPR"
console.log("EVAL_EXPR?", demoeval(test21.toString()).name); // "EVAL_EXPR"

const test22 = method2("EVAL EXPR"); // Uncaught SyntaxError: Unexpected identifier

If you want to have a dynamic function like the __call function in PHP, you could use Proxies.

const target = {};

const handler = {
  get: function (target, name) {
    return (myArg) => {
      return new Promise(resolve => setTimeout(() => resolve('some' + myArg), 600))
    }
  }
};

const proxy = new Proxy(target, handler);

(async function() {
  const result = await proxy.foo('string')
  console.log('result', result) // 'result somestring' after 600 ms
})()

You can use Dynamic Function Name and parameters like this.

1. Define function Separate and call it

   let functionName = "testFunction"; let param = {"param1":1 , "param2":2};

   var func = new Function( "return " + functionName )();

   func(param);

   function testFunction(params){ alert(params.param1); }

2. Define function code dynamic

   let functionName = "testFunction(params)"; let param = {"param1":"1" , "param2":"2"}; let functionBody = "{ alert(params.param1)}";

   var func = new Function( "return function " + functionName + functionBody )();

   func(param);

This utility function merge multiple functions into one (using a custom name), only requirement is that provided functions are properly "new lined" at start and end of its scoop.

const createFn = function(name, functions, strict=false) {

    var cr = `\n`, a = [ 'return function ' + name + '(p) {' ];

    for(var i=0, j=functions.length; i<j; i++) {
        var str = functions[i].toString();
        var s = str.indexOf(cr) + 1;
        a.push(str.substr(s, str.lastIndexOf(cr) - s));
    }
    if(strict == true) {
        a.unshift('\"use strict\";' + cr)
    }
    return new Function(a.join(cr) + cr + '}')();
}

// test
var a = function(p) {
    console.log("this is from a");
}
var b = function(p) {
    console.log("this is from b");
}
var c = function(p) {
    console.log("p == " + p);
}

var abc = createFn('aGreatName', [a,b,c])

console.log(abc) // output: function aGreatName()

abc(123)

// output
this is from a
this is from b
p == 123

I had better luck in combining Darren's answer and kyernetikos's answer.

const nameFunction = function (fn, name) {
  return Object.defineProperty(fn, 'name', {value: name, configurable: true});
};

/* __________________________________________________________________________ */

let myFunc = function oldName () {};

console.log(myFunc.name); // oldName

myFunc = nameFunction(myFunc, 'newName');

console.log(myFunc.name); // newName

Note: configurable is set to true to match the standard ES2015 spec for Function.name¹

This especially helped in getting around an error in Webpack similar to this one.

Update: I was thinking of publishing this as an npm package, but this package from sindresorhus does exactly the same thing.

1. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name

I struggled a lot with this issue. @Albin solution worked like a charm while developing, but it did not work when I changed it to production. After some debugging I realized how to achieve what I needed. I'm using ES6 with CRA (create-react-app), which means it's bundled by Webpack.

Lets say you have a file that exports the functions you need:

myFunctions.js

export function setItem(params) {
  // ...
}

export function setUser(params) {
  // ...
}

export function setPost(params) {
  // ...
}

export function setReply(params) {
  // ...
}

And you need to dynamically call these functions elsewhere:

myApiCalls.js

import * as myFunctions from 'path_to/myFunctions';
/* note that myFunctions is imported as an array,
 * which means its elements can be easily accessed
 * using an index. You can console.log(myFunctions).
 */

function accessMyFunctions(res) {
  // lets say it receives an API response
  if (res.status === 200 && res.data) {
    const { data } = res;
    // I want to read all properties in data object and 
    // call a function based on properties names.
    for (const key in data) {
      if (data.hasOwnProperty(key)) {
        // you can skip some properties that are usually embedded in
        // a normal response
        if (key !== 'success' && key !== 'msg') {
          // I'm using a function to capitalize the key, which is
          // used to dynamically create the function's name I need.
          // Note that it does not create the function, it's just a
          // way to access the desired index on myFunctions array.
          const name = `set${capitalizeFirstLetter(key)}`;
          // surround it with try/catch, otherwise all unexpected properties in
          // data object will break your code.
          try {
            // finally, use it.
            myFunctions[name](data[key]);
          } catch (error) {
            console.log(name, 'does not exist');
            console.log(error);
          }
        }
      }
    }
  }
}

Node.js JavaScript Class Based Dynamic Function Name

File: Schema.js

class Schema {
  constructor() {
    this.name = null;
  }

  virtual(name = null) {
    this.name = name;
    return this;
  }

  get(func = false) {
    if (!this.name || !func instanceof Function) {
      throw new Error("Name and function must be provided.");
    }
    // Attach the dynamic function name to the "this" Object
    this[this.name] = func;
    this.name = null;
  }
}

module.exports = Schema;

File: index.js

const Schema = require("./Schema.js");

const User = new Schema();

User.virtual("getPostCount").get(() => {
  return 10 + 10;
});

const ok = User.getPostCount();
console.log({ User });
console.log(ok);

Thank you Marcosc! Building on his answer, if you want to rename any function, use this:

// returns the function named with the passed name
function namedFunction(name, fn) {
    return new Function('fn',
        "return function " + name + "(){ return fn.apply(this,arguments)}"
    )(fn)
}

function myFunction() {
    console.log('It works!');
}

var name = 'myFunction';

window[name].call();

You was near:

this["instance_" + a] = function () {...};

{...};

I might be missing the obvious here, but what's wrong with just adding the name? functions are invoked regardless of their name. names are just used for scoping reasons. if you assign it to a variable, and it's in scope, it can be called. hat happens is your are executing a variable which happens to be a function. if you must have a name for identification reasons when debugging, insert it between the keyword function and the opening brace.

var namedFunction = function namedFunction (a,b) {return a+b};

alert(namedFunction(1,2));
alert(namedFunction.name);
alert(namedFunction.toString());

an alternative approach is to wrap the function in an outer renamed shim, which you can also pass into an outer wrapper, if you don't want to dirty the surrounding namespace. if you are wanting to actually dynamically create the function from strings (which most of these examples do), it's trivial to rename the source to do what you want. if however you want to rename existing functions without affecting their functionality when called elsewhere, a shim is the only way to achieve it.

(function(renamedFunction) {

  alert(renamedFunction(1,2));
  alert(renamedFunction.name);
  alert(renamedFunction.toString());
  alert(renamedFunction.apply(this,[1,2]));


})(function renamedFunction(){return namedFunction.apply(this,arguments);});

function namedFunction(a,b){return a+b};

This is BEST solution, better then new Function('return function name(){}')().

Eval is fastest solution:

var name = 'FuncName'
var func = eval("(function " + name + "(){})")