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);