How to programmatically distinguish an arrow function from a regular function?

Question

There is no obvious difference between an arrow function and a regular function.

({}).toString.call(function () {})
"[object Function]"
({}).toString.call(() => {})
"[object Function]"

or

console.dir( (function () {}) )

function anonymous()
    arguments: null
    caller: null
    length: 0
    name: ""
    prototype: Object
    __proto__: ()
    <function scope>

console.dir( (() => {}) )

function anonymous()
    arguments: (...)
    caller: (...)
    length: 0
    name: ""
    __proto__: ()
    <function scope>

The behaviour of the two is different though and there is a valid use case for being able to tell the two apart.

How to programmatically distinguish an arrow function from a regular function?

Answer 1

The best I can think of is using toString:

let isArrowFunction;

isArrowFunction = (fn) => {
    console.log(fn.toString());

    return fn.toString().indexOf('function') !== 0;
};

console.log(isArrowFunction(() => {}) === true);
console.log(isArrowFunction((foo: string) => {}) === true);
console.log(isArrowFunction(function () {}) === false);

See:

(function () {}).toString();
"function () {}"

(() => {}).toString();
"() => {}"

Answer 2

Uhm, the requirements are a bit weird, but I made some tests and:

typeof (() => {}).prototype === "undefined"

Is true, while:

typeof (function () {}).prototype === "undefined"

Is false, so:

function isArrow(x)
{
  return typeof (x.prototype) === "undefined"
}

Fiddle here: https://jsfiddle.net/87kn67ov/