How do you check the difference between an ECMAScript 6 class and function?

Question

In ECMAScript 6 the typeof of classes is, according to the specification, \'function\'.

However also according to the specification you are

Answer 1

You can use new.target to determine whether whether its instantiated by ES6 class function or function constructor

class Person1 {
  constructor(name) {
    this.name = name;
    console.log(new.target) // => // => [Class: Person1]
  }
}

function Person2(){
  this.name='cc'
  console.log(new.target) // => [Function: Person2]
}

Answer 2

Looking at the compiled code generated by Babel, I think there is no way you can tell if a function is used as a class. Back in the time, JavaScript didn't have classes, and every constructor was just a function. Today's JavaScript class keyword don't introduce a new concept of 'classes', it's rather a syntax sugar.

ES6 code:

// ES6
class A{}

ES5 generated by Babel:

// ES5
"use strict";

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var A = function A() {
    _classCallCheck(this, A);
};

Of course, if you are into coding conventions, you could parse the function (the class), and check if it's name starts with a capital letter.

function isClass(fn) {
    return typeof fn === 'function' && /^(?:class\s+|function\s+(?:_class|_default|[A-Z]))/.test(fn);
}

EDIT:

Browsers which already support the class keyword can use it when parsing. Otherwise, you are stuck with the capital letter one.

EDIT:

As balupton pointed out, Babel generates function _class() {} for anonymous classes. Improved regex based on that.

EDIT:

Added _default to the regex, to detect classes like export default class {}

Warning

BabelJS is heavily under development, and there is no guarantee they will not change the default function names in these cases. Really, you shouldn't rely on that.