How to determine if object is a class or a function in Javascript

Question

Say I have these two:

class Foo {}
const Bar = function() {}

// Then somewhere I have an object:
const variable = Foo;
// OR
const variable = Bar;

How can I know if variable is a function or a class? The problem here is that typeof Foo or typeof Bar both return function.

sure, but what you describe are variables, just use `typeof` — john Smith, Jun 13 '17 at 22:11
@johnSmith but `typeof Foo` or `typeof Bar` would both return `function` ... — Kousha, Jun 13 '17 at 22:11
@Kousha Wait a second, what's the question? To distinguish between a class and function's type, or to check if a variable is either a class or function? — Andrew Li, Jun 13 '17 at 22:12
There are no "classes" in javascript. There is a class syntax. All native functions are constructors, there is no way to tell them apart. However, in the latest versions of ECMAScript, constructors created using the Class syntax will throw an error if called as a function, so there is that. — RobG, Jun 13 '17 at 22:14
@Kousha Why exactly do you care what it is, and why do you need to decide dynamically? — Bergi, Jun 13 '17 at 22:34
@Bergi looks like an [XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). — Emile Bergeron, Jun 13 '17 at 22:36

Emile Bergeron · Accepted Answer · 2017-06-13T22:44:16.060

This question was to be expected with the confusion caused by the new syntactic sugar added recently.

From the MDN doc on Classes:

JavaScript classes introduced in ECMAScript 2015 are primarily syntactical sugar over JavaScript's existing prototype-based inheritance. The class syntax is not introducing a new object-oriented inheritance model to JavaScript.
[...]
Classes are in fact "special functions", and just as you can define function expressions and function declarations, the class syntax has two components: class expressions and class declarations.

The MDN doc on class expressions shows what you already know about typeof:

'use strict';
var Foo = class {}; // constructor property is optional
var Foo = class {}; // Re-declaration is allowed

typeof Foo; //returns "function"
typeof class {}; //returns "function"

Foo instanceof Object; // true
Foo instanceof Function; // true
class Foo {}; // Throws TypeError, doesn't allow re-declaration

But unlike the class expression, the class declaration doesn't allow an existing class to be declared again and will throw a type error if attempted.
[...]
Class declarations are not hoisted (unlike function declarations).

How to determine if object is a class or a function in Javascript

1 Answers1