Object vs function constructor in JavaScript

Question

I want to clear up a concept. Please tell me if my understand is correct:

Many javascript build-in objects, like Object, String, Array, Date, XMLHttpRequest, we keep saying they are objects, but they are actually constructor functions, am I right?

or these two name are used interchangeably.

Thanks

object doesn't really mean anything. Almost everything is an "object". Now these elements are constructors indeed, which are functions, and contains static methods such as Array.isArray — floribon
– floribon, Commented Feb 24, 2015 at 0:54
Oh, and XMLHttpRequest is a host object, so a very different animal to the other objects in the OP. — RobG
– RobG, Commented Feb 24, 2015 at 0:56

georg · Accepted Answer · 2015-02-24 01:05:45Z

2

Ok, to sum it up:

every object has a hidden __proto__ property
functions are objects that also have a prototype property
if, for some object O and function F, O.__proto__ == F.prototype, we say that "O is an instance of F"
"F object" is a way to refer to an object that is an instance of "F". For example:

String object: member of the Object type that is an instance of the standard built-in String constructor

and the same for other built-in and user-defined types. If you have

 function Point(x,y) { ... }
 p = new Point(10,20)

then "p is a Point object". In a casual conversation you're also allowed to say "p is a Point" although this isn't strictly correct.

answered Feb 24, 2015 at 1:05

georg

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1 Comment

Evan Davis Over a year ago

I guess this is technically true, but I don't think it actually answers the question?

Evan Davis · Accepted Answer · 2015-02-24 17:25:51Z

1

Object's prototype is the root prototype for most entities in JavaScript.

The items you list are all constructor functions, yes.

typeof Array // 'function'

Invoking a constructor returns an object.

typeof (new Array()) // 'object'
typeof (new Date()) // 'object'

edited Feb 24, 2015 at 17:25

answered Feb 24, 2015 at 1:22

Evan Davis

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4 Comments

Derek 朕會功夫 Over a year ago

I believe Object.prototype (which inherits from null) is the root prototype, not the constructor Object itself.

Evan Davis Over a year ago

@Derek朕會功夫 you are correct; I think constructors make this much more confusing than it needs to be.

RobG Over a year ago

"However, the prototype of Function is Object" nope, Function.prototype is an instance of Function, it is Function.prototype that inherits from Object.prototype (or in your notation, you might write Function.__prototype__ === Function.prototype and Function.prototype.__prototype__===Object.prototype).

Evan Davis Over a year ago

@RobG agreed; I've edited to be what I hope is the simplest possible response to OPs question. (OP isn't really asking about prototypes, but there are extensive resources here and elsewhere if and when they do.)

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