How to retrieve the constructor's name in JavaScript?

Question

Assume the following program:

var SomeConstructor = function() { };
var instance = new SomeConstructor ();

The expression instance instanceof SomeConstructor yields True, so instance has to know somehow that it was constructed by the function SomeConstructor . Is there way to retrieve the name SomeConstructor directly from instance ?

(In my problem at hand, I have a hierarchy of prototypes implementing possible signals in my application. I have to access the type of a signal which shall be equivalent to the constructor used to create that signal.)

mikemaccana · Accepted Answer · 2019-12-11 13:43:20Z

45

On Chrome (7.0.544.0 dev), if I do:

function SomeConstructor() { }

var instance = new SomeConstructor();

console.log(instance.constructor.name);

it prints 'SomeConstructor'...but if SomeConstructor is defined as an unnamed function as you have it, it will print an empty string instead.

If I print instance.constructor it prints the same thing as it does if I print SomeConstructor in the code you have. The instanceof operator need only compare these two values to see that they are equal to be able to return true.

edited Dec 11, 2019 at 13:43

mikemaccana

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answered Oct 11, 2010 at 10:15

sje397

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4 Comments

Marc Over a year ago

I thought I would need a name string of the constructor in order to use the constructor as an index of an array. However, if it is possible to use x.constructor directly as an array index, I will be fine.

sje397 Over a year ago

@Marc - the closest thing to an 'associative array' in javascript is an object. Javascript arrays are indexed using integers. If you use x.constructor as an object's property name (e.g. map[x.constructor]) then the constructor property's toString method is implicitly called.

Marc Over a year ago

Thanks! I mainly program in Python so your explanations how JavaScript does things helped me a lot. I checked the x.constructor.toString() method in the stand-alone Rhino interpreter and it gives me back the source code of the anonymous constructor.

James L. Over a year ago

C.name returns the constructor's name in a static way

phatmann · Accepted Answer · 2010-12-02 09:02:52Z

This code will get the name of the constructor, as long as it is not an anonymous function:

obj.constructor.toString().match(/function (\w*)/)[1];

Why would you need the class name? Let's say you want to save and restore class instances via JSON. You could store the class name in a "type" property, and then use a resolver function in JSON.parse to restore the objects. (See the sample code on this page).

So, in theory you could use the code above to make a generalized serializer that could handle any class instance, but parsing function strings is very inefficient. This overhead can be avoided by requiring all the classes you are going to store to provide the type explicitly:

function Foo() {}
Foo.prototype.type = 'Foo';

This seems silly and redundant, which is why I started on the quest to obtain the class name implicitly. But in the end I have to give in: there is no acceptable solution in JS :-(

Andy E · Accepted Answer · 2010-10-11 10:12:26Z

4

No. You can use x.constructor to get a direct reference to C, but it's an anonymous function so there's no way of getting its name.

If it were defined like so:

function C() { };
x = new C();

Then it would be possible to use x.constructor.toString() and parse out the name of the function from the returned string. Some browsers would also support x.constructor.name^[1].

^{[1] https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/name}

answered Oct 11, 2010 at 10:12

Andy E

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Comments

Wilt · Accepted Answer · 2019-05-29 15:49:06Z

2

You can also declare your class this:

var C = function C() { };

Now your C class constructor is a named function (no longer an anonymous function) meaning you can do:

x = new C();    

console.log(x.constructor.name); // output: C

edited May 29, 2019 at 15:49

answered Jun 27, 2016 at 8:10

Wilt

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Comments

RichN · Accepted Answer · 2010-10-11 10:23:03Z

0

This code of yours that needs to know the constructor, can it access the constructor function itself? If so, you can just do that instanceof thing.

It seems that you need to know it by name. This sounds dangerous. Someone can create another constructor that happen to have the same name and it will pass (unless that's what you want, of course).

answered Oct 11, 2010 at 10:23

RichN

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Comments

Thomas Hagström · Accepted Answer · 2018-02-23 12:37:08Z

-6

At least in React it might be

this.default.name

answered Feb 23, 2018 at 12:37

Thomas Hagström

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