How to retrieve the constructor's name in JavaScript?

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.

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 :-(

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}