确定JavaScript中对象的类

Question

What can be the best method to determine the class of an object in JavaScript. If I have to determine class of an object 'o', I can do this...

function findClass(o){
         return Object.prototype.toString.call(o).slice(8,-1);
}

But when we have our custom classes, this method doesn't help.

Answer 1

You can't really find the class of a object so easily like in Java. This is because Javascript is a prototype-based language, But you still have a few methods of determining the class:

• typeof
• instanceof
• obj.constructor
• proto.isPrototypeOf

Example:

function Foo() {}
var foo = new Foo();

typeof Foo; // == "function"
typeof foo; // == "object"

foo instanceof Foo; // == true
foo.constructor; // == Foo

Foo.prototype.isPrototypeOf(foo); // == true

Answer 2

Two Objects belongs to the same class if they both inherit from the same prototype object.

When dealing with custom classes and in order to make effective method for class checks (as suggested above Ivan Grgurevic), is very important to explicit set the constuctor and also tell which in on the proptotype.

Example:

// Class constructor
function CustomClass(type) {
  // CODE
}

// prototype, all CustomClass objects inherit form this object
Sedcard.prototype = {
  constructor: CustomClass, // explicit set constructor, backwards reference
  // Class method
};

Now you can safetly use checks such as: typeof, instanceof and direct like Foo.prototype.isPrototypeOf(foo)

Though, this is a very hard topic on JS and recommend you further lectures, like JS book "Javascript, the definitive guide" http://shop.oreilly.com/product/9780596805531.do in order to deeply understand the way it works.

A bit more about this topic:

r instanceof CustomClass: The instanceof operator does not actually check whether r was initialized by the constructor. It checks whether it inherits from its prototype. Nevertheless, the instanceof syntax reinforces the use of constructors as the public identity of a class.

The expression o instanceof c evaluates to true if o inherits from c.prototype. The inheritance need not be direct. If o inherits from an object that inherits from an object that inherits from c.prototype, the expression will still evaluate to true.

Both quotes comes from the previous book I recommended.