Class checking: "instanceof"

The instanceof operator allows to check whether an object belongs to a certain class. It also takes inheritance into account.

Such a check may be necessary in many cases, here we’ll use it for building a polymorphic function, the one that treats arguments differently depending on their type.

The instanceof operator

The syntax is:

obj instanceof Class

It returns true if obj belongs to the Class (or a class inheriting from it).

For instance:

class Rabbit {}
let rabbit = new Rabbit();

// is it an object of Rabbit class?
alert( rabbit instanceof Rabbit ); // true

It also works with constructor functions:

// instead of class
function Rabbit() {}

alert( new Rabbit() instanceof Rabbit ); // true

…And with built-in classes like Array:

let arr = [1, 2, 3];
alert( arr instanceof Array ); // true
alert( arr instanceof Object ); // true

Please note that arr also belongs to the Object class. That’s because Array prototypally inherits from Object.

The instanceof operator examines the prototype chain for the check, and is also fine-tunable using the static method Symbol.hasInstance.

The algorithm of obj instanceof Class works roughly as follows:

  1. If there’s a static method Symbol.hasInstance, then use it. Like this:

    // assume anything that canEat is an animal
    class Animal {
      static [Symbol.hasInstance](obj) {
        if (obj.canEat) return true;
      }
    }
    
    let obj = { canEat: true };
    alert(obj instanceof Animal); // true: Animal[Symbol.hasInstance](obj) is called
  2. Most classes do not have Symbol.hasInstance. In that case, check if Class.prototype equals to one of prototypes in the obj prototype chain.

    In other words, compare:

    obj.__proto__ == Class.prototype
    obj.__proto__.__proto__ == Class.prototype
    obj.__proto__.__proto__.__proto__ == Class.prototype
    ...

    In the example above Rabbit.prototype == rabbit.__proto__, so that gives the answer immediately.

    In the case of an inheritance, rabbit is an instance of the parent class as well:

    class Animal {}
    class Rabbit extends Animal {}
    
    let rabbit = new Rabbit();
    alert(rabbit instanceof Animal); // true
    // rabbit.__proto__ == Rabbit.prototype
    // rabbit.__proto__.__proto__ == Animal.prototype (match!)

Here’s the illustration of what rabbit instanceof Animal compares with Animal.prototype:

By the way, there’s also a method objA.isPrototypeOf(objB), that returns true if objA is somewhere in the chain of prototypes for objB. So the test of obj instanceof Class can be rephrased as Class.prototype.isPrototypeOf(obj).

That’s funny, but the Class constructor itself does not participate in the check! Only the chain of prototypes and Class.prototype matters.

That can lead to interesting consequences when prototype is changed.

Like here:

function Rabbit() {}
let rabbit = new Rabbit();

// changed the prototype
Rabbit.prototype = {};

// ...not a rabbit any more!
alert( rabbit instanceof Rabbit ); // false

That’s one of reasons to avoid changing prototype. Just to keep safe.

Bonus: Object toString for the type

We already know that plain objects are converted to string as [object Object]:

let obj = {};

alert(obj); // [object Object]
alert(obj.toString()); // the same

That’s their implementation of toString. But there’s a hidden feature thank makes toString actually much more powerful than that. We can use it as an extended typeof and an alternative for instanceof.

Sounds strange? Indeed. Let’s demistify.

By specification, the built-in toString can be extracted from the object and executed in the context of any other value. And its result depends on that value.

  • For a number, it will be [object Number]
  • For a boolean, it will be [object Boolean]
  • For null: [object Null]
  • For undefined: [object Undefined]
  • For arrays: [object Array]
  • …etc (customizable).

Let’s demonstrate:

// copy toString method into a variable for convenience
let objectToString = Object.prototype.toString;

// what type is this?
let arr = [];

alert( objectToString.call(arr) ); // [object Array]

Here we used call as described in the chapter Decorators and forwarding, call/apply to execute the function objectToString in the context this=arr.

Internally, the toString algorithm examines this and returns the corresponding result. More examples:

let s = Object.prototype.toString;

alert( s.call(123) ); // [object Number]
alert( s.call(null) ); // [object Null]
alert( s.call(alert) ); // [object Function]

Symbol.toStringTag

The behavior of Object toString can be customized using a special object property Symbol.toStringTag.

For instance:

let user = {
  [Symbol.toStringTag]: 'User'
};

alert( {}.toString.call(user) ); // [object User]

For most environment-specific objects, there is such a property. Here are few browser specific examples:

// toStringTag for the envinronment-specific object and class:
alert( window[Symbol.toStringTag]); // window
alert( XMLHttpRequest.prototype[Symbol.toStringTag] ); // XMLHttpRequest

alert( {}.toString.call(window) ); // [object Window]
alert( {}.toString.call(new XMLHttpRequest()) ); // [object XMLHttpRequest]

As you can see, the result is exactly Symbol.toStringTag (if exists), wrapped into [object ...].

At the end we have “typeof on steroids” that not only works for primitive data types, but also for built-in objects and even can be customized.

It can be used instead of instanceof for built-in objects when we want to get the type as a string rather than just to check.

Summary

Let’s recap the type-checking methods that we know:

works for returns
typeof primitives string
{}.toString primitives, built-in objects, objects with Symbol.toStringTag string
instanceof objects true/false

As we can see, {}.toString is technically a “more advanced” typeof.

And instanceof operator really shines when we are working with a class hierarchy and want to check for the class taking into account inheritance.

Tasks

importance: 5

Why instanceof below returns true? We can easily see that a is not created by B().

function A() {}
function B() {}

A.prototype = B.prototype = {};

let a = new A();

alert( a instanceof B ); // true

Yeah, looks strange indeed.

But instanceof does not care about the function, but rather about its prototype, that it matches against the prototype chain.

And here a.__proto__ == B.prototype, so instanceof returns true.

So, by the logic of instanceof, the prototype actually defines the type, not the constructor function.

Tutorial map

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