: "instanceof"

instanceof , , .

. , - .

instanceof

:

obj instanceof Class

true, obj Class .

:

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

//    Rabbit?
alert( rabbit instanceof Rabbit ); // true

-:

//  
function Rabbit() {}

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

, Array:

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

, , arr Object, Array Object.

instanceof . Symbol.hasInstance.

obj instanceof Class :

1. Symbol.hasInstance, : Class[Symbol.hasInstance](obj). true, false, . instanceof.

   :

   //  instanceof  ,
   //     canEat -  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)

2. Symbol.hasInstance. : , Class.prototype obj.

   , :

   obj.__proto__ === Class.prototype?
   obj.__proto__.__proto__ === Class.prototype?
   obj.__proto__.__proto__.__proto__ === Class.prototype?
   ...
   //  -   true -  true
   //      - false

   rabbit.__proto__ === Rabbit.prototype, .

   , :

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

rabbit instanceof Animal Animal.prototype:

[]

, objA.isPrototypeOf(objB), true, objA - objB. obj instanceof Class Class.prototype.isPrototypeOf(obj).

, Class ! Class.prototype.

prototype .

, , :

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

//  
Rabbit.prototype = {};

// ...  rabbit!
alert( rabbit instanceof Rabbit ); // false

: Object.prototype.toString

, [object Object]:

let obj = {};

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

toString. toString , . typeof instanceof.

? . .

toString . .

• [object Number]
• [object Boolean]
• null: [object Null]
• undefined: [object Undefined]
• : [object Array]
• .. ( ).

:

//   toString    
let objectToString = Object.prototype.toString;

//   ?
let arr = [];

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

call, , call/apply, objectToString this=arr.

, toString this . :

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

toString , Symbol.toStringTag.

:

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

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

, . :

// toStringTag     
alert( window[Symbol.toStringTag]); // window
alert( XMLHttpRequest.prototype[Symbol.toStringTag] ); // XMLHttpRequest

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

, Symbol.toStringTag ( ) [object ...].

typeof , , , .

{}.toString.call instanceof , , .

, :

, {}.toString , typeof.

instanceof , .