: "instanceof"

instanceOf . .

. (polymorphic) .

instanceof

:

obj instanceof Class

obj Class true .

:

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   
   //   (animal)    canEat    
   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.instanceof . : obj instanceOf Class 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) objA objB true . 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 (context) . .

• [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 ...] .

.

instanceof {}.toString.call .

:

{}.toString typeof .

instanceof .