Symbol.hasInstance

The Symbol.hasInstance static data property represents the well-known symbol Symbol.hasInstance. The instanceof operator looks up this symbol on its right-hand operand for the method used to determine if the constructor object recognizes an object as its instance.

Try it

class Array1 {
  static [Symbol.hasInstance](instance) {
    return Array.isArray(instance);
  }
}

console.log([] instanceof Array1);
// Expected output: true

Value

The well-known symbol Symbol.hasInstance.

Property attributes of Symbol.hasInstance
Writableno
Enumerableno
Configurableno

Description

The instanceof operator uses the following algorithm to calculate the return value of object instanceof constructor:

  1. If constructor has a [Symbol.hasInstance]() method, then call it with object as the first argument and return the result, coerced to a boolean. Throw a TypeError if constructor is not an object, or if constructor[Symbol.hasInstance] is not one of null, undefined, or a function.
  2. Otherwise, if constructor doesn't have a [Symbol.hasInstance]() method (constructor[Symbol.hasInstance] is null or undefined), then determine the result using the same algorithm as Function.prototype[Symbol.hasInstance](). Throw a TypeError if constructor is not a function.

Because all functions inherit from Function.prototype by default, most of the time, the Function.prototype[Symbol.hasInstance]() method specifies the behavior of instanceof when the right-hand side is a function.

Examples

undefined

Custom instanceof behavior

You could implement your custom instanceof behavior like this, for example:

js
class MyArray {
  static [Symbol.hasInstance](instance) {
    return Array.isArray(instance);
  }
}
console.log([] instanceof MyArray); // true
js
function MyArray() {}
Object.defineProperty(MyArray, Symbol.hasInstance, {
  value(instance) {
    return Array.isArray(instance);
  },
});
console.log([] instanceof MyArray); // true

Checking the instance of an object

Just in the same manner at which you can check if an object is an instance of a class using the instanceof keyword, we can also use Symbol.hasInstance for such checks.

js
class Animal {
  constructor() {}
}

const cat = new Animal();

console.log(Animal[Symbol.hasInstance](cat)); // true

Specifications

See also