The Symbol.hasInstance well-known symbol is used to determine if a constructor object recognizes an object as its instance. The instanceof operator's behavior can be customized by this symbol.

Property attributes of Symbol.hasInstance
Writable no
Enumerable no
Configurable no


Custom instanceof behavior

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

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

class Animal {
  constructor() {}

const cat = new Animal();

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


ECMAScript Language Specification (ECMAScript)

Browser compatibility

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See also