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.

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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(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
# sec-symbol.hasinstance

Browser compatibility

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See also