Web/JavaScript/Reference/Global objects/Symbol/hasInstance

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

Examples

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

Specifications

Specification
ECMAScript (ECMA-262)The definition of 'Symbol.hasInstance' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`hasInstance`	Chrome Full support 50	Edge Full support 15	Firefox Full support 50	IE No support No	Opera Full support 37	Safari Full support 10	WebView Android Full support 50	Chrome Android Full support 50	Firefox Android Full support 50	Opera Android Full support 37	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.5.0 Full support 6.5.0 Full support 6.0.0 Disabled' Disabled' From version 6.0.0: this feature is behind the `--harmony` runtime flag.

Legend

Full support: Full support
No support: No support
User must explicitly enable this feature.': User must explicitly enable this feature.