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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

Browser compatibilityUpdate compatibility data on GitHub

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

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/hasInstance