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.

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

Specifications

Browser compatibility

See also

• instanceof