Symbol.hasInstance

The well-known symbol @@hasInstance.

The instanceof operator uses the following algorithm to calculate the return value of object instanceof constructor:

1. If constructor has a @@hasInstance method, then call it with object as the first argument and return the result, coerced to a boolean. Throw a TypeError if constructor is not an object, or if constructor[@@hasInstance] is not one of null, undefined, or a function.
2. Otherwise, if constructor doesn't have a @@hasInstance method (constructor[@@hasInstance] is null or undefined), then determine the result using the same algorithm as Function.prototype[@@hasInstance]. Throw a TypeError if constructor is not a function.

Because all functions inherit from Function.prototype by default, most of the time, the Function.prototype[@@hasInstance] method specifies the behavior of instanceof when the right-hand side is a function.

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

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.

class Animal {
  constructor() {}
}

const cat = new Animal();

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

• instanceof
• Function.prototype[@@hasInstance]()