Symbol.hasInstance

## Value

The well-known symbol `Symbol.hasInstance`.

{{js_property_attributes(0, 0, 0)}}

## Description

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

1. If `constructor` has a `[Symbol.hasInstance]()` method, then call it with `object` as the first argument and return the result, [coerced to a boolean](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean#boolean_coercion). Throw a {{jsxref("TypeError")}} if `constructor` is not an object, or if `constructor[Symbol.hasInstance]` is not one of `null`, `undefined`, or a function.
2. Otherwise, if `constructor` doesn't have a `[Symbol.hasInstance]()` method (`constructor[Symbol.hasInstance]` is `null` or `undefined`), then determine the result using the same algorithm as [`Function.prototype[Symbol.hasInstance]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/Symbol.hasInstance). Throw a {{jsxref("TypeError")}} if `constructor` is not a function.

Because all functions inherit from `Function.prototype` by default, most of the time, the [`Function.prototype[Symbol.hasInstance]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/Symbol.hasInstance) method specifies the behavior of `instanceof` when the right-hand side is a function.

## Examples

### Custom instanceof behavior

You could implement your custom `instanceof` behavior like this, for example:

```js
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

class Animal {
  constructor() {}
}

const cat = new Animal();

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