---
title: Function.prototype[Symbol.hasInstance]()
short-title: "[Symbol.hasInstance]()"
slug: Web/JavaScript/Reference/Global_Objects/Function/Symbol.hasInstance
page-type: javascript-instance-method
browser-compat: javascript.builtins.Function.@@hasInstance
sidebar: jsref
---

The **`[Symbol.hasInstance]()`** method of {{jsxref("Function")}} instances specifies the default procedure for determining if a constructor function recognizes an object as one of the constructor's instances. It is called by the [`instanceof`](/en-US/docs/Web/JavaScript/Reference/Operators/instanceof) operator.

## Syntax

```js-nolint
func[Symbol.hasInstance](value)
```

### Parameters

- `value`
  - : The object to test. Primitive values always return `false`.

### Return value

`true` if `func.prototype` is in the prototype chain of `value`; otherwise, `false`. Always returns `false` if `value` is not an object or `this` is not a function. If `this` is a [bound function](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind), returns the result of an `instanceof` test on `value` and the underlying target function.

### Exceptions

- {{jsxref("TypeError")}}
  - : Thrown if `this` is not a bound function and `this.prototype` is not an object.

## Description

The [`instanceof`](/en-US/docs/Web/JavaScript/Reference/Operators/instanceof) operator calls the [`[Symbol.hasInstance]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/hasInstance) method of the right-hand side whenever such a method exists. Because all functions inherit from `Function.prototype` by default, they would all have the `[Symbol.hasInstance]()` method, so most of the time, the `Function.prototype[Symbol.hasInstance]()` method specifies the behavior of `instanceof` when the right-hand side is a function. This method implements the default behavior of the `instanceof` operator (the same algorithm when `constructor` has no `[Symbol.hasInstance]()` method).

Unlike most methods, the `Function.prototype[Symbol.hasInstance]()` property is non-configurable and non-writable. This is a security feature to prevent the underlying target function of a bound function from being obtainable. See [this Stack Overflow answer](https://stackoverflow.com/questions/38215027/trying-to-understand-the-official-es6-spec-regarding-symbol-hasinstance/38215392#38215392) for an example.

## Examples

### Reverting to default instanceof behavior

You would rarely need to call this method directly. Instead, this method is called by the `instanceof` operator. You should expect the two results to usually be equivalent.

```js
class Foo {}
const foo = new Foo();
console.log(foo instanceof Foo === Foo[Symbol.hasInstance](foo)); // true
```

You may want to use this method if you want to invoke the default `instanceof` behavior, but you don't know if a constructor has an overridden `[Symbol.hasInstance]()` method.

```js
class Foo {
  static [Symbol.hasInstance](value) {
    // A custom implementation
    return false;
  }
}

const foo = new Foo();
console.log(foo instanceof Foo); // false
console.log(Function.prototype[Symbol.hasInstance].call(Foo, foo)); // true
```

## Specifications

{{Specifications}}

## Browser compatibility

{{Compat}}

## See also

- [`instanceof`](/en-US/docs/Web/JavaScript/Reference/Operators/instanceof)
- {{jsxref("Symbol.hasInstance")}}