Symbol.hasInstance

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2017.

The Symbol.hasInstance static data property represents the well-known symbol Symbol.hasInstance. The instanceof operator looks up this symbol on its right-hand operand for the method used to determine if the constructor object recognizes an object as its instance.

Try it

class Array1 {
  static [Symbol.hasInstance](instance) {
    return Array.isArray(instance);
  }
}

console.log([] instanceof Array1);
// Expected output: true

Value

The well-known symbol Symbol.hasInstance.

Property attributes of Symbol.hasInstance

Writable	no
Enumerable	no
Configurable	no

Description

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

If constructor has a [Symbol.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[Symbol.hasInstance] is not one of null, undefined, or a function.
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](). 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[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

js
function MyArray() {}
Object.defineProperty(MyArray, Symbol.hasInstance, {
  value(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.

js
class Animal {
  constructor() {}
}

const cat = new Animal();

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

Specifications

Specification

ECMAScript® 2027 Language Specification
# sec-symbol.hasinstance

Browser compatibility

Enable JavaScript to view this browser compatibility table.

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This page was last modified on Jul 10, 2025 by MDN contributors.

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Standard built-in objects
Symbol
Constructor
1. Symbol()
Static methods
1. for()
2. keyFor()
Static properties
Instance methods
Instance properties
1. description
Inheritance
Object/Function
Static methods
Static properties
Instance methods
Instance properties
1. __proto__
2. constructor