The well-known symbolSymbol.hasInstance.

Theinstanceof operator uses the following algorithm to calculate the return value ofobject instanceof constructor:

1. Ifconstructor has a[Symbol.hasInstance]() method, then call it withobject as the first argument and return the result,coerced to a boolean. Throw aTypeError ifconstructor is not an object, or ifconstructor[Symbol.hasInstance] is not one ofnull,undefined, or a function.
2. Otherwise, ifconstructor doesn't have a[Symbol.hasInstance]() method (constructor[Symbol.hasInstance] isnull orundefined), then determine the result using the same algorithm asFunction.prototype[Symbol.hasInstance](). Throw aTypeError ifconstructor is not a function.

Because all functions inherit fromFunction.prototype by default, most of the time, theFunction.prototype[Symbol.hasInstance]() method specifies the behavior ofinstanceof when the right-hand side is a function.

You could implement your custominstanceof 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 theinstanceof keyword, we can also useSymbol.hasInstance for such checks.

class Animal {  constructor() {}}const cat = new Animal();console.log(Animal[Symbol.hasInstance](cat)); // true

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