why is a not an instanceof b in javascript

In JavaScript, the instanceof operator checks if an object is an instance of a specific constructor. It checks if an object has a prototype anywhere in its prototype chain that matches the prototype property of a constructor function.

So, a instanceof b will return true if a is an instance of the constructor b or any of its descendants in the prototype chain. However, a instanceof b will return false if a is not an object derived from b.

Here's an example to illustrate the issue:

index.tsx
// Define constructor functions
function Animal() {}
function Cat() {}

// Create objects
var animal = new Animal();
var cat = new Cat();

// Check the instance of the objects
console.log(cat instanceof Animal); // true
console.log(cat instanceof Cat); // true

// Create an object that is not derived from Cat or Animal
var someObject = {};

// Check the instance of the object
console.log(someObject instanceof Animal); // false
console.log(someObject instanceof Cat); // false

// Try to check if someObject is an instance of undefined
console.log(someObject instanceof undefined); // TypeError: Right-hand side of 'instanceof' is not an object

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In the last line of the example, we can see that trying to pass undefined to the instanceof operator will throw a TypeError.

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related categories

javascript
object-oriented programming
instanceof operator
inheritance
data types

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