instanceof in JavaScript: How Prototype Chain Checking Works

What is instanceof

The instanceof operator in JavaScript checks whether an object belongs to a specific class (or constructor function) in its prototype chain.

Syntax:

obj instanceof Constructor

• obj — the object we're checking.
• Constructor — constructor function or class.

The operator returns true if object obj is in the prototype chain of constructor Constructor.

Example

function Animal() {}
function Dog() {}

const rex = new Dog();

console.log(rex instanceof Dog);    // true
console.log(rex instanceof Animal); // false

If Dog inherited from Animal, then rex instanceof Animal would be true.

How it Works

The instanceof operator works like this:

• Takes obj.__proto__.
• Compares it with Constructor.prototype.
• If they match — returns true.
• If not — goes up the __proto__ chain and repeats.

Don't Confuse with typeof

typeof [] // "object"
[] instanceof Array // true

Conclusion

• Use instanceof to understand whether an object belongs to a specific type or was created via new SomeConstructor.
• It's a powerful tool, but not the only way to check type (e.g., Object.prototype.toString.call(...) is also popular).
• Be careful in cross-environment scenarios — instanceof can behave unpredictably.