What is the Acceptable Use of 'instanceof' in JavaScript?

What are the best practices for using instanceof in JavaScript?

// Example of using instanceof in JavaScript
function Animal(name) {
    this.name = name;
}

function Dog(name) {
    Animal.call(this, name);
}

Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;

const myDog = new Dog('Buddy');
console.log(myDog instanceof Dog); // true
console.log(myDog instanceof Animal); // true

The 'instanceof' operator in JavaScript checks if an object is an instance of a specific class or constructor function. Proper use of instanceof is crucial for type-checking in a dynamic language like JavaScript.

// Correct instanceof usage in an inheritance chain
class Person {}
class Employee extends Person {}

const emp = new Employee();
console.log(emp instanceof Employee); // true
console.log(emp instanceof Person); // true
console.log(emp instanceof Object); // true

Causes

• Ensuring type safety in your code.
• Implementing polymorphism effectively in inheritance chains.
• Validating complex data structures or function return types.

Solutions

• Always validate objects against the expected prototype chain using instanceof.
• Prefer using instanceof over manual type checks (like typeof) when dealing with objects.
• Keep in mind that instanceof will not work correctly with objects created from different global contexts.

Mistake: Using instanceof to check primitive types like strings or numbers instead of objects.

Solution: Use typeof for primitive types. instanceof is for checking object instances.

Mistake: Assuming instanceof works across different execution contexts (like iframes).

Solution: Create a shared reference to the constructor or use the isSameOrigin technique to avoid cross-context issues.