JavaScript Design Patterns Interview Questions

Here are some common JavaScript design patterns interview questions along with answers that can help you prepare for a senior JavaScript developer role:

1. What is a design pattern in JavaScript?

A design pattern is a reusable solution to a common problem in software design. In JavaScript, design patterns help organize code, enhance maintainability, and improve scalability. Examples include the Module Pattern, Singleton Pattern, Observer Pattern, and Factory Pattern.

2. Can you explain the Module Pattern?

The Module Pattern is used to encapsulate private variables and methods, exposing only what is necessary through a public API. It helps in organizing code and avoiding global scope pollution.

Example

const Module = (function() {
    let privateVar = "I am private";

    return {
        publicMethod: function() {
            console.log(privateVar);
        }
    };
})();

Module.publicMethod(); // "I am private"

3. What is the Singleton Pattern?

The Singleton Pattern ensures that a class has only one instance and provides a global point of access to it. This is often used for configuration settings or shared resources.

Example

const Singleton = (function() {
    let instance;

    function createInstance() {
        const object = new Object("I am the instance");
        return object;
    }

    return {
        getInstance: function() {
            if (!instance) {
                instance = createInstance();
            }
            return instance;
        }
    };
})();

const instance1 = Singleton.getInstance();
const instance2 = Singleton.getInstance();

console.log(instance1 === instance2); // true

4. What is the Observer Pattern?

The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. This is often used in event handling systems.

Example

class Subject {
    constructor() {
        this.observers = [];
    }

    subscribe(observer) {
        this.observers.push(observer);
    }

    unsubscribe(observer) {
        this.observers = this.observers.filter(obs => obs !== observer);
    }

    notify(data) {
        this.observers.forEach(observer => observer.update(data));
    }
}

class Observer {
    update(data) {
        console.log("Observer received data:", data);
    }
}

const subject = new Subject();
const observer1 = new Observer();
const observer2 = new Observer();

subject.subscribe(observer1);
subject.subscribe(observer2);

subject.notify("Hello Observers!"); // Both observers will receive this data

5. What is the Factory Pattern?

The Factory Pattern is used to create objects without specifying the exact class of the object that will be created. It provides a method for creating objects in a super class but allows subclasses to alter the type of created objects.

Example

class Car {
    constructor(make) {
        this.make = make;
    }

    drive() {
        console.log(`Driving a ${this.make}`);
    }
}

class CarFactory {
    createCar(make) {
        return new Car(make);
    }
}

const factory = new CarFactory();
const myCar = factory.createCar("Toyota");
myCar.drive(); // "Driving a Toyota"

6. What is the difference between the Prototype Pattern and the Constructor Pattern?

The Constructor Pattern uses functions to create instances of objects with the new keyword, while the Prototype Pattern uses an object as a prototype for creating new objects. The Prototype Pattern is more memory-efficient since it allows sharing properties and methods among instances.

Example of Prototype Pattern

function Car(make) {
    this.make = make;
}

Car.prototype.drive = function() {
    console.log(`Driving a ${this.make}`);
};

const myCar = new Car("Honda");
myCar.drive(); // "Driving a Honda"

7. When would you use the Decorator Pattern?

The Decorator Pattern allows adding new functionality to an object dynamically without altering its structure. It’s useful for adhering to the Single Responsibility Principle by separating concerns.

Example

function Coffee() {
    this.cost = function() {
        return 5;
    };
}

function Milk(coffee) {
    this.coffee = coffee;
}

Milk.prototype.cost = function() {
    return this.coffee.cost() + 2;
};

const myCoffee = new Coffee();
const myCoffeeWithMilk = new Milk(myCoffee);

console.log(myCoffee.cost()); // 5
console.log(myCoffeeWithMilk.cost()); // 7

8. Can you explain the concept of “this” in JavaScript and how it relates to design patterns?

In JavaScript, this refers to the context in which a function is executed. It can change based on how a function is called. Understanding this is crucial for design patterns, especially when implementing patterns that rely on callbacks or event handling. Using arrow functions can help maintain the lexical scope of this.

9. What is the purpose of the Strategy Pattern?

The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from clients that use it. This is useful for implementing different strategies for a particular behavior.

Example

class Context {
    constructor(strategy) {
        this.strategy = strategy;
    }

    executeStrategy(data) {
        return this.strategy.execute(data);
    }
}

class StrategyA {
    execute(data) {
        return `Strategy A executed with ${data}`;
    }
}

class StrategyB {
    execute(data) {
        return `Strategy B executed with ${data}`;
    }
}

const context = new Context(new StrategyA());
console.log(context.executeStrategy("input")); // "Strategy A executed with input"

context.strategy = new StrategyB();
console.log(context.executeStrategy("input")); // "Strategy B executed with input"

10. What are some best practices when implementing design patterns in JavaScript?

• Understand the problem: Ensure you have a clear understanding of the problem you are trying to solve before applying a pattern.
• Keep it simple: Don’t over-engineer. Use patterns that fit the problem without adding unnecessary complexity.
• Documentation: Document the purpose and usage of patterns in your code for better maintainability.
• Avoid global scope pollution: Use patterns like the Module Pattern to encapsulate code.
• Test: Ensure your implementations are well-tested, especially when using patterns that involve multiple objects interacting.