Tricky JavaScript Design Pattern Interview Questions- Navigating the Depths:

Umar Farooque Khan
3 min readNov 24, 2023

JavaScript design patterns are a crucial aspect of modern web development, providing developers with reusable and proven solutions to common problems. During interviews, candidates are often tested not only on their understanding of foundational design patterns but also on their ability to navigate more intricate scenarios. In this context, tricky interview questions are designed to evaluate a candidate’s depth of knowledge, problem-solving skills, and the ability to apply design patterns in nuanced situations. Let’s explore some challenging questions that delve into advanced aspects of Singleton, Module, Observer, Factory, and Decorator patterns, aiming to uncover the candidate’s ability to think critically and strategically in a JavaScript context.

Question:

How would you make a Singleton pattern that is both lazy-loaded and thread-safe?

Answer:

One approach is to use a combination of lazy initialization and double-check locking in the getInstance method.

Example:

var Singleton = (function () {
var instance;

return {
getInstance: function () {
if (!instance) {
// Double-check locking
synchronized (this) {
if (!instance) {
instance = createInstance();
}
}
}
return instance;
}
};
})();

Question:

How can you achieve private members in the Module pattern while still allowing some level of customization?

Answer:

One approach is to use a revealing module pattern, revealing only the necessary methods while keeping others private.

Example:

var Module = (function () {
var privateVariable = 'I am private';

function privateMethod() {
console.log('This is private');
}

function publicMethod() {
privateMethod();
console.log('This is public');
}

return {
publicMethod: publicMethod
};
})();

Question:

How can you implement an Observer pattern where observers can subscribe to specific events rather than a generic update?

Answer:

You can extend the Observer pattern to include event types, and observers can subscribe to specific events.

Example:

function Subject() {
this.observers = {};

this.addObserver = function (event, observer) {
if (!this.observers[event]) {
this.observers[event] = [];
}
this.observers[event].push(observer);
};

this.notifyObservers = function (event) {
if (this.observers[event]) {
this.observers[event].forEach(function (observer) {
observer.update();
});
}
};
}

Question:

How can you implement a Factory pattern that dynamically selects the appropriate subclass based on certain conditions?

Answer:

Modify the Factory to accept parameters and make the decision inside the Factory method.

Example:

function AnimalFactory() {
this.createAnimal = function (type, name) {
if (type === 'Dog') {
return new Dog(name);
} else if (type === 'Cat') {
return new Cat(name);
}
// Handle other cases or throw an error
};
}

Question:

Can you implement a Decorator pattern where decorators can be removed or applied dynamically?

Answer:

You can make decorators removable by including an undo method in each decorator that reverts the changes it made.

Example:

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

function MilkDecorator(coffee) {
var cost = coffee.cost();

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

coffee.undo = function () {
coffee.cost = function () {
return cost;
};
};

return coffee;
}

These questions aim to assess a candidate’s ability to handle nuanced aspects of JavaScript design patterns, combining theoretical knowledge with practical problem-solving skills.

Conclusion :

In the dynamic landscape of JavaScript design patterns, the journey to expertise goes beyond foundational knowledge. Tricky interview questions, focusing on Singleton, Module, Observer, Factory, and Decorator patterns, serve as a litmus test for a candidate’s ability to navigate the intricate realms of practical application. These queries not only assess theoretical understanding but also spotlight a developer’s capacity to unravel complexities in real-world scenarios. Demonstrating prowess in these advanced concepts signifies an adeptness that transcends conventional skill sets, setting candidates apart as strategic thinkers in the evolving realm of JavaScript development. As technology advances, the ability to tackle these intricate challenges becomes a hallmark of true proficiency, emphasizing the importance of staying agile and mastering the deeper intricacies of design patterns.

Here are some additional high-quality tutorials for you to explore:

  1. Unraveling JavaScript Design Patterns
  2. JavaScript interview Question and Answer
  3. Node Js Interview Question and Answer
  4. JavaScript Tricky Question

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Umar Farooque Khan

Experienced software developer with a passion for clean code and problem-solving. Full-stack expertise in web development. Lifelong learner and team player.