Top JavaScript Design Pattern Interview Questions and Answers

Design patterns are proven solutions to common programming problems. They provide developers with reusable templates to tackle challenges around code structure maintainability and scaling.

Mastering design patterns is essential for passing JavaScript coding interviews and demonstrating your design abilities. In this comprehensive guide, we’ll overview popular JavaScript design patterns and sample interview questions with model answers.

Overview of JavaScript Design Patterns

Some key design patterns that web developers should know include

• Singleton – Restricts instantiation of a class to a single object instance. Useful for things like modal dialogs.

• Factory – Creates objects without specifying the exact class to instantiate. Helps decouple code dependencies.

• Observer – Allows objects to subscribe to event notifications from other objects. Handy for state changes.

• Decorator – Dynamically adds behaviors to existing objects at run time. Avoids subclassing just to extend functionality.

• Module – Encapsulates code into independent, interchangeable modules with public/private access modifiers.

• Mixin – Allows objects to borrow (“mix in”) functions from other objects. Supports code reuse and customization.

Understanding these and other patterns will prepare you to handle common JavaScript design questions. Study up on use cases, class diagrams, and real code examples for each pattern.

8 JavaScript Design Pattern Interview Questions and Answers

Let’s look at some typical JavaScript design pattern interview questions along with sample responses:

Q1. What is a design pattern?

A design pattern is a reusable solution to a common programming problem within a certain context. Design patterns provide templates to follow in order to optimize code structure, promote reuse, and manage complexity.

Some key benefits of using design patterns are they improve code maintainability and readability by leveraging proven techniques. Design patterns also save time by avoiding re-inventing solutions and provide shared vocabulary to discuss design concepts.

Q2. What are the benefits of using design patterns in JavaScript?

Some benefits of using design patterns with JavaScript include:

• Design patterns provide tried and tested development paradigms for JavaScript to help optimize code structure.

• They offer reusable templates that are optimized for performance and scalability.

• Design patterns improve maintainability of JavaScript codebases as projects grow larger.

• Patterns enable loose coupling of components, which reduces dependencies between code modules.

• They improve developers’ shared vocabulary and understanding of common abstractions.

• Design patterns support clean separation of concerns within JavaScript code.

• They reduce complex custom code by leveraging well-understood techniques.

Q3. What is the Singleton pattern?

The Singleton pattern restricts instantiation of a class to a single object instance. It ensures a class has at most one instance at runtime and provides global access to that instance.

Some typical uses of the Singleton pattern include:

• Managing global application state
• Accessing shared resources like databases or file systems
• Implementing centralized event dispatching mechanisms
• Providing access to single-instance entities like modal dialogs

The Singleton provides controlled access to shared resources while saving memory from unnecessary object duplication.

Q4. How do you implement the Singleton pattern in JavaScript?

Here is one common way to implement the Singleton pattern in JavaScript:

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 instanceA = Singleton.getInstance(); const instanceB = Singleton.getInstance();console.log(instanceA === instanceB); // true

This uses a closure to encapsulate the instance creation logic within the enclosing function. The getInstance() method checks if an instance already exists, creating one if not and caching it for subsequent lookups. This ensures at most a single instance is created.

Q5. What is the Observer pattern?

The Observer pattern defines one-to-many relationships between objects, such that when one object (the subject) changes state, the other dependent objects (observers) are automatically notified.

The Observer pattern facilitates loose coupling by abstracting away direct dependencies between the subject and observers. Observers simply subscribe to event notifications and can update independently based on their own requirements.

This pattern is useful for event handling, state monitoring, and push-based updates between objects in a publish-subscribe model.

Q6. When would you use the Decorator pattern in JavaScript?

The Decorator pattern lets you dynamically modify an object at runtime by wrapping it within a Decorator class. This avoids having to rely on heavy subclassing just to extend basic functionality.

Here are some cases where the Decorator pattern is handy in JavaScript:

• Adding new windowing behaviors like scrollbars or resizing without subclassing Window
• Extending prototype objects with new methods at runtime
• Implementing row striping, pagination, or sorting for tables/lists
• Enhancing UI components with added behaviors like drag-and-drop
• Adding stackable functionality like retries or logging to network requests

The core benefit of the Decorator is enhancing flexibility – new behaviors can be added, removed, or reordered on the fly.

Q7. Explain the Module design pattern in JavaScript

The Module pattern provides encapsulation and information hiding, keeping implementation details private and exposing a public API. Modules only share the minimum necessary to allow for loose coupling between components.

In JavaScript, modules can be implemented by wrapping code inside an immediately invoked function expression (IIFE):

const Module = (function() {  const privateMethod = function() {    // private method  };    const publicMethod = function() {    // can call private method    privateMethod();   };  return {    publicMethod: publicMethod  };  })();Module.publicMethod(); 

By storing methods and variables inside a closure, only the returned object is exposed. This reveals public methods while restricting access to private code.

Q8. When is it better to use inheritance vs composition in JavaScript?

Inheritance models “is a” relationships, allowing new classes to inherit behaviors from parent classes. Composition models “has a” relationships – objects contain or reference other objects to reuse their capabilities.

Composition is generally preferable in JavaScript because:

• Composition provides more flexibility than inheritance. Objects can absorb any functionality dynamically.

• Mocking and testing is easier without deep inheritance hierarchies.

• Composition avoids tight coupling between classes that inheritance can create.

• Multiple inheritance is possible in JavaScript via mixins.

However, inheritance can be useful when:

• There is an obvious subclass relationship between classes (e.g. Dog extends Animal).

• You need to share code across several subclasses.

• You want to model complex hierarchical domains.

Preparing strong answers to questions like these will get you well on your way to mastering the JavaScript design pattern interview!