Top 10 Senior TypeScript Developer Interview Questions & Answers in 2024

1. How can you optimize the performance of a TypeScript application, especially in terms of bundle size and runtime efficiency?

To optimize TypeScript application performance, several techniques can be applied. Using tree-shaking to eliminate unused code, employing code splitting to reduce bundle size, and optimizing imports can help. Additionally, leveraging tools like Webpack Bundle Analyzer and performance profiling tools like Lighthouse or Chrome DevTools aids in identifying bottlenecks.

2. Explain the concept of dependency injection in TypeScript and how it can be implemented in an application.

Dependency injection is a design pattern where the dependencies of a class are injected from the outside rather than created internally. In TypeScript, frameworks like Angular provide built-in support for dependency injection. Custom implementations can use techniques like constructor injection or property injection to inject dependencies. Dependency injection enhances code modularity, testability, and maintainability.

3. Discuss the use of Web Workers in TypeScript for concurrent execution and how they contribute to better performance.

Web Workers enable concurrent execution of scripts in the background, improving the responsiveness of web applications. In TypeScript, Web Workers can be utilized for CPU-intensive tasks without affecting the main thread. The worker_threads module in Node.js allows similar functionality on the server-side. This approach enhances the application's performance by utilizing multiple threads.

4. How does TypeScript handle state management in large-scale applications, and what are the advantages of using state management libraries?

In large-scale TypeScript applications, state management becomes crucial. Libraries like Redux or MobX are commonly used to manage the application state. They provide a predictable and centralized state container, ensuring a clear flow of data and making it easier to manage complex application states.

5. Explain the role of decorators in design patterns, such as the Singleton pattern, and provide an example in a TypeScript context.

Decorators in TypeScript can be used to implement design patterns like the Singleton pattern, ensuring a class has only one instance. Example:

function singleton(target: any) {
    let instance: any;
    return function () {
        if (!instance) {
            instance = new target();
        }
        return instance;
    };
}

@singleton
class MyApp {
    // Class implementation
}

This decorator ensures that only one instance of MyApp is created.

6. Discuss the challenges and solutions when integrating TypeScript with third-party JavaScript libraries lacking type definitions.

Integrating TypeScript with libraries lacking type definitions requires declaration files or type definitions. Challenges include potential mismatches and runtime errors. Solutions involve creating custom declaration files or using tools like tsc --allowJs to gradually migrate to TypeScript while maintaining compatibility.

7. How can you implement server-side rendering (SSR) in a TypeScript-based Node.js application, and what are the benefits of SSR?

Implementing SSR in a TypeScript Node.js application involves using frameworks like Next.js. SSR enhances performance by rendering HTML on the server, reducing the load on the client. Benefits include improved SEO, faster initial page loads, and better user experience.

8. Discuss the principles of functional programming in TypeScript and provide examples of how they can be applied in real-world scenarios.

Functional programming principles, such as immutability and higher-order functions, can be applied in TypeScript for cleaner and more maintainable code. Example:

const numbers = [1, 2, 3, 4];

// Immutability
const doubledNumbers = numbers.map(num => num * 2);

// Higher-order function
const sum = numbers.reduce((acc, num) => acc + num, 0);

These principles lead to more predictable and testable code.

9. Explain the concept of "TypeScript Decorator Factories" and provide an example of their usage.

Decorator factories in TypeScript allow the creation of parameterized decorators. They are functions that return decorator functions. Example:

function logParameter(target: any, key: string, index: number) {
    const metadataKey = `log_${key}_parameters`;
    if (Array.isArray(target[metadataKey])) {
        target[metadataKey].push(index);
    } else {
        target[metadataKey] = [index];
    }
}

class MyClass {
    @logParameter
    greet(message: string): void {
        console.log(message);
    }
}

Here, the logParameter decorator factory logs the index of the parameter when the greet method is called.

10. Discuss the role of "intersection types" and "union types" in advanced TypeScript type system usage.

Intersection types (&) and union types (|) are powerful tools in TypeScript for creating complex types. Intersection types combine multiple types, while union types represent a value that could be of multiple types. They are extensively used in scenarios like creating flexible APIs, modeling complex data structures, and enhancing code expressiveness and safety.