Top 10 TypeScript Developer Interview Questions & Answers in 2024

1. Explain the difference between "interface" and "type" in TypeScript.

In TypeScript, both "interface" and "type" can be used to define shapes for objects. However, there are subtle differences. An "interface" can be extended or implemented, while a "type" can be used to create union types or intersection types. Generally, when defining shapes for objects, it's common to use "interface," and for more complex scenarios, "type" is preferred.

2. Describe the key features of TypeScript decorators and provide an example of their usage.

TypeScript decorators are used for adding metadata or behavior to classes, methods, or properties. They are a form of metaprogramming and are commonly used in frameworks like Angular. Key features include being applicable to classes, methods, and properties, and they execute top-down during runtime. An example usage is the @NgModule decorator in Angular to define a module.

3. How does TypeScript handle null and undefined, and what are the differences between them?

In TypeScript, null and undefined are both values that represent the absence of a value. However, they have different use cases. null is often used to intentionally indicate the absence of any object value, while undefined is usually the default value of uninitialized variables. TypeScript has strict null checks to help catch potential errors related to null and undefined, reducing the chance of runtime errors.

4. Explain the concept of generics in TypeScript and provide a practical example.

Generics in TypeScript allow you to create flexible and reusable components. They enable you to write functions and classes that can work with any data type. For example:

function identity<T>(arg: T): T {
    return arg;
}

let result: number = identity(10);

Here, the identity function uses a generic type T and returns the same type it receives as an argument.

5. How does TypeScript support mixin patterns, and why might they be useful in your code?

Mixins in TypeScript are achieved using a combination of intersection types and utility functions. They allow you to combine multiple classes to create a new class with the combined functionality. This is useful for reusing and composing code in a modular way without the need for deep hierarchies.

6. What are conditional types in TypeScript, and how can they be applied in practical scenarios?

Conditional types in TypeScript allow you to create types that depend on other types. They are often used in generics to create dynamic types based on certain conditions. For example:

type NonNullable<T> = T extends null | undefined ? never : T;

This type ensures that the result is never null or undefined.

7. Discuss the benefits of using async/await in TypeScript and provide an example.

Async/await in TypeScript simplifies asynchronous code by allowing developers to write asynchronous logic in a synchronous-looking manner. It enhances code readability and maintainability. Example:

async function fetchData(): Promise<string> {
    const response = await fetch('<https://example.com/data>');
    const data = await response.text();
    return data;
}

8. How does TypeScript support static typing for React applications, and what are the advantages?

TypeScript can be used with React through the tsx file extension. It provides static typing for props, state, and component lifecycles. This helps catch errors during development, improves code documentation, and enhances collaboration in large codebases.

9. Explain the role of declaration files (.d.ts) in TypeScript and when they are necessary.

Declaration files in TypeScript provide type information for JavaScript libraries that don't have built-in TypeScript support. They are necessary when using external libraries or modules in your TypeScript project. Declaration files define the shape of the code, allowing TypeScript to perform type checking without having access to the actual implementation.

10. How does TypeScript handle module systems, and what are the differences between CommonJS and ES6 import/export?

TypeScript supports both CommonJS and ES6 module systems. CommonJS uses require() and module.exports, while ES6 uses import and export. ES6 modules are more statically analyzable and have a more straightforward syntax. TypeScript provides type-checking support for both module systems, but ES6 modules are generally preferred for modern web development.