Top 50 TypeScript Interview Questions Explained: A Comprehensive Guide

Are you preparing for an upcoming TypeScript interview? Look no further! In this article, we’ll dive deep into the top 50 TypeScript interview questions, providing you with a comprehensive understanding of this powerful language. Whether you’re a beginner or an experienced TypeScript developer, this guide will equip you with the knowledge necessary to ace your interview and showcase your expertise.

Introduction to TypeScript

TypeScript is a statically-typed superset of JavaScript, developed and maintained by Microsoft. It introduces features like classes, interfaces, and modules, making it easier to build large-scale applications. TypeScript code is transpiled into plain JavaScript, allowing it to run on any platform that supports JavaScript.

Beginner-Level TypeScript Interview Questions

1. What is TypeScript, and why should we use it?
   TypeScript is a superset of JavaScript that adds optional static typing, making it easier to catch errors during development rather than at runtime. It provides better tooling, improved code organization, and better support for large-scale projects.

2. What are the key differences between TypeScript and JavaScript?

   • TypeScript is a statically-typed language, while JavaScript is dynamically-typed.
   • TypeScript supports features like classes, interfaces, and modules, while JavaScript (ES5) does not.
   • TypeScript code needs to be transpiled into JavaScript before execution.

3. How do you declare variables in TypeScript?
   Variables in TypeScript can be declared using let, const, or var. Additionally, you need to specify the variable’s type explicitly or let TypeScript infer it.

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   let name: string = 'John'; // Explicit type declarationconst age = 30; // Type inference (number)

4. What are the different data types supported in TypeScript?
   TypeScript supports various data types, including:

   • Primitive types: number, string, boolean, null, undefined, symbol
   • Non-primitive types: object, array, tuple, enum, any, void
   • User-defined types: class, interface, union, intersection

5. Explain the concept of type inference in TypeScript.
   Type inference is a mechanism where TypeScript automatically infers the type of a variable based on its value or the expression used to initialize it. This feature reduces the need for explicit type annotations and makes the code more concise.

Intermediate-Level TypeScript Interview Questions

1. What are interfaces in TypeScript, and how are they used?
   Interfaces in TypeScript define the structure of an object or a class. They describe the shape of the data, specifying the properties, methods, and their types. Interfaces are used for type-checking and can be implemented by classes.

2. Explain the difference between interfaces and classes in TypeScript.
   Interfaces are used to define the structure of an object or a class, while classes are used to create instances of objects. Interfaces cannot have implementation details, whereas classes can have properties, methods, and constructor logic.

3. How do you implement inheritance in TypeScript?
   Inheritance in TypeScript is achieved using the extends keyword. A child class can extend a parent class and inherit its properties and methods.

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   class Animal {  name: string;  constructor(name: string) {    this.name = name;  }}class Dog extends Animal {  breed: string;  constructor(name: string, breed: string) {    super(name);    this.breed = breed;  }}

4. What is the purpose of the tsconfig.json file?
   The tsconfig.json file is a configuration file used by the TypeScript compiler. It specifies various options and settings, such as the root directory, compiler options, and files to be included or excluded during compilation.

5. How do you define and use modules in TypeScript?
   TypeScript supports both internal modules (namespaces) and external modules (ES6 modules). External modules are defined using the export and import keywords.

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   // module.tsexport class MyClass {}// main.tsimport { MyClass } from './module';

Advanced-Level TypeScript Interview Questions

1. What are decorators in TypeScript, and how are they used?
   Decorators are a way to annotate or modify classes, properties, methods, and parameters in TypeScript. They are functions that take the target object as an argument and can modify its behavior or add metadata.

2. Explain the concept of generics in TypeScript.
   Generics in TypeScript allow you to create reusable components that can work with different data types. They provide a way to define type variables that can be used throughout the code, making it more flexible and type-safe.

3. How do you handle null and undefined values in TypeScript?
   TypeScript has strict null checks by default, which means you need to handle null and undefined values explicitly. You can use type guards, non-null assertion operators (!), or optional chaining (?.) to safely work with nullable values.

4. What are type aliases in TypeScript, and how are they different from interfaces?
   Type aliases are a way to create a new name for an existing type or a combination of types. They are similar to interfaces but can also represent primitive types, unions, tuples, and other types that interfaces cannot represent directly.

5. Explain the difference between let, const, and var in TypeScript.

   • let is a block-scoped variable declaration that allows reassignment.
   • const is a block-scoped variable declaration that cannot be reassigned.
   • var is a function-scoped or globally-scoped variable declaration that allows reassignment.

6. What is the purpose of the declare keyword in TypeScript?
   The declare keyword is used to declare variables or functions that are defined in external libraries or JavaScript files that TypeScript doesn’t have access to. It helps TypeScript understand the types of these external entities and avoid compilation errors.

7. How do you create and use enums in TypeScript?
   Enums in TypeScript are a way to define a set of named constants. They provide a convenient way to represent a group of related values.

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   enum Color {  Red,  Green,  Blue}let favoriteColor: Color = Color.Green;

8. What are mapped types in TypeScript, and how are they used?
   Mapped types in TypeScript allow you to create new types based on existing types by applying a mapping function to the properties of the original type. They are useful for creating utility types or transforming existing types into new shapes.

9. Explain the concept of intersection types in TypeScript.
   Intersection types in TypeScript combine multiple types into a single type, taking the union of all properties and methods from the combined types. They are useful when you need an object to conform to multiple type constraints.

10. How do you handle optional properties and parameters in TypeScript?
    Optional properties and parameters can be defined using the ? symbol after the property or parameter name. This allows you to handle cases where the property or parameter may or may not be present.

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    interface Person {  name: string;  age?: number; // Optional property}function greet(name: string, message?: string) { // Optional parameter  // ...}

Conclusion

Congratulations! You’ve made it through the top 50 TypeScript interview questions. This comprehensive guide has covered a wide range of topics, from beginner-level concepts to advanced TypeScript features. By mastering these questions, you’ll be well-prepared to showcase your TypeScript knowledge and impress potential employers.

Remember, practice is key to solidifying your understanding of TypeScript. Keep coding, exploring new features, and staying up-to-date with the latest developments in the TypeScript ecosystem. Good luck with your upcoming interviews!