Mastering TypeScript: Your Complete Interview Guide

TypeScript has rapidly gained popularity since its inception by Microsoft, becoming an essential tool for developers involved in web and application development. It extends JavaScript by introducing static typing, which aids in building large-scale applications more effectively. If you’re preparing for a TypeScript developer interview, familiarizing yourself with commonly asked questions is crucial. This guide provides a comprehensive list of interview questions categorized by skill level.

Beginner Level TypeScript Interview Questions

1. What is TypeScript?

TypeScript is a superset of JavaScript developed by Anders Hejlsberg at Microsoft. It introduces optional static typing, enabling developers to spot and fix errors during compilation rather than at runtime. As TypeScript is an extension of JavaScript, every JavaScript program is also a valid TypeScript program, making it an attractive option for developers transitioning from JavaScript to TypeScript.

2. Explain Arrays in TypeScript.

In TypeScript, arrays are collections of values of the same data type. They are ordered and indexed, starting at index 0. You can declare an array using:
typescript
var arrayName: datatype[]; // Declaration
arrayName = [val1, val2, valn]; // Initialization

Example:
typescript
let values: number[] = [10, 20, 30];

3. List the Applications of TypeScript.

• Building both client-side and server-side JavaScript applications.
• Used in large-scale enterprise systems.
• Acts as a more functional alternative to JavaScript, allowing for error checking during development.

4. List the Advantages of TypeScript.

• Early detection of errors during development and compilation.
• Compatible with any JavaScript engine.
• Provides IntelliSense for active code hints.
• Supports strong static typing.

5. List the Disadvantages of TypeScript.

• Longer compilation time compared to plain JavaScript.
• Additional compilation step needed to convert TypeScript to JavaScript.
• Type system can be complex for beginners.

6. Explain the features of Arrays in TypeScript.

• Arrays have sequential memory allocations.
• They are immutable once created.
• Array elements should be declared before use.
• You can update the values of elements, even though you cannot erase them.

7. How to declare a variable in TypeScript?

You can declare a variable using let or const:
typescript
let a = 10; // Example with let
const PI = 3.14; // Example with const

8. Name the access modifiers supported in TypeScript.

• Protected: Accessible only within the class and its subclasses.
• Private: Accessible only within the class itself.
• Public: Accessible anywhere there is an instance of the class.

9. Explain Loops in TypeScript.

TypeScript supports various loops:

• For Loop: For definite iteration.
• While Loop: For indefinite iteration based on conditions.
• Do…While Loop: Similar to the while loop but guarantees at least one execution.

10. What is the use of the tsconfig.json file?

The tsconfig.json file specifies compiler options for a TypeScript project. Its presence indicates that the folder is the root of a TypeScript project.

Intermediate Level TypeScript Interview Questions

11. What are Anonymous Functions in TypeScript?

Anonymous functions, or functions without a name, can be dynamically created at runtime and assigned to variables for later use:
typescript
const msg = function() {
return "Hello, World!";
};

12. How to Install TypeScript?

Install TypeScript globally using npm:
bash
npm install -g typescript

13. What are Decorators?

Decorators provide a way to modify classes and methods. They are prefixed with @ and serve as a way to add metadata:
typescript
function log(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
console.log(${propertyKey} was called!);
}

14. Explain Classes in TypeScript.

Classes in TypeScript provide a way to create objects and are the blueprint for creating instances. They support inheritance, encapsulation, and polymorphism:
typescript
class Animal {
constructor(public name: string) {}
}

15. What are Rest Parameters in TypeScript?

Rest parameters allow a function to accept an indefinite number of arguments. They are specified with three dots (...) followed by a name:
typescript
function greet(greeting: string, …names: string[]) {
return ${greeting} ${names.join(", ")};
}

Advanced Level TypeScript Interview Questions

16. What is Type Assertion?

Type assertion allows you to tell the compiler that you know more about the type of a variable than it does. There are two syntaxes:
typescript
let someValue: unknown = "this is a string";
let strLength: number = (someValue as string).length;

17. What are Conditional Types in TypeScript?

Conditional types allow for defining a type based on a condition:
typescript
type TypeCheck = T extends string ? "It’s a string" : "It’s not a string";

18. What are Mapped Types?

Mapped types allow you to create new types by mapping over existing ones, often used to make properties optional or readonly:
typescript
type Partial = { [K in keyof T]?: T[K] };

19. Explain Recursive Type Aliases.

These allow a type to refer to itself. For instance:
typescript
type ValueOrArray = T | Array<ValueOrArray>;

20. What is the Awaited Type?

Awaited is a utility type that provides type inference for promises, helping to unwrap promises:
typescript
type Result = Awaited<Promise>; // Result is inferred as number

This extensive collection of TypeScript interview questions and answers should serve as a solid foundation for your interview preparation. Aim to understand the principles behind the concepts, not just memorize the answers, for a more effective learning experience.