Top 50 TypeScript Interview Questions for Beginner to Advanced-Level

Introduction

TypeScript is one of the fastest growing web development technologies today due to its powerful typing system and seamless integration with JavaScript. As TypeScript adoption grows, knowledge of TypeScript is becoming an increasingly important skill when hiring for web developer roles.

In this guide, we have compiled 50 of the most commonly asked TypeScript interview questions ranging from basic to advanced levels. Whether you’re a candidate preparing for an interview or an interviewer looking to evaluate TypeScript skills, these questions will help assess a candidate’s overall TypeScript proficiency.

Basic TypeScript Interview Questions

Here are some common TypeScript interview questions for beginner to intermediate level:

Q1. What is TypeScript?

TypeScript is a typed superset of JavaScript developed by Microsoft that compiles to plain JavaScript. It adds optional static typing, classes, interfaces and other features to JavaScript.

Q2. What are the benefits of using TypeScript over plain JavaScript?

Key benefits of TypeScript:

• Optional typing allows catching errors during compile time
• Improved intellisense, autocompletion, refactoring in IDEs
• Modern ES6+ features like classes, modules that compile down for older browsers
• Non-JavaScript features like interfaces, generics
• Increased productivity for large JavaScript codebases

Q3. What are the different data types supported by TypeScript?

Some of the commonly used data types in TypeScript:

• Primitive – string, number, boolean, void, null, undefined
• Object – Any JavaScript objects/classes
• Array – number[], string[] etc
• Tuple – [string, number]
• Enum – enum {NEW, OLD}
• Any – any (allow anything)
• Union – number | string (either number or string)

Q4. What is difference between var, let and const in TypeScript?

• var – Function scoped, can be redeclared and mutated
• let – Block scoped, can be mutated but not redeclared
• const – Block scoped, cannot be changed or redeclared

Q5. What are the advantages of TypeScript over JavaScript?

Some key advantages of TypeScript over plain JavaScript:

• Type safety identifies bugs during compile time
• Object-oriented features like classes make code more maintainable
• Tooling support improves developer workflow in IDEs
• Non-JavaScript features like generics increase expressiveness
• Modern ES6+ features compile down for older browsers
• Additional non-standard features like enums

Q6. What is generics in TypeScript and how is it useful?

Generics allow creating reusable components by abstracting concrete types. For example:

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

This echo function works with any type instead of just string or number. Generics provide increased flexibility and code reuse.

Q7. What is type inference in TypeScript?

Type inference refers to TypeScript’s ability to deduce the types in certain scenarios when they are not explicitly provided. For example:

let age = 30; // Type inferred as number

TypeScript will infer age as a number type based on the initialization value. This reduces explicit type annotations in many cases.

Q8. How are interfaces useful in TypeScript?

Interfaces define contracts in the form of property and method signatures. They are useful to:

• Declare structured object types
• Declare reusable method types
• Get design-time errors if contracts are not satisfied
• Enable duck typing with structural subtyping

For example:

interface User {
  name: string
  id: number
}

This User interface shape can be reused across multiple classes and objects.

Q9. What is a tuple type in TypeScript?

A tuple type defines an array with fixed number of elements and known types. For example:

let user: [string, number];
user = ["John", 30];

The user tuple declares a 2-element array with the first element as string and second element as number.

Q10. How do you explicitly set a type in TypeScript?

The type can be explicitly set using “: type” or “as type” syntax during variable declaration:

let id: number = 1; 

let name = "John" as string;

This syntax removes any ambiguity and makes the intended type clear.

Intermediate TypeScript Interview Questions

Let’s continue with some TypeScript interview questions for intermediate level:

Q11. What is type casting in TypeScript?

Type casting allows explicitly converting a variable from one type to another to satisfy type requirements. For example:

let str = "1";
let num = str as number;

Here num gets type casted to a number from the original string type.

Q12. What is a class in TypeScript and how is it different from interfaces?

Classes define object blueprint including fields, constructor, and methods. Interfaces act like contracts and do not contain implementation. Key differences:

• Classes can be instantiated, interfaces cannot
• Classes can have concrete implementation, interfaces define signatures only
• Classes can extend from base classes and implement interfaces
• Interfaces can extend other interfaces

Q13. How are Enums useful in TypeScript?

Enums allow defining a set of named constants. For example:

enum Direction {
  Up,
  Down, 
  Left,
  Right
}

This improves code readability by using Direction.Up vs raw values. It also prevents invalid assignments that are not any of the defined enums.

Q14. What are the differences between var, let and const in TypeScript?

• var – Function scoped, reassignable
• let – Block scoped, reassignable
• const – Block scoped, cannot be reassigned

Q15. How does module system work in TypeScript?

TypeScript has inbuilt support for modules that can contain logically grouped code in different files. For example:

File1.ts

export const name = "John";

File2.ts

import { name } from "./File1";

console.log(name); // "John"

Modules avoid polluting global namespace and allow organizing code at scale.

Q16. What are ambient declarations in TypeScript?

Ambient declarations allow describing existing global declarations like objects and functions to get TypeScript benefits without changing existing code.

For example, we can create a declare block for jQuery:

declare let $: any;

$.ajax("/data");

This allows passing TypeScript’s correctness checks without errors from undefined $ reference.

Q17. What is non-null assertion operator (postfix !) in TypeScript?

The non-null assertion operator allows overriding strict null checks on a specific variable. For example:

function handler(str?: string) {
  // str could be undefined  
  str!.trim(); // Override strict null check
}

The ! operator indicates str should not be null in this specific case. This prevents null reference errors.

Q18. What is the difference between == and === in TypeScript?

• == does equality with coercion (e.g. compares different types)
• === does strict equality without coercion (e.g. requires same type)

Using === prevents unintended type coercion issues and is safer.

Q19. How do you enable strict null checks in TypeScript?

Strict null check can be enabled in tsconfig.json:

{
  "strictNullChecks": true 
}

This prevents undefined/null variables from being accessed without checking first. This helps avoid common bugs.

Q20. What are the advantages of strict null checks?

Strict null checking improves overall code quality and robustness by requiring explicit handling of null/undefined values. Some benefits:

• Surface reference errors during compile time rather than runtime
• Reduce ambiguous use of null/undefined in application code
• Encourage handling of edge cases upfront
• Improve developer experience with better editor tooling

Overall it reduces bugs caused by null references in large codebases.

Advanced TypeScript Interview Questions

Finally, let’s go through some advanced TypeScript interview questions:

Q21. What are control flow analysis errors in TypeScript?

These refer to logical errors flagged by the TypeScript compiler related to unreachable code and redundant conditionals.

For example:

function test(x: number) {
  if (x > 0) {
    return "hello";
  } else if (x < 0) { 
    return "world";
  }

  return "bye"; // Unreachable
}

The return "bye" will never be executed based on earlier return statements. TypeScript flags these cases as control flow errors.

Q22. What are the differences between type aliases and interfaces in TypeScript?

Both allow declaring object types, but key differences:

• Interfaces can be extended or implemented by classes, type aliases cannot.
• Interface signatures can be merged, type aliases cannot.
• Type aliases can use primitives and unions, interfaces only object types.

So in summary, interfaces focus on contracts that can be implemented. Type aliases are more for opaque types.

Q23. How do you enable strict property initialization in TypeScript?

Strict property initialization requires explicitly initializing class properties – either in constructor or inline. It prevents inconsistent state due to uninitialized values.

Enable via:

{
  "strictPropertyInitialization": true  
}

For example:

class User {
  name: string // Error - property not initialized

  constructor() {
    this.name = "John"; // Ok
  }
}

Q24. What are function overloads in TypeScript?

Function overloading allows defining multiple function signatures with the same name but different parameters. The suitable overload is chosen based on parameters passed.

For example:

// Overloaded sum()
function sum(a: number, b: number): number;
function sum(a: string, b: string): string;
function sum(a: any, b: any) {
  return a + b;
}

This allows reuse of sum name while setting precise param types for each case.

Q25. What are the differences between namespace and module in TypeScript?

Namespaces group code logically into global scope identifiers. Modules organize code into files that import/export. Key differences:

• Namespaces pollute global scope, modules are local.
• Namespaces group wide classes, modules group code files.
• Modules can import other modules, namespaces cannot.

Q26. What is optional chaining ? operator in TypeScript?

The ?. optional chaining operator allows safely accessing nested properties/methods even if parent reference is undefined.

For example:

let user = getUser(); 

// No error even if user is null
user?.address?.country;

This avoids tedious null checking at each level. The expression short-circuits on first undefined reference.

Q27. What is Declaration Merging in TypeScript?

Declaration merging allows a single name to refer to both a variable and a function. For example:

// Declare global variable
declare let someVar: any; 

// Declare global function
declare function someVar(): void;

// Use as both variable and function
someVar = 123;
someVar();

The compiler merges the variable and function into one identifier someVar.

Q28. What are Index Signatures in TypeScript?

Index signatures define the types of indexed properties in objects and interfaces. For example:

interface User {
  // Index with string keys = string values
  [prop: string]: string; 
}

const user: User = {}; 

user.name = "John"; 
user.age = "20"; // Error

This allows enforcing types for dynamically indexed objects.

Q29. What is destructuring assignment in TypeScript?

Destructuring allows unpacking arrays/objects into individual variables for convenient access.

For example:

const arr = [1, 2];

// Array destructure
const [x, y] = arr; 

// Object destructure 
const { prop1, prop2 } = { prop1: 123, prop2: "abc" };

Destructuring cleans up code by avoiding repetitive array/object access.

Q30. What are Tagged Templates in TypeScript?

Tagged Templates allow interpreting template literals with a special tag function. For example:

function echo(strs: string[], vals: string[]) {
  return strs.join("") + vals.join("");
}

const name = "John";
const text = echo`Hello ${name}, welcome!`;

// text = "Hello John, welcome!"

This allows reusing template logic without cluttering code with strings.

Q31. How do you enable noImplicitAny in TypeScript?

The noImplicitAny flag prevents implicitly returning the any type and forces explicit typing:

// tsconfig.json
{
  "noImplicitAny": true
}

This improves type safety and prevents bugs caused by ambiguous any types.

Q32. What is non-null assertion operator (postfix !) in TypeScript?

The non-null assertion operator allows overriding strict null checks on a specific variable. For example:

function handler(str?: string) {
  // str could be undefined
  str!.trim(); // Override strict null check  
}

The ! asserts that str should not be null in this case to prevent errors.

Q33. What are the differences between type aliases and interfaces in TypeScript?

• Interfaces can be extended or implemented, aliases cannot.
• Interface merging is possible but not for aliases.
• Aliases can use primitives and unions, interfaces only objects.

Q34. How do you enable noUnusedLocals and noUnusedParameters in TypeScript?

These flags prevent unused locals and params by enabling compiler checks:

// tsconfig.json 
{
  "noUnusedLocals": true,
  "noUnusedParameters": true
}

This improves code quality by eliminating dead code and unused variables/params.

Q35. What are some benefits of strictBindCallApply in TypeScript?

Enabling strictBindCallApply makes bind, call and apply methods more type safe. Some benefits:

• Avoid errors from too few/many params passed
• Unify signatures of bound and original method
• Intellisense displays context types for this

Q36. How do you enable strictPropertyInitialization in TypeScript?

It forces explicit property initialization, preventing inconsistent state:

// tsconfig.json
{
  "strictPropertyInitialization": true 
}

For example:

class User {
  name: string // Error - property not initialized

  constructor() {
    this.name = "John"; // Ok
  }
}

Q37. What compiler checks are enabled with strict flag in TypeScript?

Enabling "strict": true sets:

• noImplicitAny
• strictNullChecks
• strictFunctionTypes
• strictBindCallApply
• strictPropertyInitialization

This opt-in stricter checking often surfaces bugs early.

Q38. How do you enable noImplicitReturns in TypeScript?

noImplicitReturns forces functions to always return a value:

// tsconfig.json
{
  "noImplicitReturns": true
}

For example:

function foo(x) {
  if (x) {
    return true;
  } 
  // Error - function may end without returning
}

This avoids bugs from missing returns especially with complex logic.

Q39. What is the utility type Partial<T> in TypeScript?

Partial makes all properties on T optional:

interface User {
  name: string;
  age: number; 
}

function update(user: Partial<User>) {
  // user may have partial fields  
}

This allows objects with partial fields instead of full objects.

Q40. What is the difference between Pick and Partial in TypeScript?

• Pick accepts some properties from an object
• Partial makes all properties optional

For example:

interface User {
  name: string;
  age: number;
  active: boolean;
}

type UserUpdate = Pick<User, 'name' | 'age'>; 

type UserPartial = Partial<User>;

Pick creates a type with subset of properties while Partial makes a type optional.

Q41. What is the Record<K,T> utility type in TypeScript?

Record allows creating a mapped type from a key type K to a value type T:

interface PageInfo {
  title: string;
}

type Pages = Record<string, PageInfo> 

const pages: Pages = {
  home: { title: "Home" },
  about: { title: "About" },
};

This dynamically generates types from record-like data.

Q42. What are some differences between type guards and type assertions in TypeScript?

Type guards actively check conditions and narrow types:

function isNumber(x: any): x is number {
  return typeof x === "number";
}

Type assertions just forcefully cast without checks:

let x = {} as number;

Prefer type guards where possible as they are safer.

Q43. What are Mapped Types in TypeScript?

Mapped types generate new object types by transforming properties from existing types.

For example:

type Optional<T> = {
  [P in keyof T]?: T[P]; // Make all props optional
};

type UserPartial = Optional<User>;

This creates new types without repeating declarations.

Q44. What are some advantages of using Discriminated Unions in TypeScript?

Discriminated unions attach a discriminator field to differentiate union members.

Some advantages:

• Narrow types based on discriminator during runtime
• Avoid need for explicit casts/guards
• Distinguish unions clearly for tooling
• Prevent holes in unions with a catch-all case

For example:

interface Rect {  
  kind: "rect";
  width: number;
}

interface Circle {
  kind: "circle";
  radius: number;
}

type Shape = Rect | Circle;

The kind field discriminates the shape types.

Q45. How do you enable noPropertyAccessFromIndexSignature in TypeScript?

This flag requires explicit property access for indexable types:

// tsconfig.json
{ 
  "noPropertyAccessFromIndexSignature": true 
}

For example, this will error:

interface Props {
  [key: string]: any;
  foo: number; 
}

const props: Props = {};
props.foo = 123; // Error - explicit props['foo'] needed

This helps avoid accidental misuse of index signatures.

Q46. What are some key principles for writing good generic functions in TypeScript?

Some principles for solid generics:

• Reuse and apply DRY principle
• Type parameters should appear twice at least
• Avoid over-engineering – keep it simple
• Name generic types meaningfully (T is generic, KV is key-value)
• Add constraints to generic types where applicable
• Use default types for generic type parameters

Q47. How do you mark a property as optional in TypeScript?

Using ? after property name marks it as optional:

interface User {
  name: string;
  age?: number; // optional
}

const user: User = { name: "John" }; // no error

Optional properties allow partial initialization.

Q48. What are some benefits of using Tuple types in TypeScript?

Tuples provide an array type with fixed length and known element types. Benefits:

• Convey semantics – e.g. pair, key-value
• Avoid misuse/misalignment of array values
• Intellisense indicates purpose of each index
• Enforce length and positional typing

For example:

type NameAge = [string, number];

const user: NameAge = ["John", 30];

NameAge makes the intent clearer than just string[].

Q49. How do you enable strictFunctionTypes in TypeScript?

strictFunctionTypes makes function type checking stricter:

// tsconfig.json
{
  "strictFunctionTypes": true  
}

This catches errors when function types are used inconsistently.

Q50. What is the Omit<T,K> utility type in TypeScript?

Omit creates a type that excludes certain properties:

interface User {
  name: string;
  age: number;
  active: boolean; 
}

type UserUpdate = Omit<User, 'active'>;
// Has name and age only

This helps create defensive copies or partial versions of types.

Conclusion

This guide covered TypeScript interview questions ranging from basics like types and interfaces to advanced concepts like declaration merging and tagged templates. The depth of knowledge required depends on the specific role expectations.

However, having a grasp of TypeScript’s key capabilities and being able to discuss tradeoffs between different techniques puts you ahead. We hope these questions help assess and improve your TypeScript skills. Let us know if you have any other favorite TypeScript interview topics we should cover!