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30+ TypeScript Interview Questions And Answers

TypeScript is a powerful, typed superset of JavaScript that compiles to plain JavaScript. It's used to add static type definitions to JavaScript, which can help with maintaining large codebases and catching errors early. Below are carefully crafted TypeScript interview questions to assess candidates' understanding and skills.


Most asked TypeScript interview questions



What is TypeScript and how does it differ from JavaScript?

TypeScript is a typed superset of JavaScript that compiles down to plain JavaScript. It offers static typing, interfaces, and classes, which JavaScript does not have by default.


Can you explain what is meant by 'static typing'?

Static typing means that variable types are known at compile time. This allows for better tooling and error checking before code is run.


Write a TypeScript interface for a User object with username and password fields.

interface IUser {
  username: string;
  password: string;

The IUser interface would ensure that any User object has both username and password fields as strings.


How do you declare a variable in TypeScript?

In TypeScript, declare a variable using let or const followed by a colon and the type, like let variableName: type.


What is a 'tuple' in TypeScript?

A tuple is an array with fixed size and known datatypes of elements. Use it to store a collection of values of varied types.


How do you make a property optional in an interface?

interface OptionalProp {
  mandatory: string;
  optional?: number;

You can make a property optional in an interface using ? after the property name. It means the property is not required to exist.


What is the use of 'enums' in TypeScript?

Enums are a feature in TypeScript that allow for defining a set of named constants, which can improve the readability and maintainability of your code.


Can TypeScript code be executed directly in a browser?

No, TypeScript cannot be directly executed in a browser. It needs to be transpiled to JavaScript first, which browsers can run.


What is the 'any' type in TypeScript?

The any type is a powerful way to opt-out of TypeScript's static type checking. It's a type that represents any JavaScript value, with no specific type assignment.


Guess what the following TypeScript code does:

let result: any;
result = 'Hello, World!';

This code will compile but result in a runtime error because it attempts to invoke a string as if it were a function.


Explain the difference between 'let' and 'const' in TypeScript.

In TypeScript, let is used to declare a variable with block scope while const is used to declare a block scope variable that cannot be reassigned.


What is a 'type assertion' in TypeScript?

Type assertion is like a type cast in other languages, used when you have more information about the type of a variable. It's done using as syntax or angle brackets.


Why would you use '{@link TypeName}' in your comments?

The '{@link TypeName}' tag in comments is used to link to a specific type or member in the documentation, improving the developer's understanding.


How do you compile a TypeScript file to JavaScript?

Compile a TypeScript file to JavaScript using the TypeScript compiler (tsc). Run tsc file.ts in the command line.


Can you use 'null' and 'undefined' in TypeScript?

Yes, in TypeScript, null and undefined are legal values and have their own types. They can be assigned to variables explicitly typed with them or to variables of type any.

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Describe Type Inference in TypeScript.

Type inference is TypeScript's capability to deduce and establish types based on variable values without explicit type annotations, making the code cleaner and less verbose.


What are union types and how are they used?

Union types allow variables to be of different types. You use the | symbol to indicate that a variable can be one of several types, improving flexibility in your functions and variables.


Explain Decorators in TypeScript and their use cases.

Decorators are a special kind of declaration that can be attached to a class declaration, method, accessor, property, or parameter. They are used for meta-programming, such as annotating or modifying classes and members at design time.


How can you overload functions in TypeScript?

function add(x: string, y: string): string;
function add(x: number, y: number): number;
function add(x: any, y: any): any {
  return x + y;

Function overloading in TypeScript is done by writing multiple function signatures (types) before the actual function implementation. This can serve different return types based on the input types.


What does the 'never' type represent?

The never type in TypeScript represents values that never occur. It's used for functions that do not return a value or always throw an exception.


Explain how 'this' works in TypeScript. Provide an example.

class Example {
  function() {
const example = new Example();

In TypeScript, this refers to the object a function is a method of. Proper usage is critical for class methods to access properties and other methods of the class instance.


What is the difference between 'type' and 'interface' in TypeScript?

Both 'type' and 'interface' can define the shape of an object or function, but interfaces are more extensible through declaration merging. 'Type' can, however, represent a union or tuple.


Guess what the following TypeScript code does: const items = ['John', 'Jane', 'Joe']; console.log(items['1']);

const items = ['John', 'Jane', 'Joe'];

The code accesses the second element in the array items using an index. It outputs: Jane.


How do generics enhance the capabilities of TypeScript?

Generics introduce a way to create reusable, type-safe components by allowing you to parameterize types. They work much like variables, but for types.


What is a type guard in TypeScript?

A type guard is a runtime check that guarantees the type of a variable within a scope. It can be done using typeof, instanceof, or custom type guard functions.


How do you make properties of a class readonly in TypeScript?

You can declare class properties as readonly. It prevents them from being changed after they're set, typically within a constructor.


What is 'declaration merging'?

Declaration merging is a TypeScript feature where the compiler merges two or more separate declarations declared with the same name into a single definition.


Describe how a TypeScript enum is compiled into JavaScript

A TypeScript enum gets compiled into a JavaScript object that maps both enum keys to values and values to keys, allowing you to use enums at runtime.


Share an example of a TypeScript Indexed Access Type and explain its use case.

interface Person { name: string; age: number; }
type NameType = Person['name'];  // string

Indexed Access Types allow you to look up a specific property on an interface, extracting its type. They are useful when needing to reference nested types.


Can you provide an example of a type alias in TypeScript and when it might be useful?

A type alias defines a new name for an existing type. It's useful when you want to use complex types multiple times without repeating yourself.

type StringOrNumber = string | number;

TypeScript Interview Tips

Understand the Concept, not just the Solution

When preparing for tough TypeScript questions, focus on grasping the underlying concepts rather than just memorizing solutions. Being able to explain the 'why' behind a code snippet or solution is far more valuable than rote memorization. Employers are looking for developers who can think critically and solve problems, not just recite answers.

For example, if asked about types, don't just list them; explain how they contribute to TypeScript's overall reliability. Consider how you can apply your knowledge to a range of scenarios. This depth of understanding will help you remain adaptable and articulate during an interview.

Practice Code Writing and Reading

Becoming adept at reading and writing TypeScript is essential. Practice by examining well-written TypeScript code from open-source projects or by contributing to them. Try to rewrite JavaScript code in TypeScript and notice how the added type information changes your approach to structuring components.

When you're familiar with common patterns and idioms, you can better demonstrate your capability to interviewers. Being comfortable with reading code also enables you to understand and answer code-related interview questions quickly and accurately.

Relate Experience with TypeScript Features

Connect your past experiences to the TypeScript features you discuss. Rather than just stating you've used a certain feature, describe how you've implemented it in a project and what benefits it offered. This gives life to your knowledge and shows that you have practical experience.

For instance, if you've used TypeScript's interfaces, discuss a situation where they were particularly helpful or how they improved your development process. This real-world application is what interviewers need to hear to understand your expertise.

Stay Up-To-Date and Show Enthusiasm

TypeScript is an evolving language, so staying current with the latest features and best practices is important. Discuss recent advances or features that you're excited about. This shows your passion for technology and your proactive approach to learning.

You might talk about how TypeScript's latest improvements have influenced your work or your eagerness to try out new features in upcoming projects. Your enthusiasm can be contagious and make a positive impression on your interviewer.

Explain Your Reasoning and Problem-Solving Approach

During an interview, explain the reasoning behind your answers. If you're unsure about a solution, talk through your thought process and the steps you'd take to find an answer. This demonstrates your rational thinking and problem-solving skills, which are crucial for developers.

For example, if presented with a complex coding problem, discuss how you'd simplify it, what tools you'd use, and how you'd verify your solution. Interviewers appreciate candidates who can confront challenges methodically and effectively.


How much do TypeScript developers make?

TypeScript developers' salaries vary vastly depending on the region, experience, and the complexity of the project, with a general range starting from approximately $45/hour. Read more: How much do TypeScript developers make?

Is TypeScript better than JavaScript?

TypeScript is often seen as an enhancement to JavaScript, offering static typing and advanced features for large-scale and complex applications. Read more: Is TypeScript better than JavaScript?

What do TypeScript developers do?

TypeScript developers write, test, and maintain front-end applications, ensuring type-safe and scalable code that aligns with modern development practices. Read more: What do TypeScript developers do?

Why using FireHire for hiring TypeScript developers is the best choice?

FireHire assures expedited access to pre-vetted senior TypeScript developers, risk-free hiring with a satisfaction guarantee, and an efficient process tailored to your startup's needs.

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