Top 20 TypeScript Interview Questions and Answers for 2023

Which are the common TypeScript interview questions? What are the object-oriented terms supported by TS? What is the difference between JavaScript and TypeScript?

Looking for answers to these questions? Keep reading.

What is TypeScript?

TypeScript is a programming language and a strict syntactical superset of JavaScript. TypeScript adds optional static typing to the language, develops large applications, and transpiles to JavaScript.

A TypeScript program is written in a file with the .ts extension, and the compiler subsequently converts it to JavaScript. The file can also be written in any code editor. However, your platform must have the compiler installed. Following installation, the TypeScript code is converted into a normal JavaScript file with the command tsc filename>.ts. Here is the sample syntax:

var message:string = “Welcome to Turing!”

console.log(message)

Basic-level TypeScript interview questions

1. What are the Benefits of TypeScript in app development?
   

   The benefits of TypeScript in app development are as follows:
   1. Any JavaScript engine or browser can execute TypeScript since it is quick, easy to learn, and runs quickly. 
   2. TypeScript shares the same syntax and semantics as JavaScript. This feature facilitates the speedier writing of front-end code.
   3. TypeScript also seamlessly integrates with existing JavaScript frameworks and libraries. 
   4. Existing JavaScript libraries like Jquery, D3.js, and more are supported via the Definition file, which has the.d.ts extension. 
   5. TypeScript includes ES6 and ES7 capabilities that can function in JavaScript ES5 engines like Node.js.
      

2. What are the disadvantages of TypeScript?
   

   Here are a few disadvantages of TypeScript:
   1. Coding in TypeScript requires a lengthy compilation process. 
   2. TypeScript doesn’t support abstract classes. 
   3. A compilation step is necessary to convert TypeScript code into JavaScript if you want to run the TypeScript application in the browser. 
   4. Any third-party library must have a definition file for use.
   5. Type definition files’ quality is not very good. 
      

3. What are the Components of TypeScript?
   

   The TypeScript language is divided into three main layers. Each layer is further divided into sublayers or components. There are three different types of components in TypeScript, they are as follows:
   1. Language
   2. The TypeScript Compiler
   3. The TypeScript Language Service
      

4. Explain a few features of Arrays in TypeScript.
   

   Here are a few features of Arrays in TypeScript: 
   1. Arrays can’t be changed. This action implies that an array cannot be resized once it has been constructed. 
   2. Each memory block represents an element of an array. 
   3. An array element’s subscript or index is a special number that serves as the element’s identifier. 
   4. Like variables, arrays must be declared before use. Developers can use the ‘var’ keyword to declare an array.
   5. The process of adding elements to an array is ‘array initialization.’ 
      

5. How to declare a variable in TypeScript?
   

   There are two ways to declare variables: let and const. However, the following conditions must meet to declare a valuable:
   1. Variable names must contain alphabetic characters and numbers. 
   2. Variable names cannot contain spaces or other special characters, with the exception of the underscore (_) and the dollar ($) symbol. 
   3. The initial character in a variable name cannot be a digit.
      
      Sample syntax:
      let var a=10;
      or
      function f() {
      var message = “Hello, Turing!”;
      return message;
      }
      

6. Which access modifiers are supported in TypeScript?
   

   The access modifiers supported by TypeScript are as follows: 
   1. Protected Access Modifier
   2. Private Access Modifier
   3. Public Access Modifier
      

7. What are loop statements in Typescript?
   

   A loop statement allows repetitive execution of a statement or a series of statements. To handle looping requirements, TypeScript provides many loops, such as the following: 
   1. For Loop: 
   2. While Loop
   3. Do…While Loop
      

8. Define static typing.
   

   Static typing is a compiler with recognisable variables, arguments, and object members at compile time. Static typing aids in the early detection of faults in codes. 
   

9. Can TypeScript be used for backend development?
   

   Yes. By combining TypeScript with Node.js, backend applications can benefit from the added security that the language provides.
   

10. How will you check if a variable is null, or undefined in TypeScript?
    

    If (value) returns true – if value is not null, undefined, empty, false, 0 or NaN.
    
    Related post: IT Hiring Trends 2022-23: 10 Most In-Demand Skills in US Companies

Advanced-level TypeScript interview questions

1. What is Type Assertion?
   

   Similar to a type cast in other languages, a type assertion doesn’t do any further data reorganization or verification. It is only used by the compiler and has no impact on runtime. TypeScript assumes that you have carried out the particular checks as a programmer.
   

2. What are Recursive Type Aliases?
   

   There has never been much scope for ‘recursively’ referencing type aliases. This is necessary because each type of alias must be able to replace whatever it aliases with itself. Some recursive aliases are rejected by the compiler since they aren’t always feasible. 
   
   Although type aliases cannot be recursive, interfaces can, though their expressiveness is constrained. Creating a type alias and extracting the type’s recursive components into interfaces are combined to do that.
   
   Here is the sample syntax:
   type ValueOrArray<T> = T | ArrayOfValueOrArray<T>;
   interface ArrayOfValueOrArray<T> extends Array<ValueOrArray<T>> {}
   

3. How to get ‘declaration’ files automatically?
   

   Set the declaration compiler option true in the tsconfig.json.
   
   Here is a sample syntax:
   {
   “compilerOptions”: {
   …
   “declaration”: true,
   }
   }
   

4. How to overload a function?
   

   Use the same function name again above the original function without the brackets {}. Change the number of arguments, the argument types or/and the return type.
   
   Here is an example:
   function add(x: string, y: string): string;
   function add(x: number, y: number): number {
   return x + y;
   }
   

5. Explain Modules in TypeScript.
   

   A module organizes TypeScript code. Modules are classified into two types. They are as follows:
   
   
   1. Internal Modules: Internal modules were previously used to logically group classes, interfaces, and functions into a single unit that could then be exported into a different module. However, in the most recent version of TypeScript, this logical grouping is referred to as namespace.
   2. External Modules: External modules allow developers to describe and load dependencies between many external js files in TypeScript.
      

6. How to make all properties of an interface optional?
   

   By using the partial mapped type, developers can make all properties of an interface optional.
   
   Here is a sample syntax: 
   interface Person {
   name: string;
   age: number;
   }
   type PartialPerson = Partial<Person>; // Same as next lines
   interface PartialPerson {
   name?: string;
   age?: number;
   }
   

7. Where can decorators be applied?
   

   Decorators can be applied by using classes, properties, methods, and method arguments.
   
   Here is an example:
   @MyClassDecorator
   export class Person {
   …
   @MyPropertyDecorator myProperty: string;
   }
   

8. What is the use of the Record?
   

   Record allows developers to create a typed map.
   
   Here is an example:
   let Person = Record<string, number> = {};
   Person.age = 20;
   

9. How can you access classes outside a module?
   

   You can use the export keyword in front of the class to access classes outside a module.
   
   Here is an example:
   export class Person {}
   

10. What are Distributive Conditional Types?
    

    Conditional types or distributive conditional types have a checked type that is a bare type parameter. Distributive conditional types are automatically distributed over union types after instantiation.
    
    For example, an instantiation of T extends U ? X : Y with the type argument A | B | C for T is resolved as (A extends U ? X : Y) | (B extends U ? X : Y) | (C extends U ? X : Y).
    
    Related post: 10 JavaScript Interview Questions and Answers You Must Know

So, how should you start preparing for TypeScript interview questions?

First, understand that TypeScript is one of the best front-end web development tools, as it provides a powerful way of working with JavaScript. 

So, hiring managers are likely to ask about front-end development terms, such as HTML, CSS, frameworks, responsive design, version control, browser developer tools, web performance, user interface, and more. Also, they are likely to deep dive into primary subtopics of TypeScript, such as classes, components, arrays, functions, modules, interfaces, and generic and static typing.  

Read and practice all topics mentioned above. Also, use the above-mentioned questions to revise the fundamentals. You can also explore practical applications and build projects using free online coding environments to practice more. 

If you are interested in applying for a high-paying US job, visit Turing.com. Turing offers long-term remote software jobs with great compensation. 

FAQs

1. What are the object-oriented terms supported by TS?
   TypeScript supports the following object-oriented terms:
   1. Modules
   2. Classes
   3. Interfaces
   4. Inheritance
   5. Data Types
   6. Member functions
      
      
2. What is the difference between JavaScript and TypeScript?
   JavaScript is the web programming language, whereas TypeScript is an object-oriented programming language created. Also, JavaScript is a server-side programming language that helps in creating interactive web pages, whereas TypeScript is an open-source language that helps in creating large-scale web programs.