Top 50 TypeScript Interview Questions With Answers for 2024

Beginner Level TypeScript Interview Questions

Here is a list of all possible TypeScript interview questions asked to freshers and their answers.

1. What is TypeScript?

• TypeScript is a superset of JavaScript. It is an object-oriented and tightly typed programming language. TypeScript code is transformed to JavaScript, which may be used in any environment that supports JavaScript, including browsers, Node.js, and your own applications.
• Anders Hejlsberg of MICROSOFT created TypeScript. TypeScript was created in response to the limitations of JavaScript for constructing large-scale applications within Microsoft and among its external customers. There was a demand for bespoke tooling to make developing JavaScript components easier due to the complexity of working with complicated JavaScript code.
• TypeScript is a variant of JavaScript with a few more features. TypeScript extends JavaScript with extra syntax to provide a more robust interface with your editor. TypeScript is a scripting language that is compatible with JavaScript and relies on type inference to deliver advanced functionality without the need for additional code.

2. Explain Arrays in TypeScript

A collection of values of the same data type is called an array. It's a kind that's been defined by the user. To store values of the same kind, you use arrays. Arrays are collections of values that are ordered and indexed. The indexing begins at zero, with the first element having index 0, the second having index 1, and so on.

Syntax: 

var array_name[:datatype];        //declaration 

array_name = [val1,val2,valn..]   //initialization

Example:

let values: number[] = [];

values[0] = 10;

values[1] = 20;

values[2] = 30;

3. List the Applications of TypeScript

• Both client-side and server-side JavaScript applications can be built with TypeScript.
• TypeScript is used to create both client-side and server-side JavaScript applications.
• Because TypeScript adds more functionality and provides errors directly in the code, it can be used instead of JavaScript.
• TypeScript is a programming language that is used to create large-scale enterprise systems.

4. List the Advantages of TypeScript

• Problems are highlighted throughout development and at compilation time.
• Typescript can be run in any browser or JavaScript engine.
• A namespace notion is created by declaring a module.
• IntelliSense is a TypeScript feature that provides active hints as you type.
• Strongly typed or static typing is supported. The advantage of TypeScript is that it is strictly typed or allows for static typing. Because of static typing, it may confirm type correctness at compilation time.

5. List the disadvantages of TypeScript

• It takes a long time to compile TypeScript code.
• Abstract classes are not supported in TypeScript.
• Converting TypeScript to JavaScript requires an additional compilation step.
• Its type scheme is extremely complicated.

Want a Top Software Development Job? Start Here!

Full Stack Developer - MERN StackExplore Program

6. Explain the features of Arrays in TypeScript

• An array declaration allocates memory blocks in a sequential order.
• Arrays are immutable. This means that once an array is created, it cannot be resized.
• An array element is represented by each memory block.
• The subscript/index of an array element is a unique number that identifies the element.
• Arrays, like variables, should be declared before being used. Declare an array with the var keyword.
• The term "array initialization" refers to the process of filling an array with elements.
• The values of array elements can be updated or modified, but they cannot be erased.

7. How to declare a variable in TypeScript?

• Let and const are the two methods to declare variables.
• Alphabets and numeric digits can both be used in variable names.
• Except for the underscore (_) and the dollar ($) sign, they cannot contain spaces or special characters.
• A digit can’t be the first character in a variable name.

let var a=10;

 or

function f() {

  var message = "Hello, world!";

  return message;

}

8. Name the access modifiers supported in TypeScript.

 The access modifiers supported by TypeScript are:

• Protected- All child classes and members have access to them, but the instance does not.
• Private- Only members have access
• Public- Members, child classes, and instances of the class have access to the public modifier.

9. Explain Loops in Typescript

A loop statement allows to repeat the execution of a statement or a series of statements. To handle looping requirements, TypeScript provides a variety of loops.

• For Loop

The for loop is a definite loop implementation. A definite loop is defined as a loop with a fixed number of iterations.

• While Loop

When the condition supplied evaluates to true, the while loop executes the instructions.

• Do..While Loop

The do...while loop is identical to the while loop, except that the condition isn't evaluated the first time the loop runs.

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

The tsconfig.json file is a JSON format file where you may specify several options to inform the compiler how to compile a project. The presence of this file in the directory implies that it is the TypeScript project root.

11. How to convert a string to a number in TypeScript?

You can convert a string to a number by using parseInt(), parseFloat() and Number(‘’) Method.

Prepare Yourself to Answer All Questions!

Automation Testing Masters ProgramExplore Program

12. What is meant by contextual typing?

When one side of the equation has type but the other does not, the TypeScript compiler determines the form. You can skip typing on the right side because TypeScript will figure it out for you. This reduces the amount of time spent typing code.

13. What is JSX?

It's a syntax that's similar to XML and can be embedded. It must be converted into TypeScript that is valid. The JSX file with the.tsx extension is used.

JSX has an XML-like syntax that can be embedded. It is intended to be turned into legitimate JavaScript, though the semantics of that transformation will vary depending on the implementation. TypeScript allows you to embed JSX, type verify it, and compile it to JavaScript immediately.

14. Define static typing

Static typing refers to a compiler that has recognisable variables, arguments, and object members at compile time. This aids in the early detection of faults. 

15. What are import and export keywords in TypeScript?

• Import keyword is used to export declaration. Example: export * from module
• Any variable, function, or type alias can be exported with the export keyword

16. Can TypeScript be used for the backend?

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

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

if(value) return true if value is not null, undefined, empty, false, 0 or NaN.

18. What are getters/setters?

Getters and setters prevent access to an object's members.They allow you to have more precise control over how a member interacts with each object. A getter method starts with keyword ‘get’ and setter method starts with keyword ‘set’

19. How can a class constant be implemented in TypeScript?

Class properties cannot be declared with the const keyword. The keyword 'const' cannot be used in a class member.

20. What is the Declare Keyword in TypeScript?

Since JavaScript lacks a TypeScript declaration, the declare keyword is used to include it in a TypeScript file without causing a compilation issue. Ambient methods and declarations use the term to define a variable that already exists.

21. What is an Interface with reference to TypeScript?

The interface specifies the syntax that classes must use. All of the members of an interface are implemented by a class that implements it. It's possible to refer to it, but not to use it. A type-checking interface is used by the TypeScript compiler.

22. Describe ‘as’ syntax in TypeScript.

In TypeScript, the 'as' syntax is used for Type assertion. It was created because the original syntax was incompatible with JSX. Only as-style assertions can be used with JSX and TypeScript.

Example:

let stdid: any=007

let stdid= id as number;

23. Define Lambda function.

For defining function expressions, TypeScript provides a shortcut syntax. A lambda function is an unnamed anonymous function. 

Example:

let sum=(a: num, b: num): num=>{ return a+b;}

console.log(sum(5,10)); //returns 15

Here, ?=>? is a lambda operator.

24. How to create objects in Typescript?

Objects are collections of keys and values that resemble a dictionary. The keys must be one-of-a-kind. They resemble arrays and are sometimes referred to as associative arrays. An array, on the other hand, employs numbers to index the values, whereas an object lets you use any type as the key.

An Object type in TypeScript refers to any value with properties. It can be defined simply by specifying the properties and the kinds of those properties. As an example,

let pt: { x: number; y: number } = {

  x: 10,

  y: 20

};

25. Explain Different Data Types in Typescript?

Learn 15+ In-Demand Tools and Skills!

Automation Testing Masters ProgramExplore Program

26. What is meant by Type Inference?

When you don't specify an explicit type for a variable, TypeScript can infer it. Type inference is the term for this. This is normally done during the declaration, when the variables or parameters are initialized.

TypeScript recognises that the variable koo is a string, even if you don't mention string as a type.

let koo = "Hello world";

console.log(typeof koo);  // "string"

27. Explain Tuples in Typescript With Example

Tuples are a collection of values that are diverse. It allows for the storage of many fields of various sorts. Tuples can also be used as function parameters.

There are instances when it is necessary to save a collection of values of various types. Arrays will not suffice in this situation. TypeScript provides a data type called tuple that aids in this endeavor.

Syntax:

var tuple_name = [value c,value b,value c,…value n]

For Example:

var yourtuple = [12,"Hi"];

Intermediate Level TypeScript Interview Questions

After seeing the beginner level TypeScript Interview questions, look at the Intermediate level TypeScript Interview Questions.

28. What is Anonymous Function in TypeScript?

Anonymous functions are those that have no identifier (function name) attached to them. At runtime, these functions are dynamically declared. Anonymous functions, like normal functions, can accept inputs and return outputs. After its initial creation, an anonymous function is normally inaccessible. An anonymous function can be assigned to variables. 

Syntax: 

var res = function( [arguments] ) { ... }

Example:

var msg = function() { 

   return "hello world";  

} 

console.log(msg())

29. How to Install Typescript?

TypeScript is a JavaScript programming language for use in applications. TypeScript extends JavaScript with optional types that support tools for large-scale JavaScript applications in any browser, on any host, and on any operating system. TypeScript compiles to legible JavaScript that adheres to industry standards. 

You can install TypeScript globally with npm, which means you can run the tsc command from anywhere in your terminal.

For the latest stable version:

npm install -g typescript

30. What are Decorators?

The Decorator is a type of declaration that is used to decorate a class declaration, method, accessor, property, or argument. Decorators take the form @expression, where expression must evaluate to a function that will be called with information about the decorated declaration when it is called at runtime.

Example: 

function color(value: string) {

 // this is the decorator factory, it sets up

 // the returned decorator function

 return function (target) {

   // this is the decorator

   // do something with 'target' and 'value'...

 };

}

31. What are Mixins?

Combining simpler partial classes is a popular approach of constructing classes from reusable components. For languages like Scala, you may be familiar with the concept of mixins or characteristics.

To extend a base class, the design relies on generics and class inheritance. The finest mixin support in TypeScript is provided through the class expression pattern.

We have a class where mixins are applied on top of it.

class Sprite {

 name = "";

 x = 0;

 y = 0;

 constructor(name: string) {

   this.name = name;

 }

}

Unleash a High-paying Automation Testing Job!

Automation Testing Masters ProgramExplore Program

32. Explain Classes in TypeScript.

In terms of OOPs, a class is a template for producing objects. Object-oriented programming elements like as classes, interfaces, polymorphism, and data binding are all supported by TypeScript. The term "object" refers to a physical entity. Classes were not supported in JavaScript ES5 or earlier. This is a feature that Typescript inherits from ES6.

A class is a collection of items with similar characteristics. Fields, methods, constructors, Blocks, Nested classes, and interfaces are all included in the class.

Syntax to declare a class:

class class_Name{    

    field;    

    method;    

} 

33. What is Namespace and how to declare it?

The namespace is used to group functionalities logically. To enable a single or a group of linked functionalities, a namespace can include interfaces, classes, functions, and variables.

The namespace keyword, followed by the namespace name, can be used to construct a namespace. Curly brackets can be used to define all interfaces, classes, and other objects.

Syntax: 

namespace <name>

{   

}

34. What are Rest Parameters in Typescript?

• You can use rest parameters when the number of parameters that a function will get is unknown or varies.
• The rest parameter can take zero or more parameters. The compiler will generate an array of arguments containing the name of the rest parameter.

let Greet = (greeting: string, ...names: string[]) => {

  return greeting + " " + names.join(", ") + "!";

}

Greet("Hi!", "John", "Sam"); // returns "Hi John, Sam"

Greet("Hi!");// returns "Hi !

35. What are Objects in TypeScript?

An object is a type of instance that consists of a collection of key-value pairs. Scalar values, functions, and even arrays of other objects can be used as values.

Syntax:

var object_name = { 

   key1: “value”, //scalar value 

   key2: “value”,  

   key3: function() {

      //functions 

   }, 

   key4:[“contentA”, “contentB”] //collection  

};

36. Explain Type Aliases 

Type aliases give a type a new name. Type aliases are similar to interfaces in that they can be used to name primitives, unions, tuples, and any other kinds that you'd have to define by hand otherwise.

Aliasing doesn't truly create a new type; instead, it gives that type a new name. Aliasing a primitive isn't very useful, however it can be used for documentation purposes.

Type aliases, like interfaces, can be general; all you have to do is add type parameters and utilise them on the right side of the alias declaration.

type Container<T> = { value: T }; 

37. Explain Modules in TypeScript

A module is created with the intention of organizing TypeScript code. Modules are classified as follows:

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. In the most recent version of TypeScript, this logical grouping is referred to as namespace.

External Modules: External modules allow you to describe and load dependencies between many external js files in TypeScript.

38. What are Mapped Types ?

Taking an existing type and making each of its properties optional is a typical undertaking.

Because this happens frequently enough in JavaScript, TypeScript has a feature called mapped types that allows you to define new types based on existing ones. The new type turns each property in the old type in the same way into a mapped type. You can, for example, make all properties optional or of the readonly type. 

It's important to note that this syntax refers to a type rather than a member. You can use an intersection type to add more members:

Now, take a look at the simple mapped type and its parts:

type Keys = "option1" | "option2";

type Flags = { [K in Keys]: boolean };

The syntax is similar to that of index signatures with a for.. in the middle. There are three sections in total:

The type variable K is assigned to each property one by one.

The literal union of strings The names of the properties to iterate over are stored in keys.

The property's type as a result.

Want a Top Software Development Job? Start Here!

Full Stack Developer - MERN StackExplore Program

39. What are Conditional Types in TypeScript ?

Based on a condition given as a type relationship test, a conditional type chooses one of two alternative types:

T extends U ? X : Y

When T can be assigned to U, the type is X, and when it can't, the type is Y.

Because the condition depends on one or more type variables, a conditional type T extends U? X: Y and is either resolved to X or Y or delayed. Whether to resolve to X or Y, or to defer, when T or U contains type variables is determined by whether the type system has enough information to conclude that T is always assignable to U.

40. What are Distributive Conditional Types?

Distributive conditional types are conditional types in which the checked type is a bare type parameter. During instantiation, distributive conditional types are automatically distributed over union types.

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).

41. Explain how TypeScript files can be supported from Node Modules

TypeScript includes a series of declaration files to guarantee that TypeScript and JavaScript support works well right out of the box (.d.ts files). The various APIs in the JavaScript language, as well as the standard browser DOM APIs, are represented in these declaration files. While there are some fair defaults based on your target, you can configure the lib setting in the tsconfig.json to specify which declaration files your program uses.

TypeScript has a feature similar to @types/ support that allows you to override a specific built-in lib. TypeScript will check for a scoped @typescript/lib-* package in node modules when selecting which lib files to include. After that, you can use your package manager to install a specific package to take over for a particular library.

42. Explain the Components of Typescript

Internally, the TypeScript language is separated into three layers. Each of these layers is further subdivided into components. These layers are as follows:

• Language
• The TypeScript Compiler 
• Language Services for TypeScript

Language: It is written in TypeScript and includes TypeScript language components. It includes syntax, keywords, and type annotations.

TypeScript Compiler (TSC): The TypeScript compiler (TSC) converts TypeScript programmes into JavaScript code. It also converts your TypeScript code to JavaScript code, parsing and type-checking it.

The TypeScript Language Services:  Provides information that allows editors and other tools to deliver greater assistance capabilities like automated refactoring and IntelliSense. It adds a layer of abstraction to the core-compiler pipeline. It allows you to do things like code formatting and outlining, colorization, statement completion, signature help, and so on.

43. What are Object-Oriented Principles supported by TypeScript?

The object-oriented principles supported by TypeScript are Encapsulation, Inheritance, Polymorphism and Abstraction 

Encapsulation is a major component of Object Oriented Programming, and it is a method of structuring code so that each block of code has its own set of access points for external code.

TypeScript makes creating an object model and inheritance chain a breeze. To construct classes, simply use the standard class keyword. The extended keyword causes the stated base class to be inherited by the child class.

Polymorphism occurs when many classes inherit from the same parent and override the same functionality. Each of those kid classes is now responsible for implementing a property or method, but they may do so in their own unique way.

Abstraction is a method of modelling objects in a system that separates the responsibilities of the class or type from the code that inherits it.

44. Explain the Drawbacks of using Declaration Files with Typescript

There are two drawbacks to using these declaration files with TypeScript:

Since you upgrade TypeScript, you must also deal with changes to TypeScript's built-in declaration files, which can be difficult when the DOM APIs change so regularly.

Customizing these files to meet your needs and the demands of your project's dependencies is difficult (e.g. if your dependencies declare that they use the DOM APIs, you might also be forced into using the DOM APIs).

Advanced Level TypeScript Interview Questions

So those were all Intermediate level TypeScript Interview Questions, now look at the Advanced level TypeScript Interview Questions

45. How to compile Typescript with Visual Studio Code?

• Visual Studio Code includes TypeScript language support but does not include the TypeScript compiler.
• You need to install the TypeScript compiler either globally or in your workspace to transpile TypeScript source code to JavaScript
• The easiest way to install TypeScript is through npm, the Node.js Package Manager.       If you have npm installed, you can install TypeScript globally (-g) on your computer by:

npm install -g typescript

• You can test your install by checking the version or help.

tsc --version

46.  What are the Recent Advancements in TypeScript?

• TypeScript 4.2 has been released, and it includes more flexible type annotations, tougher checks, additional configuration choices, and a few breaking changes.
• Rest arguments can now be placed anywhere in a triple type. In type error messages, type aliases are no longer enlarged, resulting in a better developer experience.
• TypeScript 4.2 brings the language one step closer to its aim of accurately typing JavaScript at any size, anywhere JavaScript is used.

47. Explain the Awaited Type and Promise Improvements

The Awaited type is a new utility type introduced in TypeScript 4.5. This type is intended to represent activities such as await in async functions and the.then() method on Promises - notably, the way they recursively unwrap Promises.

// A = string

type A = Awaited<Promise<string>>;

// B = number

type B = Awaited<Promise<Promise<number>>>;

// C = boolean | number

type C = Awaited<boolean | Promise<number>>;

Existing APIs, such as JavaScript built-ins like Promise.all, Promise.race, and others, can benefit from the Awaited type. In fact, some of Promise.all's inference concerns provided a foundation for Awaited.

Promise.all combines certain traits with Awaited to produce far superior inference results.

Want a Top Software Development Job? Start Here!

Full Stack Developer - MERN StackExplore Program

48. Explain how TypeScript files can be supported from Node Modules

TypeScript includes a series of declaration files to guarantee that TypeScript and JavaScript support works well right out of the box (.d.ts files). The various APIs in the JavaScript language, as well as the standard browser DOM APIs, are represented in these declaration files. While there are some fair defaults based on your target, you can configure the lib setting in the tsconfig.json to specify which declaration files your program uses.

TypeScript has a feature similar to @types/ support that allows you to override a specific built-in lib. TypeScript will check for a scoped @typescript/lib-* package in node modules when selecting which lib files to include. After that, you can use your package manager to install a specific package to take over for a particular library.

49. What is Type Assertion? Explain its types 

You can find yourself in a scenario where you know a type for an entity that is more specific than its present type.

A type assertion is similar to a type cast in other languages, but it does not do any additional data verification or restructuring. It has no effect on runtime and is only used by the compiler. TypeScript expects that you, the programmer, have completed any necessary specific checks.

There are two types of type assertions.

One is the as-syntax:

let someValue: unknown = "this is a string";

let strLength: number = (someValue as string).length;

The other version is the “angle-bracket” syntax:

let someValue: unknown = "this is a string";

let strLength: number = (<string>someValue).length;

Both samples are identical. selecting one over the other is basically a matter of preference; however, only as-style assertions are allowed when combining TypeScript with JSX. 

50. What is Recursive Type Aliases?

The ability to "recursively" reference type aliases has always been limited. The reason for this is because each type alias must be capable of substituting itself for whatever it aliases. Because this isn't always possible, the compiler rejects some recursive aliases.

Interfaces can be recursive, but their expressiveness is limited, and type aliases cannot. That involves combining the two: creating a type alias and extracting the type's recursive portions into interfaces. It's effective.

type ValueOrArray<T> = T | ArrayOfValueOrArray<T>;

interface ArrayOfValueOrArray<T> extends Array<ValueOrArray<T>> {}

Related Software Developer Interview Guides

Next Steps

In this tutorial, you saw Beginner Level TypeScript Interview Questions, Intermediate Level TypeScript Interview Questions, and Advance Level TypeScript Interview Questions. Hope this tutorial has helped you understand the types of TypeScript Interview Questions asked and how to respond to them. 

However, if you want to learn TypeScript and possibly make a career out of it, certification will be helpful. if you study these programming languages and work as a developer or programmer. Learn typescript by enrolling in the  Full Stack Developer - MERN Stack.

Do you have any queries regarding TypeScript Interview Questions that you'd want to ask us? Please post any questions or comments on the TypeScript Interview Questions in the comments area. Our professionals will respond as quickly as possible!