Top 50 Typescript Interview Questions and Answer for 2021

In this era of web development, JavaScript is essential along with client-side technologies or frameworks such as Gulp, AngularJS, Yeoman, Grunt, etc. An essential part of JavaScript- TypeScript lets you write the code your way and address issues with JavaScript. In this blog, we provide you with a list of the most frequently asked typescript interview questions.

If you plan to have a career as a PHP developer or an Angular developer and wish to know skills related to web development, this is the right time as the technology is blossoming. On the other hand these 50 typescript interview questions will provide you with in-depth knowledge of the concept and help you prepare for your upcoming interviews.

Top Typescript Interview Questions

Let us begin with the top 50 typescript interview questions.

1. Why is TypeScript beneficial?

TypeScript is simple, fast, easy to learn and runs on any JavaScript engine or browser. It has the same semantics and syntax as JavaScript. Also, it helps in fast front-end coding.

2. Give the steps of compiling TypeScript files.

 Any JavaScript file is a TypeScript file. So, to compile any .ts file into .js file, use the following command: tsc <TypeScript File Name>

3. How to declare a variable in TypeScript?

 Four way of variable declaration are:

  1. var[identifier]: [type-annotation]=value
  2. var[identifier]: [type-annotation]
  3. var[identifier]=value
  4. var[identifier]

4. List access modifiers supported by TypeScript.

 The supported access modifiers are:

  1. Protected- all child classes and class members can access them, but the instance cannot.
  2. Private- only accessible by members
  3. Public- accessible by members, child classes and instance of the class.

5. Does TypeScript support function overloading?

 Yes, function overloading is supported, but implementation is odd. On overloading, there is only one implementation but multiple signatures.

6. Tell how you will debug a TypeScript file.

 A .js source map file is needed to debug the file. Compile the .ts file with -sourcemap flag for generating source map file.

Command: $tsc -sourcemap file.ts


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7. What is the purpose of the tsconfig.json file?

tsconfig.json file is a JSON format file in which we can specify different options to tell the compiler how to compile a given project. This file in the directory indicates that the directory is TypeScript's project root.

8. What is meant by Ambient Declarations in TypeScripts?

Ambient declarations tell the compiler about actual source code that exists elsewhere; these files are like doc files. If the ambient declaration file is not updated, they cause compilation errors.

9. Describe ‘as’ syntax in TypeScript.

The ‘as’ is an additional syntax in TypeScript for Type assertion. It was introduced as the original syntax conflicted with JSX. While using JSX with TypeScript, only as-style assertions can be used. Example:

let stdid: any=007

let stdid= id as number;

10. Define Lambda function.

TypeScript provides shorthand syntax for defining function expressions. A lambda function is an anonymous function without a name. For example:

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

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

Here, ?=>? is a lambda operator.

11. Tell about the disadvantages of TypeScript.

TypeScript takes more time to compile code, does not support abstract classes, and requires a compilation step to run the typescript application in the browser.

12. How can you combine multiple .ts files into a single .js file?

Using the outFILE compiling option as follows:

$ tsc --outFile out.js file1.ts file2.ts file3.ts  

The result is stored in a single 'out.js' file.

13. What is an interface with reference to TypeScript?

The interface defines the syntax for classes to follow. A class implementing an interface implements all its members. It can be referenced but not instantiated. TypeScript compiler uses an interface for type-checking. 

14. Tell something about inheritance.

It is a mechanism that acquires behaviours and properties of a class from another class. This allows creating classes from an existing class, and this class is called the derived class.

15. What is meant by Mixins in JavaScript?

Mixins are a way of building classes from reusable components by combining simpler partial classes.

16. What is the Declare Keyword in TypeScript?

As JavaScript does not have TypeScript declaration, to use it in the TypeScript file without any compilation error, declare keyword is used. The keyword is used for ambient methods and declarations to define a variable that exists elsewhere.

17. What are object-oriented principles supported by TypeScript?

All objected-oriented principles – Encapsulation, Inheritance, Polymorphism and Abstraction are supported by TypeScript.

18. What do you understand by TypeScript Map file?

TypeScript Map file is a source map file that holds information about original files. It lets tools map between the TypeScript source and emitted JavaScript code.

19. What is Enum in TypeScript?

Enumerations or Enum are a TypeScript data type that allows defining named constants. Using enums can make it easier to create a set of distant cases. It is a collection of related string or numeric values.

20. What is method overriding?

If the child class has the same methods as declared in parent class, this is called method overriding. The method must have the same name and same parameters as the parent class.

21. What are the built-in types in TypeScript?

The primitive types are Number type, string type, Boolean type, null type and undefined type. The syntax for these are as follows:

var name_of_variable: type/ var name_of_variable: type= null/ var name_of_variable: type=undefined.

22. What is the minimum requirement of TypeScript installation?

TypeScript is installed with the help of npm and node. npm installs all tools and libraries.

23. Does Native JavaScript support modules?

Modules are not supported by Native JavaScript currently. We require an external common to create and work with modules.

24. What are decorators in TypeScript? 

Decorators are functions that support modifying or annotating classes. It is an experimental feature of TypeScript. Decorators can be enabled by editing tsconfig.json or using the command line.

25. Define an implicit module.

In TypeScript, the module is used to create related functions, variables, interfaces and classes. Implicit and Explicit are categories of TypeScript.

26. What are some of the features of classes in TypeScript?

TypeScript classes impart benefits of object-oriented techniques like abstraction and encapsulation. The classes in TypeScript work across browsers and platforms by the TypeScript compiler.

27. How can we convert a string to a number with TypeScript?

This can be done by using parseInt(), parseFloat(), Number(‘’) or by using unary operator like * and +.

28. Give the use of the 'Record' type.

Record lets us create a typed ‘map’. For example:

let student= Record<number,string>= {};


29. What is meant by contextual typing?

When there is type on one side of the equation but not on the other, the TypeScript compiler finds out the form. We can omit typings on the right side as TypeScript can work it out automatically. This decreases the amount of work needed to type code.

30. What is JSX?

It is an XML like syntax that can be embedded. It needs to be translated into valid TypeScript. We use the JSX file with .tsx extension.

31. How will you create a TypeScript object?

An object stores a set of key-value pairs. The syntax is as follows:

var name_of_obj= { key1: “val1”, key 2: ‘val2”, key3: function()}

32. Define static typing.

Static typing refers to a compiler having variables, parameters and object members that are recognizable at the compilation time. This helps in detecting early errors. Static typing is an important concept that supports many TypeScript Interview questions.

33. How is a typescript function called?

TypeScript is compiled into JavaScript using a JS function. The code of structure changes a bit during the process. If there are multiple modules, they become a part of the path calling the specific function.

34. Define super keyword.

Super keyword is a TypeScript keyword used by developers in expressions for base class properties reference and base class constructor. Only derived class constructors can make this call. 

35. What are export and import keyword in TypeScript?

The export keyword is used to export any variable, function, and type alias. Import keyword is used to export declaration. Example: export * from 'module.'

36. What is an optimal parameter in TypeScript?

The optimal parameter is used to declare optional parameters in a function. A parameter can be marked optional by using '?' at the end of it. Example: par 2 is an optimal parameter.

function functionName(par1: string, par2?: string) {


37. What is a Type of assertion?

Like a typecast in other languages, a type assertion performs no special checking. It is used purely by the compiler and has no runtime impact. However, TypeScript assumes that necessary special checks have been performed.

38. Is TypeScript an Optionally Statically Typed Language?

This is because the compiler ignores the variable type optionally. We can assign any value using any data type to the variable, and there is no error checking during compilation.

39. Tell us about TypeScript Definition Manager.

To work with external libraries in TypeScript, TypeScript definition files are needed. TypeScript Definition Manager is a package manager to install these definition files directly from a community-driven repository.

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40. What is a type in TypeScript?

The type represents the value's type that is being used in our programs. It supports the simplest unit of data such as strings, numbers, and additional types too.

41. Can TypeScript be used for the backend?

TypeScript can be generally used for backend applications by using Node.js and having additional safety that the language brings.

42. 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.

43. What are getters/setters?

Getters/setters intercept access to a member of the object. They give us a way of having finer-grained control over how a member accesses each object.

44. Name different components of TypeScript.

Language- the most critical part, compiler- open-source and cross-platform, language service- VS Code, Visual Studio, Sublime, etc.

45. Do TypeScript files need to be combined?

TypeScript is a language extension that the browser cannot interpret. Thus, it needs to be converted to JavaScript. This is called compiling.

46. How can a base class constructor be called from child class?

It can be called using super().

47. In which case should we use the interface and when should classes be used?

If you wish to create an instance of a custom object with the benefits of type checking, the class makes sense. If interfaces are not created, we have interfaces at our disposal, allowing us to virtually type-check code.

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

The const keyword cannot be used to declare class properties. A class member cannot have the 'const’ keyword.

49. Name the object-oriented terms supported by TypeScript.

The supported terms are modules, classes, data types, interfaces and member functions.

50. Can classes in TypeScript be used in the same place as interfaces?

Yes, as classes create types, they can be used in the same place as the interfaces.

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We have reached the end of the TypeScript interview questions article. We hope these questions will help you in your journey of becoming a full-stack developer.

Check out our Post Graduate Program in Full Stack Web Development, in collaboration with Caltech University, that offers live training and real-life project experience, making you proficient in skills with front-end and back-end web technologies, including jQuery, NodeJS, Angular, MongoDB, and so much more. Add another star to your resume along with preparing with these top 50 TypeScript interview questions. Start learning now!

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