30 Best TypeScript Interview Questions

Are you prepping for a career in Web Development? This is a fast-rising field with a lot of prospects.

A lot of interviews for web developers include questions about JavaScript. With technologies like AngularJs, Grunt, and Gulp for better user experience and client-side frameworks, it is important to have good knowledge about the subject.

If you are preparing yourself for a career in Web Development and JavaScript, you’ll need to gain an understanding of TypeScript. It is a crucial part of JavaScript and allows you to code easily. It also addresses a lot of JavaScript issues.

These TypeScript interview questions and answers are common in interviews and will help you understand the concepts as well as the kind of questions you are in store for.

Dive into these top TypeScript interview questions right now, and prepare for the future!

1. What is TypeScript?

TypeScript is a free and open-source programming language. It was designed and developed by Microsoft. It is accepted as a superset of JavaScript.

Compiling Typescript with the tsc <filename>.ts command gives clean and simple JavaScript code for any browser. It allows the use of development tools and practices and is thus preferred by developers. It is programmed in C# or Java and is written with a .ts extension.

2. How is TypeScript different from JavaScript?

  • TypeScript is an Object-oriented language, unlike javascript, which is a scripting language.
  • Typescript supports static typing and optional parameter function while JavaScript does not.
  • Typescript also gives support for modules, unlike JavaScript.
  • TypeScript supports interfaces and Es6, while JavaScript does not.
  • Javascript uses functions and prototype-based inheritance, thereby building up reusable components. TypeScript, on the other hand, allows the programmer to use classes.

3. What are some of the features of TypeScript?

Some of the main features of TypeScript are:

  • Can be compiled in all JavaScript versions
  • Used for cross-browser development
  • It is an Open Source project
  • Superset of JavaScript
  • TypeScript 2.6 covers error suppression comments
  • Easy to debug and predictable

4. Define and describe “namespace” in TypeScript.

Namespace groups different functionality logically. This ensures the maintenance of legacy typescript code internally. It also supports the encapsulation of features and objects that share certain relationships. They are known as internal modules and include classes, variables, and interfaces to support a group of functionalities that are related.

5. List the components of TypeScript.

TypeScript components can be divided into three:

  • Language –  comprises of the keywords, syntax, and type annotations.
  • Compiler – converts instructions from TypeScript to its equivalent in JavaScript.
  • Language Service – supports the typical editor operations. This exposes a layer around the core compiler pipeline, namely the editor applications.

6. What are TypeScript decorators?

Decorators are TypeScript declarations that can be applied to classes, methods, accessors, property, or parameters. They are prefixed with @expression, where the expression is evaluated to a function that is called at runtime. This gives information about the decorator declaration. They are used to add annotation and metadata.

7. What are TypeScript Variables, and how are they created?

TypeScript Variables are named spaces in memory. They store a particular value. Variables have to be declared before they can be used.

8. Explain interfaces with reference to TypeScript

Interfaces are a contract to define the syntax that will be used for an entity. TypeScript interfaces define properties, methods, events, and members. Only the declaration of these is present. Interfaces are responsible for deriving classes and give a standard structure for these classes. They use the keyword “interface.”

9. How do you create a TypeScript variable?

  • Variables are declared by using the var keyword before the variable name.
  • They should contain alphabets and/or numeric digits.
  • It cannot contain spaces or special characters other than ‘_’ and ‘$.’
  • It cannot begin with a digit.

You can declare type and value, type but no value, value without type, or neither.

10. What are the internal modules in Typescript?

Internal modules group classes, interfaces, functions, and variables into a single unit. This can then be used to export them to a different module. They are local or exported members of other modules, declared using ModuleDeclarations that specify their name and body.

11. Is function overloading supported by TypeScript?

Yes, function overloading is supported by TypeScript. You only have one implementation, but with different signatures.

12. What are the built-in types present in TypeScript

The primitive types of TypeScript are of 5 kinds –

  • Number
    • Represents number tuple values, double-precision floating points
  • String
    • The sequence of characters generally stored as Unicode UTF-16, the same as the JavaScript primitive type.
  • Boolean
    • Used to represent a logical value. The output is either true or false.
  • Null
    • Represents a null literal and impossible to directly refer to the null type value itself.
  • Undefined
    • A subtype of all types, undefined literal.

13. How is a TypeScript file compiled?

A typeScript file can be used to write complex yet manageable code in AppBuilder. These are the steps to compile it –

  • TypeScript engine has to be enabled. Click your name on the title bar and select options.
  • Right-click on the TS file being compiled in the project navigator.
  • Choose compile to JavaScript.
  • A <script> reference for the JS file has to be added in HTML code, if needed.
  • Via command line, type tsc <TypeScript filename> to compile.

14. Describe TypeScript mixins

Mixins are a way to build up classes from components that can be reused. This is done by combining simple partial classes.

15. Define duck-typing

Duck typing allows you to check the type compatibility in case of complex variable types. This checks the type match between objects, and a compile-time error occurs if the objects contain more or less number of properties. 

16. What are TypeScript optional properties?

The interface has many properties, but not every property is required. You can pass the object into the interface using the ( ? ) question mark symbol.

17. What do TypeScript type assertions do?

TypeScript uses assertions to typecast. It does have runtime support but does not have any impact on runtime. Unlike C# or Java, there is no type checking or restructuring involved in this case. It just gives a hint to the compiler as to how their code should be analyzed.

18. Can multiple TypeScript files be compiled into a single file?

Yes, multiple TypeScript files can be compiled into a single file.

19. What are external modules in TypeScript?

External modules hide the internal statements of module definitions. The show the methods and parameters that are associated with the variable declared. They are loaded separately and are referenced with external module names.

The external module is written as a source file separately. It contains at least one export or import declaration.

20. Predict the output of the below code:

let color: string[] = [“green”,”blue”,”red”];

console.log(color.pop());

Output – Red

21. What are non-relative module imports?

A non-relative import has to be resolved in relation to the baseUrl, or via path mapping. This is used while importing from external dependencies.

22. What is the role of a tsconfig.json file in TypeScript?

The tsconfig.json is a JSON file where we specify options as to how the current project has to be compiled.

23. Explain TypeScript generics

Generics provides an option to create reusable components. These can work with a variety of data types without any blow to the performance or productivity.

24. How do we check for null and undefined in TypeScript?

We can use a juggling check for null and undefined.

if (x == null) { 

25. What are the features of an interface statement?

An interface declaration introduces a named object and can have many merged declarations. It creates a new name used everywhere, and it can be named in an extends or implements clause.

26. What are type statements?

Type alias declaration introduces a name for any type. This includes primitive, union, and intersection. It cannot be named in extends or implements, and they do not create new names.

27. Define TypeScript Map files

A TypeScript Map file acts as a source file with information about original files.

28. Define rest parameters

Rest parameters are used when you have to pass zero or more values to a function. The parameter in question has to be prefixed by the three-dot characters (‘…’) while declaring. It makes sure the function can have a variable number of arguments instead of the arguments object.

 Only one rest parameter can be used in one function, and it must be of an array type. It must also be the last parameter in the list.

E.g

function product(…b: number[ ]){

let result=1;

for (var i = 0; i < b.length; i++) {   

result *= b[i]; 

}

console.log(result);

}

let result1=product(2,3) //Output 6

Let result2=product(2,3,4)//Output 24

29. What is relative module imports?

A relative import is used in the case of our own modules. This means they are used for modules that will definitely maintain their relative location during runtime. It starts with /, ./ or ../.

30. What is the use of the lambda or arrow function?

The Es6 version of TypeScript provides an opportunity for shorthand syntax. This is used to define anonymous functions. The Lambda function is a nameless function, while the arrow function excludes the function keyword.

E.g:

let product= (a: number, b: number): number => {return a * b;}

console.log(sum(2,3); //returns 6

?=>? is a lambda operator, and (a * b) is the function body

These TypeScript interview questions will help you brush up on the major concepts and breeze through your interview!

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