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Top TypeScript Interview Questions (2023)

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Introduction

Websites are an essential part of our day to day life, and you are reading this article on one of them. TypeScript is a popular language that is widely used in the development of websites. As the demand for TypeScript developers increases, industries are searching for candidates with the right knowledge and expertise in the language.

typescript interview questions

This article discusses the typescript interview questions. These questions are frequently asked in many Typescript developers' interviews. If you are a Typescript developer and are looking for an opportunity in this domain. You should go through this article and assess your knowledge of Typescript. So without any further ado, let’s get started. 

Beginner-Level Typescript Interview Questions 

1. What do you know about TypeScript?

TypeScript is an open-source programming language that is a superset of JavaScript. It adds optional static typing and other features to JavaScript, making it easier to write and maintain large-scale web applications. TypeScript code is compiled into JavaScript and can run on any browser or platform that supports JavaScript.

2. What are the features of Typescript?

Some important features of TypeScript are:

  • Enhanced developer productivity: TypeScript provides developers with features such as static typing, interfaces, and code completion, which can catch errors and enhance developer productivity.
     
  • Better code quality: TypeScript provides static typing, which helps to catch errors before runtime and can lead to better code quality.
     
  • Improved scalability: TypeScript is designed to scale to large applications, making it easier to manage and maintain large codebases.
     
  • Improved tooling: TypeScript works well with modern IDEs and can improve tooling support for development.
     
  • Compatibility with JavaScript: TypeScript is a superset of JavaScript, which means that it can be used alongside existing JavaScript codebases without needing to rewrite everything in a new language.

3. What are arrays in TypeScript?

 In TypeScript, an array is a collection of elements of the same type, which can be stored and accessed using a single variable name. Here's an example of declaring and initialising an array in TypeScript:

let numbers: number[] = [1, 2, 3, 4, 5];


In the example above, we declare an array called "numbers" that can only store values of type "number." We also initialise the array with five numbers.
Arrays in TypeScript can be accessed and manipulated using various methods and properties. Here is an example:

let numbers: number[] = [1, 2, 3, 4, 5];

// Accessing array elements
console.log(numbers[0]); // 1
console.log(numbers[3]); // 4

4. What is the “any” type in Typescript, and when to use it?

In TypeScript, the "any" type is a special type that can be used to represent any type of value, similar to how "var" is used in JavaScript. The "any" type can be used to opt out of type-checking and type inference, allowing a value to be assigned to a variable without the compiler performing any type checks.
Here's an example of using the "any" type:

let myVariable: any = "Hello World";
myVariable = 42;
myVariable = true;


In the example above, we declare a variable called "myVariable" with the "any" type, and we assign it different types of values. Because "myVariable" is of type "any," the TypeScript compiler does not perform any type checks on the assigned values.

5. Write the syntax of a function with the type annotations in typescript.

TypeScript function with type annotations:

function greet(name: string, age: number): string {
return `Hello, ${name}! You are ${age} years old.`;
}


In this function, ‘name’ and ‘age’ are the parameters with type annotations ‘string’ and ‘number’, respectively. The ‘: string’ after the parameter list indicates that the function returns a string.

6. What are the access modifiers supported in the typescript?

TypeScript supports the following access modifiers:

access modifiers
  • public: The member is accessible from anywhere. This is the default access modifier, and members declared as public can be accessed from within the class as well as from outside the class.
     
  • private: The member is accessible only within the same class. Members declared as private cannot be accessed from outside the class.
     
  • protected: The member is accessible within the same class and its subclasses. Members declared as protected cannot be accessed from outside the class and its subclasses.
     
  • read-only: The member can only be read, not written to. The value of the member must be set in the constructor or within the class, but it cannot be changed outside the class.

7. What are loops in Typescript?

In TypeScript, loops are used to repeatedly execute a block of code a specified number of times or until a certain condition is met. The following are the different types of loops in TypeScript:

  • for loop: This is a traditional loop that allows you to repeat a block of code a specified number of times.
     
  • for in loop: This loop is used to iterate over the properties of an object.
     
  • for of loop: This loop is used to iterate over the values of an array or any other iterable object.
     
  • while loop: This loop repeatedly executes a block of code as long as a specified condition is true.
     
  • do while loop: This loop is similar to the while loop, but the code block is executed at least once, even if the condition is false.

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

In TypeScript, you can convert a string to a number using the Number() function or by using the unary plus operator +.

Here's an example using Number():

let str = "123";
let num = Number(str);
console.log(typeof num); // Output: "number"

 

And here's an example using the unary plus operator:

let str = "123";
let num = +str;
console.log(typeof num); // Output: "number"

9. Can you explain the typeof operator?

The typeof operator in TypeScript is used to determine the data type of a variable or an expression at runtime. It returns a string representing the data type. For example, typeof variableName would return "number", "string", "boolean", etc., depending on the variable's type.

10. What is undefined in TypeScript?

In TypeScript, undefined is a value that represents the absence of a value or a variable that has been declared but has not been assigned a value.

Here's an example:

let x: number;
console.log(x); // Output: undefined

 

In the example above, the variable ‘x’ is declared with a type of ‘number’, but it has not been assigned a value, so it has the value of ‘undefined’.

11. What is JSX?

JSX is a syntax extension for JavaScript that allows you to write HTML-like code within your JavaScript code. It was introduced by Facebook as part of their React library and has since become a popular way of writing components for modern web applications. JSX allows you to write code that looks like HTML, but it gets compiled to JavaScript at runtime.

Intermediate-Level Typescript Interview Questions

12. What is contextual typing?

Contextual typing, also known as "type inference", is a feature in TypeScript. Here, the type of a value is automatically determined based on how it is used in the code. The TypeScript compiler can infer the type without explicit declaration, reducing the need for manual annotations and making the code more concise.

13. What is the role of the never type in TypeScript?

The never type in TypeScript is used to represent values that never occur. It's typically used as the return type for functions that always throw exceptions or never return, making it useful for catching unreachable code and ensuring type safety.

14. Can we combine multiple .ts files into a single .js file?

Yes, you can combine multiple .ts files into a single .js file using TypeScript. This process is called "bundling" and can be done using various tools such as Webpack, Rollup, or Parcel.

When you bundle multiple .ts files into a single .js file, the TypeScript compiler compiles each file into JavaScript and then combines the output into a single .js file. This allows you to split your application into multiple modular .ts files, which can make your code easier to maintain and reuse. 

15. Explain how enums work in TypeScript?

Enums in TypeScript are a way to declare a set of named constants. They are used to represent a group of named values that are related to each other. An enum is a collection of constant values that are assigned to a name.

Here's an example of how to define an enum in TypeScript:

enum Color {
  Red,
  Green,
  Blue
}

let backgroundColor = Color.Blue;

 

In this example, we define an enum called ‘Color' with three constant values: ‘Red', ‘Green’, and ‘Blue’. We can then use the ‘Color’ enum in our code by assigning its values to a variable.

16. What are the object-oriented terms supported by TypeScript?

TypeScript supports many of the object-oriented concepts found in other programming languages, including:

object-oriented concepts
  • Classes: TypeScript supports the definition of classes, which are blueprint objects that can be used to create instances of objects with properties and methods.
     
  • Interfaces: Interfaces define a contract that can be implemented by classes. They define the structure and shape of an object, including its properties and methods.
     
  • Inheritance: TypeScript supports inheritance, which allows you to create new classes that inherit the properties and methods of an existing class. This allows you to create reusable code and reduces duplication.
     
  • Polymorphism: Polymorphism is the ability of an object to take on different forms based on the context in which it is used. TypeScript supports polymorphism through the use of interfaces and inheritance.
     
  • Abstraction: Abstraction is the process of hiding the underlying complexity of an object and exposing only the relevant information to the user. TypeScript supports abstraction through the use of classes and interfaces.
     
  • Encapsulation: Encapsulation is the process of wrapping data and behaviour within a single unit or object. TypeScript supports encapsulation through the use of classes and objects.
     
  • Constructors: A constructor is a special type of method that is called when an object is created from a class. TypeScript supports constructors, which allow you to initialise the properties of an object when it is created.

17. What is the lambda function in typescript?

A lambda function, also known as an anonymous function, is a function expression that is not bound to an identifier. In TypeScript, lambda functions are used to write concise and expressive code.

A lambda function has the following syntax:

(parameters) => { statements }

 

Lambda functions are commonly used in TypeScript for callbacks and event handlers, as well as for functional programming constructs such as mapping, filtering, and reducing arrays. They provide a way to write compact, readable code and can simplify your codebase by eliminating the need for named functions in certain scenarios.

18. What are the rest parameters in TypeScript?

Rest parameters are used in the function definition to specify that the function can accept a variable number of arguments. They are defined using three dots ‘...’ followed by a parameter name, which acts as an array that collects all the remaining arguments passed to the function.

Here's an example of how to define a function with a rest parameter in TypeScript:

function printNumbers(first: number, second: number, ...others: number[]) {
  console.log(first);
  console.log(second);
  console.log(others);
}

printNumbers(1, 2, 3, 4, 5);

 

Output:

1
2
[3,4,5]

 

In this example, the ‘printNumbers’ function takes two required parameters, ‘first’ and ‘second’, and a rest parameter ‘others’, which collects all the remaining arguments passed to the function.

19. How can variables be declared in TypeScript using keywords?

Variables in TypeScript can be declared using keywords like let, const, and var. For example, let myVar: number = 42; declare a variable named myVar with the type number.

20. How can I define properties as optional in TypeScript interfaces?

In TypeScript, you can define properties as optional in interfaces by using the "?" syntax. For example, interface MyInterface { prop1: string; prop2?: number; } defines an interface with prop1 as a required property and prop2 as an optional property.

21. Does TypeScript support function overloading?

Yes, TypeScript supports function overloading. Function overloading allows you to create multiple functions with the same name but with different parameter lists. This way, you can handle different types of inputs in a single function and provide a better user experience.

In TypeScript, function overloading is achieved by declaring multiple function signatures with the same name but with different parameters. The number and type determine the correct function to be executed, and the order of the arguments passed to the function at the time of its invocation.

22. What is Type Inference?

Type inference is a feature of TypeScript that allows the compiler to deduce the type of a variable based on the value assigned to it. This allows you to write code with fewer type annotations, making it easier to read and write while still ensuring type safety.

23. What is the class syntax in Typescript?

Classes are one of the core concepts in object-oriented programming (OOP), and TypeScript supports classes just like any other object-oriented programming language.

A TypeScript class is a blueprint for creating objects (instances), providing initial values for state (member variables or properties), and implementations of behaviour (member functions or methods).

Here's an example of how to define a simple class in TypeScript:

class Person {
  name: string;
  age: number;

  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }

  greet() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
  }
}

let alan = new Person("Alankrit Srivastava", 22);
alan.greet();

 

Output:

“Hello, my name is Alankrit Srivastava and I am 22 years old.”

 

In this example, the ‘Person’ class is defined with two properties, ‘name’ and ‘age’, and a constructor method that initialises the properties. The class also has a ‘greet’ method that outputs a greeting message.

24. What are Mixins in Typescript?

Mixins in TypeScript are a pattern that allows you to reuse code between classes by combining properties and methods from multiple classes into a single class. This makes it easier to build reusable and extendable classes while avoiding redundant code and making it easier to maintain and update. Mixins are typically implemented using a function that extends an existing class with properties and methods from one or more other classes.

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Advanced-Level Typescript Interview Questions

25. What are mapped types in Typescript?

Mapped types in TypeScript allow you to transform one type into another by mapping over its properties. They provide a way to create new types based on an existing type, with each property being transformed according to a given set of rules.

The syntax for a mapped type in TypeScript is:

{ [P in K]: T }

 

Where ‘P’ is the property name, ‘K’ is a union of keys in the original type, and ‘T’ is the transformed type of the property. The ‘in’ keyword is used to iterate over the keys in the union.

26. What is parameter destructuring in Typescript?

 Parameter destructuring in TypeScript is a technique that allows you to destructure objects or arrays passed as function parameters and assign their properties or elements to variables with specific names. It is similar to parameter destructuring in JavaScript but provides additional features such as type annotations.

Here's an example of parameter destructuring in TypeScript:

interface Person {
  name: string;
  age: number;
  address: string;
}

function printPersonInfo({ name, age }: Person) {
  console.log(`Name: ${name}, Age: ${age}`);
}

const person: Person = { name: Sherlock, age: 34, address: '221 B Bakers Street.' };
printPersonInfo(person);

 

In this example, we define an interface ‘Person’ with three properties: ‘name’, ‘age’, and ‘address’. We then define a function ‘printPersonInfo’ that accepts a single parameter of type ‘Person’ and uses destructuring to extract the ‘name’ and ‘age’ properties into separate variables. Finally, we create a ‘Person’ object and pass it to the ‘printPersonInfo’ function. 

27. What is the significance of the tsconfig.json file in TypeScript development?

The tsconfig.json file is a configuration file used in TypeScript development. It specifies compiler options and settings for a TypeScript project, making it easier to manage and build TypeScript code. It helps enforce coding standards, manage module resolution, and streamline the TypeScript development process.

28. What is TypeScript Definition Manager, and why do we need it?

TypeScript Definition Manager (TSD) is a package manager that was used in the early days of TypeScript to manage external type definitions for JavaScript libraries and frameworks. It allowed TypeScript developers to easily download and install type definitions for popular JavaScript libraries such as jQuery, React, and AngularJS and use them in their TypeScript projects.

TSD has since been deprecated in favour of the newer and more powerful DefinitelyTyped project, which is now the official repository for TypeScript type definitions. DefinitelyTyped provides a wide range of high-quality type definitions for thousands of popular JavaScript libraries and frameworks and is actively maintained and updated by a large community of contributors.

29. What is Type Assertion?

Type assertion in TypeScript is a way to tell the TypeScript compiler the specific type of a value, even when the compiler cannot infer it automatically. It allows you to treat an object as if it were of a different type without actually changing the object's underlying type.

type assertion

There are two types of type assertion in TypeScript:

  • Angle bracket notation: This type of type assertion is also known as the "angle bracket syntax" and is used to assert a specific type to a value. This is done by placing the type that you want to assert inside angle brackets (‘<>’) after the value you want to assert.
     
  • As keyword notation: This type of type assertion is also known as the "as syntax" and is used to assert a specific type to a value. This is done by placing the ‘as’ keyword before the type that you want to assert after the value you want to assert.
     

30. What is the purpose of noImplicitAny?

A noImplicitAny option is a compiler option in TypeScript that enforces stricter type checking by disallowing the use of implicit any types in the code. When this option is enabled, the TypeScript compiler will issue an error whenever a variable or function parameter does not have an explicit type and cannot be inferred.

The purpose of noImplicitAny is to help catch potential type errors at compile time before they cause issues at runtime. By requiring explicit typing, the compiler ensures that every value has a well-defined type, which can prevent bugs and make the code more maintainable and easier to reason about. 

31. What are type aliases? How do you create one?

In TypeScript, a type alias is a name that is given to a type. It allows you to define a custom name for an existing type, making it easier to refer to that type throughout your code.

To create a type alias, you can use the type keyword followed by the alias name and the type it refers to. Here's an example:

type User = {
  name: string;
  age: number;
  email: string;
}

 

In this example, we've created a type alias named ‘User’, which refers to an object type with three properties: ‘name’, ‘age’, and ‘email’.

32. How to enforce strict null checks in TypeScript?

To enforce strict null checks in TypeScript, you can use the ‘strictNullChecks’ compiler option. This option is included in the strict set of compiler options, which also includes ‘--noImplicitAny’, ‘--noImplicitThis’, ‘--strictBindCallApply’, and ‘--strictFunctionTypes’.

Enabling ‘strictNullChecks’ ensures that null and undefined values are not used in places where they are not allowed. This helps catch potential bugs and makes your code more type-safe.

33. What are triple-slash directives?

Triple-slash directives are special comments that can be used in TypeScript files. It is used to provide additional information to the compiler. These comments begin with three forward slashes (‘///’) and are used to reference external files, configure the compiler, and provide type definitions for JavaScript files.

Triple-slash directives are processed by the TypeScript compiler and are not part of the JavaScript language. When the compiler encounters a triple-slash directive, it reads the contents of the comment and acts on the information it contains. 

34. What are Ambients in TypeScripts

In TypeScript, Ambients are declarations that describe the shape of code outside of the current TypeScript project, typically used to provide type information for external code. They can be defined using the ‘declare’ keyword.

Ambient declarations in TypeScript are used to provide type information for external code that doesn't have type information available in TypeScript, allowing developers to use this code in a type-safe manner. This enables better integration with external libraries and APIs, improving code quality and reliability. 

35. What are some of the utility types provided by TypeScript?

TypeScript provides several built-in utility types that can help simplify type declarations and improve code readability. Some of the most commonly used utility types include:

utility types in Typescript
  1. Partial<T>: This utility type takes a type T and makes all of its properties optional. It can be useful when defining interfaces that allow for optional properties.
     
  2. Readonly<T>: This utility type takes a type T and makes all of its properties read-only. It can be useful when working with immutable data.
     
  3. Pick<T, K>: This utility type takes a type T and a set of keys K and creates a new type that includes only the specified keys. 
     
  4. Omit<T, K>: This utility type takes a type T and a set of keys K and creates a new type that excludes the specified keys. 
     
  5. Record<K, T>: This utility type creates a new type that represents an object with keys of type K and values of type T. 
     
  6. Exclude<T, U> and Extract<T, U>: These two utility types can be used to filter a union type based on whether its members are assignable to another type. 

Frequently Asked Questions

What are the disadvantages of TypeScript?

TypeScript is not independent(Based on JavaScript), making the initial setup challenging. Finding skilled TypeScript developers is also a difficult process. There are typically higher numbers of code lines and less readability in TypeScript.

Why is TS better than JS?

JavaScript more effectively supports smaller apps, while TypeScript better serves larger applications. In terms of language features, reference checking, and project scalability, TypeScript is superior to JavaScript. JavaScript does not allow static typing, but TypeScript does.

Is TypeScript case-sensitive or not?

TypeScript is case-sensitive. This indicates that TypeScript distinguishes among characters in upper- and lowercase. A consistent capitalization of letters must be used while typing all language keywords, variables, function names, and other identifiers. 

Which TypeScript types are immutable?

In TypeScript, all primitive data types, such as strings, numbers, booleans, etc., are immutable, meaning that the value cannot be changed more than once. Additionally, it means that passing them to functions has no negative impacts.

Conclusion

This article discussed the most asked typescript interview questions and their answers. We covered all aspects and topics from where you can expect questions in Typescript. 

We hope that this article has helped you with various Typescript interview questions. You can refer to our articles on similar topics for further reading:

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