Most popular TypeScript interview questions and answers 2024

If you are looking for TypeScript developer interview questions, go through this curated list of TypeScript interview questions and answers for acing that interview.

Most popular TypeScript interview questions and answers 2024

Last updated on May 19, 2024

TypeScript is crucial in modern software development due to its ability to enhance JavaScript with static typing. By providing a robust type system, TypeScript helps catch errors during development, making code more reliable and maintainable. It improves code documentation, enabling developers to understand and work with codebases more efficiently. Furthermore, its tooling support and compatibility with popular frameworks and libraries make it an invaluable choice for building scalable and error-resistant web applications, contributing significantly to overall code quality and developer productivity.

Whether you're a seasoned developer looking to brush up on your TypeScript knowledge or a hiring manager seeking to evaluate prospective candidates, this blog is your go-to resource. We've curated a diverse set of questions that cover TypeScript's fundamentals, advanced concepts, and real-world application scenarios. Dive in to enhance your TypeScript expertise or use these questions to identify top talent who can drive your projects to success.

Basic TypeScript interview questions and answers


Explain TypeScript and its use?

TypeScript is an open-source programming language developed and maintained by Microsoft, which acts as a statically-typed superset of JavaScript. By extending JavaScript with type annotations and interfaces, it enables developers to catch errors during the compilation process rather than at runtime, thus ensuring safer and more efficient code.

TypeScript is also highly scalable, allowing for the development of large applications with features like namespaces, decorators, and abstract classes. One of its most significant strengths is its seamless integration with the JavaScript ecosystem and modern frameworks like Angular, React, or Vue.js, making it an invaluable tool for any developer looking to create maintainable and robust web applications.


Can you tell the names of some of the built-in types in TypeScript?

In TypeScript, there are several built-in types available for developers to ensure safer, more accurate code. Some of the most common built-in types include:

any: Represents any type of value and allows a variable to store multiple types.

boolean: Represents a boolean value, which can be either true or false.

number: Represents any numeric value, including integers and floating-point numbers.

string: Represents a sequence of characters, commonly used for storing textual data.

null: Represents the absence of a value intentionally assigned to a variable.

undefined: Represents a variable that has not yet been assigned a value.

void: Represents the lack of a type, commonly used as the return type for functions that do not return a value.


Give an example of generics in TypeScript?

Generics in TypeScript allow for creating reusable and flexible components without compromising on type safety. They enable you to create functions, classes, or interfaces that can work with different types while still providing the benefits of static typing. Here's an example using generics in a TypeScript function:

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The generic function identity allows you to work with different types without losing type information. In this case, numIdentity will have the type number, and strIdentity will have the type string.


Tell me about Modules in TypeScript.

Modules in TypeScript provide a way to organize and encapsulate code, making it easier to develop, maintain, and test large applications. They allow you to split your code into separate files, keeping each file focused on a specific set of functionality or a single responsibility. Modules are designed to manage dependencies and avoid naming conflicts by using distinct namespaces and imports/exports.

There are two main types of modules in TypeScript:

External modules: Also known as "file modules," they are one module per source file. When you create a new TypeScript file and work with the import and export keywords, you are automatically creating an external module. These are the most commonly used TypeScript modules.

Ambient modules: These are used to provide type information for third-party libraries or existing global variables. They are typically declared using a .d.ts file and may be provided by the library developers themselves or created by the community. The declare module syntax is used to define an ambient module.


Why can TypeScript be chosen over JavaScript?

TypeScript offers several advantages over JavaScript, making it a preferable choice for many developers seeking to build large-scale and maintainable applications. One of the most noticeable benefits of TypeScript is the tools and IDEs that provide a rich environment that helps to find the errors easily. Some other benefits include:

Advanced autocompletion and IntelliSense: TypeScript provides enhanced autocompletion, code navigation, and refactoring capabilities through better integration with editors like Visual Studio Code, making the development process faster and more efficient.

Large-scale application support: Features like classes, interfaces, generics, and namespaces in TypeScript facilitate the design of modular code, making it well-suited for large and complex applications.

Improved code readability: TypeScript's type annotations and advanced OOP features bring clarity to codebases and make it easier to understand, debug, and maintain code written by other developers.

Strict null checks: TypeScript enforces strict null checks, which can help catch null or undefined values during compilation, reducing runtime errors related to missing or inaccessible data.


Do TypeScript files need compilation?

Yes, as TypeScript can’t be interpreted by browsers. Compiling is needed to convert TypeScript to JavaScript. And for compilation, you need the TypeScript compiler. TypeScript has all the features and functionality of JavaScript, with some added features. It improves developer experience, provides features like type aliases, abstract classes, tuple, generics, etc, and it allows developers to add type safety to their projects.


List some benefits of TypeScript?

TypeScript offers several benefits that enhance the development process, improve code quality, and streamline collaboration. Some of these benefits include:

Static typing: TypeScript adds static typing to JavaScript, which catches type-related errors at compile-time instead of runtime. This allows developers to identify and fix issues early in the development process, reducing bugs and improving overall code quality.

Code maintainability: TypeScript's static types, interfaces, and other OOP features contribute to better code readability and maintainability, making it easier for developers to understand and work with shared codebases.

Better tooling and editor support: TypeScript provides excellent integration with popular IDEs, resulting in advanced autocompletion, code navigation, and refactoring tools. This improves the development experience and boosts productivity.

Large-scale application support: TypeScript's features, such as generics, namespaces, and modules, foster the creation of modular and scalable code, making it an ideal choice for large and complex applications.

JavaScript ecosystem compatibility: TypeScript is fully compatible with JavaScript libraries, frameworks, and tools, enabling seamless integration into existing development workflows.


In TypeScript, can we call the base class constructor from the child class ?

Yes, in TypeScript, you can call the base class constructor from a child class. When you create a child class that extends a base class, you can use the super() function within the child class constructor to invoke the parent class constructor.

Here's an example:

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In this example, the Dog class extends the Animal class. When you create a new Dog instance, the Dog constructor calls the Animal constructor using super(name). This ensures that the parent class's properties are properly initialized before initializing the child class's properties.


What is the difference between TypeScript and a statically typed language?

TypeScript is optionally statically typed, which means you can tell the compiler to ignore a variable's type. We can assign any type of value to the variable using any data type. During compilation, TypeScript will not perform any error checking.


Is string interpolation possible in TypeScript?

Yes, string interpolation is possible in TypeScript, and it is also known as template literals or template strings. It allows you to embed expressions within string literals, using backticks (`) instead of single or double quotes. To include a variable or expression within the string, use the ${expression} syntax. This feature makes it easier to create more readable and concise strings without resorting to string concatenation.

Here's an example of string interpolation in TypeScript:

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In this example, the message variable uses a template literal to construct a complete string with interpolated variables firstName, lastName, and age. The output will be:
Hello, my name is John Doe and I'm 30 years old.


How do the extensions .ts and .tsx differ in TypeScript?

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Differentiate between Classes and Interfaces in TypeScript?

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Explain the Interface in TypeScript?

In TypeScript, an interface is a fundamental concept that allows you to define the shape, structure, and contract of an object or a class without implementing its actual functionality. Interfaces let you define custom types by specifying the properties and their respective types that an object should have, and the class that implements the interface must adhere to these specifications. Interfaces can also be used with functions to define specific parameters and return types.

Interfaces can have optional properties, read-only properties, and method signatures as well as support extending and implementing multiple interfaces.


Explain Decorators in TypeScript.

Decorators in TypeScript are a special kind of declarative syntax used to add metadata or modify the behavior of classes, methods, properties, or parameters. They provide a convenient way to apply reusable logic or behavior across multiple parts of your code without having to manually modify each target element. Decorators are built using higher-order functions, which receive the target element (such as a class or property) as an argument and return a modified version of it or perform specific actions on it.


When should you use interfaces or classes in TypeScript?

In TypeScript, both interfaces and classes are used to define custom types and structures. However, you should choose between interfaces and classes based on your specific requirements:

Use Interfaces when:

  • You want to define the shape, structure, and contract for how an object or a class should look, without implementing its actual functionality.
  • You want to enforce specific properties, methods, or their types on an object or a class.
  • You need a lightweight approach that only focuses on the type-checking aspect during development.
  • You want to describe the structure and requirements for function parameters.

Use Classes when:

  • You want to encapsulate methods and properties within a single entity, including providing a concrete implementation for them.
  • You want to use object-oriented concepts like inheritance, abstraction, and encapsulation.
  • You need to create instances of the class to interact with.
  • You want to use instance-specific state and methods based on concrete implementations.


How can you use class constants in TypeScript?

You can declare class constants in TypeScript using the keywords " readonly ", const and readonly static. The readonly keyword marks a property as unchangeable; you cannot change the value of the property once it is set. Const keyword declares a constant outside the class and uses it inside the class. But if you want it to be accessible only inside the class, declare it privately.


In TypeScript, what can you do to check null and undefined?

In TypeScript, null and undefined are separate but related types representing the absence or uninitialized state of a value. To check for null or undefined, you can use various language constructs and techniques:

  • Equality checks Use the equality operator == or inequality operator != to check for both null and undefined at the same time. You can also use the strict equality === or strict inequality !== operators followed by explicit checks for null and undefined.

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  • Optional Chaining Operator The optional chaining operator ?. can be used to access properties and methods on an object that may be null or undefined. If the object is null or undefined, the result of the expression will be undefined.
    const result = obj?.property; // result will be undefined if obj is null or undefined


Are all object-oriented principles supported by TypeScript?

Yes, it supports the OOP principles such as,

  • Inheritance
  • Abstraction
  • Polymorphism
  • Encapsulation


Also, tell me some OOP terms supported by TypeScript.

TypeScript, being a statically-typed superset of JavaScript, supports many Object-Oriented Programming (OOP) concepts to help you build more structured, maintainable, and reusable code. Some of the important OOP terms supported by TypeScript include:

Classes: Classes in TypeScript serve as blueprints for creating objects that share similar properties and methods. They consist of a constructor, properties, and member functions.

Inheritance: TypeScript allows you to inherit properties and behaviors from a parent (base) class to a child (derived) class through the extends keyword, which enables code reusability and simplifies complex code structures.

Interfaces: Interfaces are used to define the shape, structure, and contract for objects, classes, and functions, specifying properties and methods that should be implemented by classes that adopt the interface.

Abstraction: TypeScript supports the creation of abstract classes, which serve as base classes that cannot be instantiated directly and often include unimplemented methods and properties.

Encapsulation: TypeScript provides access modifiers (public, private, and protected) to control the visibility and accessibility of class properties and methods, ensuring only intended parts of an object or a class are exposed.


Explain Getters/Setters in TypeScript.

In TypeScript, getters and setters are special class methods used to control the access and modification of object properties. They provide a flexible and encapsulated way to work with properties while giving you the ability to add additional logic around retrieval and assignment.

Getters are accessor methods used to retrieve the value of a property. They are defined using the get keyword followed by the method name, and they have no parameters. Getters are accessed like regular properties, but they invoke the getter function internally to compute the result.

Setters are accessor methods used to update the value of a property. They are defined using the set keyword followed by the method name, and they contain a single parameter representing the new value. Setters are assigned values like regular properties, but they invoke the setter function internally to perform additional logic.


Can TypeScript be used on the back-end and how?

Apart from the front-end or browser, TypeScript can also be used for the back-end. TypeScript can be used on the back-end for building server-side applications and services. One popular approach for using TypeScript on the back-end is with Node.js. With TypeScript and Node.js, you can create scalable and maintainable server-side applications while enjoying the benefits of static typing and other TypeScript features.


Explain TypeScript Map file?

A TypeScript Map file, also known as a Source Map, is a JSON file generated during the compilation process of TypeScript to JavaScript. It serves as a bridge between the original TypeScript source code and the compiled JavaScript code, allowing developers to easily debug and trace back to the TypeScript code while working directly in a browser's developer console.

A Map file essentially maps the compiled JavaScript code back to its corresponding TypeScript code, enabling support for features like breakpoints, code stepping, variable inspection, and call stacks in the TypeScript code during debugging. This is particularly useful when using minified, obfuscated, or transpiled code because it preserves the original structure and naming, making it more comprehensible for developers.


Can you explain the difference between String and string in TypeScript?

In TypeScript, String and string both represent textual data, but they have different meanings and use cases:

string: This is the simplest, preferred, and most commonly used type for representing textual data in TypeScript. It's a primitive type that is automatically assigned to string literals. When you use the string type, TypeScript performs type checking and ensures that all operations and assignments are valid for strings.

let text: string = 'Hello, TypeScript!';

String: This is a built-in global object of JavaScript and wraps around the simple string type to provide additional methods and functionalities. The String object acts as a constructor for creating new strings and as a namespace for various utility methods to manipulate or inspect strings. However, in most scenarios, it is not necessary to use the String object, as TypeScript and JavaScript automatically convert primitive strings to String objects when required.

let wrappedText: String = new String('Wrapped TypeScript!');

In practice, you should generally use the string type for working with textual data in TypeScript, as it provides type checking and simpler, more efficient code. In contrast, avoid using the String object unless you have a specific use case that requires the additional functionality it provides.


Explain Type Erasure in TypeScript?

Type erasure in TypeScript refers to the process in which TypeScript's type information is removed during the transpilation (compilation) to JavaScript. Since JavaScript doesn't have a native type system like TypeScript, TypeScript compiler (tsc) eliminates all type annotations and corresponding type-checks when generating the resulting JavaScript code.

Type erasure has the following consequences and implications:

Runtime type information: TypeScript's type system only exists at compile-time for static type checking. Once compiled to JavaScript, there’s no information about types at runtime, which means you cannot perform type-checks or type-related operations during the execution of the code.

Compatibility: Type erasure ensures that the generated JavaScript code remains compatible with existing JavaScript libraries, frameworks, and codebases. TypeScript aims to be a strict superset of JavaScript, and type erasure helps maintain that compatibility.

Performance impact: Type erasure causes TypeScript's static type checking to have no overhead at runtime. The generated JavaScript code doesn't include any additional constructs related to type-checking, which means there is no impact on the overall runtime performance of the application.


How can you create an enum with string values?

In TypeScript, you can create an enum with string values by assigning a string to each enum member. Unlike numeric enums, string enums don't have auto-incrementing behavior. You need to define string values manually for each member. Here's an example of creating an enum with string values:

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In this example, we define a Color enum with string values for each member. When using the Color enum, the TypeScript compiler will enforce the string values for type safety, ensuring that you can't accidentally assign an incorrect string value.


List some features of TypeScript.

TypeScript brings several powerful features to JavaScript development, which help in creating safer, more maintainable, and scalable code. Here are some key features of TypeScript:

Static Typing: TypeScript introduces static typing, providing type checking during the development stage rather than at runtime. This leads to early detection of errors and better developer tooling.

Interfaces: Interfaces define the shape, structure, and contract of objects or classes, allowing for custom types and enforcing a consistent structure across your codebase.

Classes: TypeScript supports object-oriented programming concepts like inheritance, encapsulation, and abstraction through classes and their related features, such as constructors, access modifiers, and abstract classes.

Generics: Generics enable you to create highly reusable and flexible components that work with a variety of types without sacrificing type safety.

Type Inference: TypeScript automatically infers types of variables and expressions when they are not explicitly specified.


Tell me the Components of TypeScript?

TypeScript consists of several components that work together to provide powerful features for developers building JavaScript applications:

Language− TypeScript extends JavaScript by adding optional static typing, interfaces, classes, decorators, namespaces, and many other features that JavaScript doesn't have natively.

Compiler − TypeScript has a dedicated transpiler (tsc) that compiles TypeScript code into plain JavaScript. The TypeScript compiler ensures type safety, checks for errors, and transpiles TypeScript into JavaScript, which can run across different browsers and platforms.

Language Service: The TypeScript Language Service provides editor support, such as IntelliSense, code completion, syntax highlighting, and refactoring, for modern development environments like Visual Studio, VSCode, WebStorm, etc. These advanced editing features enable developers to write code efficiently and productively.

Type Definitions: TypeScript uses type definition files (.d.ts) to provide type information about external libraries or JavaScript code. Type definitions help TypeScript developers access type information for third-party libraries like jQuery, React, or Express that are written in JavaScript. The DefinitelyTyped repository is a popular source of type definitions for various JavaScript libraries.

Tooling: TypeScript has robust integration with popular build tools, test runners, task runners, and bundlers, such as Webpack, Rollup, Gulp, Grunt, Babel, and Jest. These integrations ensure a smooth development, testing, and build process for TypeScript developers.


How will you install TypeScript?

To install TypeScript, you need to have Node.js and npm (Node Package Manager) installed on your machine. With Node.js and npm installed, you can follow these steps to install TypeScript:

Open up your terminal or command prompt.

Run the following command to install TypeScript globally:

npm install -g typescript

This command will install the latest version of TypeScript as a global package, making it available on your entire system for use in multiple projects. The -g flag ensures that the package is installed globally.

After the installation is complete, you can check the installed version of TypeScript by running:

tsc --version

This command will display the TypeScript version on your system, confirming that the installation was successful.

Now, you have TypeScript installed and ready to use. To compile a TypeScript file, you can use the tsc command followed by the filename, like this:

tsc myFile.ts

This command will compile your TypeScript file (myFile.ts) into a JavaScript file with the same name (myFile.js).


List the steps you will use to compile TypeScript files?

To compile TypeScript files, follow these steps:

Install TypeScript: First, install TypeScript globally on your system using npm (Node.js package manager) by running npm install -g typescript. If you don't have Node.js installed, you can download it from

Write TypeScript code: Create a TypeScript file (.ts extension) and write your TypeScript code in it. For example, create a file called app.ts and add the following code:const message: string = 'Hello, TypeScript!'; console.log(message);.

Compile TypeScript code: Run the TypeScript compiler (tsc) on your TypeScript file by executing tsc app.ts. This command will compile the app.ts file and generate a corresponding JavaScript file, app.js, in the same directory.

Execute compiled JavaScript: Once the TypeScript files have been successfully compiled into JavaScript, you can execute the .js files using Node.js or include them in your HTML files for use in a browser environment. To run the compiled JavaScript file using Node.js, execute node app.js (or node ./dist/app.js if you are using a tsconfig.json file with outDir).


Explain Variables in TypeScript and how to create them?

In TypeScript, variables are memory locations that store values of various data types. To declare and initialize a variable in TypeScript, you use the let or const keyword followed by the variable name, an optional type annotation, and the assignment operator (=) to provide an initial value.

Creating Variables:

Using let keyword: The let keyword is used to create mutable variables that can be reassigned later in your code. It follows block-scoping rules, making the variable available only within the scope in which it is defined.

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Using const keyword: The const keyword creates constant, immutable variables that must be initialized at the time of declaration and cannot be reassigned afterwards. This enforces the value remains constant throughout the execution of the program.

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Type annotations:

In TypeScript, you can explicitly set a type for a variable using type annotations, which are written after the variable name followed by a colon. This helps catch type-related errors at compile-time.

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If you do not explicitly set a type for a variable, TypeScript will infer the type based on the initial assigned value, which is known as "type inference".

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List the ways you can declare a variable?

var [identifier] : [type-annotation] = value; //Here we declare type and value in a single statement

var [identifier] : [type-annotation]; // Here we declare type without value

var [identifier] = value; //declare without type

var [identifier]; //Here we declare without value and type


Can we compile .ts automatically with real-time changes in the .ts file?

Yes, you can automatically compile a TypeScript file (.ts) whenever there are real-time changes by using the TypeScript compiler's --watch option. The tsc (TypeScript Compiler) will watch for modifications in the .ts file(s) and automatically recompile them whenever changes are detected.

To use the --watch option, open your terminal or command prompt, navigate to the directory containing the .ts file(s), and run the following command:

tsc --watch your-file.ts

Replace your-file.ts with the name of your TypeScript file.

If you want to watch and compile multiple .ts files or an entire project, you can also specify the --watch option in your project's tsconfig.json settings file. Set up a tsconfig.json file in the root directory of your TypeScript project (if you don't already have one) and include the following:

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After setting up the tsconfig.json file, you can run the TypeScript compiler with just:


This will start the compiler in watch mode, and it will recompile the specified TypeScript files automatically whenever changes are detected.


List the access modifiers that TypeScript supports.

Public: members of the class including child classes and the instances of the class have access to it.

Protected: Here, the members of the class including the child classes can access except the instances of the class.

Private: accessible only to the members of the class.

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Intermediate TypeScript interview questions and answers


Why do we say that TypeScript is an optionally statically typed language?

TypeScript is referred to as an "optionally statically typed language" because it allows developers to mix both static and dynamic typing within the same codebase. TypeScript provides static type-checking through type annotations, interfaces, and generics, enabling safer and more efficient code development. However, it doesn't enforce static typing for every variable or function, as it's not a requirement for all TypeScript code.

The language offers a great deal of flexibility, making it easy for developers to migrate from JavaScript and incrementally add static types to their existing JavaScript projects. TypeScript gracefully handles dynamic typing with the built-in any type, which effectively bypasses static type-checking. By using the any type, developers can tell the compiler to treat a particular variable or object as dynamically-typed, retaining JavaScript's dynamic nature when necessary.

Thus, TypeScript's ability to balance static and dynamic typing, where developers can choose to use one or the other as per their requirements, is what makes it "optionally statically typed."


Explain modules in TypeScript?

In TypeScript, modules are a way to organize and encapsulate related code, making it easier to reason about, maintain, and manage. They allow developers to split code into separate files, with each module exporting and importing specific components, such as classes, functions, variables, or interfaces. By using modules, you can avoid naming collisions and control the visibility of your code parts.

Modules in TypeScript can adhere to one of the two popular module systems: CommonJS (used mainly in Node.js applications) or ES Modules (used in modern JavaScript running in browsers).

Exporting Components: To export a component (class, function, variable, or interface) from a module, you use the export keyword:

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Importing Components: To import and use a component from another module, you use the import statement:

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Aside from the CommonJS and ES Modules systems, TypeScript supports other module systems, such as AMD, SystemJS, and UMD, enabling compatibility with various runtime environments.


Differentiate between the internal module and the external module?

In TypeScript, the concepts of internal and external modules have evolved over time. The terms have been mostly replaced with "namespaces" and "modules", respectively. Here's a brief comparison between the two:

typescript internal vs external module.webp


Explain namespace in TypeScript and its declaration

Also, known as internal modules, Namespace group functionalities in a logical manner. Also, it encapsulates the features and objects that have common relationships. Also, a namespace can include interfaces, classes, functions, and variables to support some other functionalities.


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Is function overloading supported in TypeScript?

Yes, TypeScript supports function overloading, allowing you to define multiple function signatures with the same function name but differing parameters. Function overloading enables you to have several ways to call the same function with different argument types and combinations while maintaining type checking capabilities.

To implement function overloading in TypeScript, you need to define the different overloaded function signatures without their implementation, followed by the actual function implementation with a more generic signature. The implementation must be flexible enough to handle all supported overloads, and TypeScript will enforce the implementation to be compatible with the provided overloaded signatures.


Explain Mixins?

Mixins are a way of creating classes from components that can be reused. Basically, they are built up by combining simpler partial classes. So this means instead of class A extending class B to take its functionality, B takes class A and reuters a new class with additional functionality, And, thus function B is a mixin here.


TypeScript supports optional parameters in function, can you explain how?

In the TypeScript compiler, an error is thrown if the function is invoked without giving the exact number and types of parameters as are given in the function signature. To declare an optional parameter, use the ? symbol after the parameter name in the function signature.


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Explain the Scope variable?

In TypeScript, the scope of a variable refers to the context or region of code in which the variable is accessible and can be used. Proper scoping is essential for managing variables effectively, preventing naming conflicts and keeping code maintainable.

TypeScript has three types of variable scopes:

Global scope: Variables declared outside any function, class, or module are considered global. They can be accessed from any part of the code in the same script file or other script files included in the same application. Global variables should be used sparingly, as they can lead to naming conflicts and difficulty in managing code.

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Local scope: Variables declared within a function are limited to the function's scope, meaning they can only be accessed within the function in which they are declared. These scopes are also known as local scopes.

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Block scope: Variables declared with let and const keywords are bound to the block in which they are declared (within curly braces {}). These variables are only accessible within the block and any nested blocks enclosed by that block.

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Explain to me the debugging process for a TypeScript file.

First of all, for the debugging process, you will need .js source map file. Then you compile the .ts file with the – sourcemap flag to generate a source map file.

$ tsc -sourcemap file 11.ts

After this we get file11.js and And the last line of file11.js would be a reference of the source map file.



Explain TypeScript Definition Manager and its use.

In the TypeScript ecosystem, the TypeScript Definition Manager (TSD) was a popular command-line tool used to manage TypeScript declaration files (.d.ts) which contain type definitions for external JavaScript libraries and frameworks. Providing accurate type information for external libraries allows TypeScript developers to leverage the advantages of static typing and catch potential type-related errors during compilation.

However, TypeScript Definition Manager (TSD) has been deprecated, and it is now replaced by DefinitelyTyped and @types scoped packages alongside the npm package manager.

DefinitelyTyped is a large repository containing high-quality TypeScript declaration files for numerous JavaScript libraries and frameworks. The @types scoped packages refer to individual type declaration packages generated from DefinitelyTyped, which can be easily installed using the regular npm package manager.


Tell me the steps to include a Type Definition File.

First, install TSD

$ npm install tsd -g

Go to TypeScript directory and launch new TypeScript project

$ tsd init

Now install the definition file for jquery

tsd query jquery --action install

Now, update the TypeScript file to include the definition file to point to the jQuery definition.

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Now, compile again.


Explain TypeScript Declare Keyword?

The declare keyword in TypeScript is used to inform the TypeScript compiler about the existence of a variable, function, class, or module that is defined externally, usually in some external JavaScript code or a library. This allows TypeScript to provide type checking and intellisense for those elements without actually implementing their behavior within your TypeScript code.

When defining TypeScript declaration files (commonly with the .d.ts extension), the declare keyword is often used to provide type information about external JavaScript code, allowing you to use it safely and efficiently in your TypeScript projects. These declaration files act as a bridge between the external JavaScript library and TypeScript, enabling proper type checking and code completion.


Explain tsconfig.json file?

The tsconfig.json file is a configuration file used in TypeScript projects to specify various compiler options and settings. It enables developers to control options like specifying the output folder for the generated JavaScript code, module resolution strategy, strict type checking, source mapping, and features specific to various ECMAScript versions. The TypeScript compiler uses this configuration file to understand how to compile and transpile TypeScript code into JavaScript.


Tell me about Generics in TypeScript?

Generics is a tool that provides a way of creating reusable components. It provides components that can work with varied data types instead of a single data type. Besides all this, Generics also provides type safety without affecting performance or productivity. Generics also allow the creation of generic classes, generic functions, generic methods, and generic interfaces.


Differentiate between interface and type statements?

In TypeScript, both interface and type statements can be used to define custom types, and sometimes their use can overlap. However, there are key differences between the two, which make them suitable for different situations.

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Tell me about JSX in TypeScript.

JSX (JavaScript XML) is a syntax extension for JavaScript, primarily used with React, that allows you to write HTML-like code within your JavaScript or TypeScript code. TypeScript supports JSX and provides a seamless way to use JSX syntax alongside TypeScript's type checking and other features.

To use JSX in TypeScript, you need to configure your tsconfig.json file to specify some options, like:

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Here, the "jsx" option is set to "react", which transpiles JSX content into React.createElement() calls, compatible with the React library.

TypeScript also allows you to leverage its type system in your JSX code to ensure type safety when using React components:

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In this example, we define a Greeting functional component with a strongly-typed prop using the GreetingProps interface. The FC (Functional Component) generic type is imported from the react library and used to specify the type of our components.

Using TypeScript with JSX ensures safer, maintainable, and more efficient React code, since the TypeScript compiler checks for correct component usage, valid properties, and accurate types during development.


List some JSX modes in TypeScript?

In TypeScript, JSX is a syntax extension for JavaScript that resembles XML, commonly used in UI frameworks like React. TypeScript supports various JSX modes, which determine how JSX expressions are compiled and emitted, as specified by the jsx option in the tsconfig.json file or as a command-line flag.

Below are some of the supported JSX modes in TypeScript:

preserve: Keeps the JSX syntax intact in the output files, with the file extension changed to .jsx or .tsx. Mostly used with tools like Babel for further transpilation.

react: Transforms JSX syntax into JavaScript calls, specifically to React.createElement(). This is the commonly used mode for React applications.

react-jsx: Convert JSX syntax to JavaScript calls using the jsx imported-from or local factory function with the React version 17 or higher. Creates more optimized and compact code.

react-jsxdev: Similar to react-jsx, but with additional checks to enable React development mode features during development. Should only be used in development builds.

react-native: Identical to the preserve mode, but does not change the file extension, leaving it as .js. Used for building React Native applications.

When using TypeScript with JSX, make sure to set the specific JSX mode you're using in the tsconfig.json file based on the target platform/framework.


Explain Ambients in TypeScripts and their use?

These declarations inform the compiler about the actual source code that exists elsewhere. If these codes do not exist at runtime and one tries to use them, it will break and not give a warning either.

These declaration files are similar to doc if the source changes, the docs need to keep updated. However, if the ambient declaration file is not updated, you get a compilation error.


Explain Type assertions in TypeScript?

Type assertions in TypeScript are a way to explicitly inform the compiler that you, as a developer, are certain about the type of a specific variable or expression. By asserting the type, you essentially override TypeScript's inferred type, giving you more control over how the compiler treats that variable or expression. Type assertions do not change the value or runtime behavior of the variable, but they can affect the type information during compile-time, allowing additional functionality or avoiding compile-time errors


Explain Rest parameters?

In TypeScript, Rest parameters provide a clean and concise way to handle an indefinite number of function arguments as an array. By using the rest parameter syntax, denoted by three dots ... preceding the parameter name, you can represent any remaining arguments in a function as an array of elements. Rest parameters offer an alternative to the arguments object in JavaScript, allowing you to capture all extra arguments while maintaining type safety and providing better code readability.

Here's an example of Rest parameters in TypeScript:

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In this example, the ...names rest parameter captures the remaining arguments and treats them as an array of strings (string[]). The displayGreeting function outputs a greeting message and a list of names separated by a comma.

It's important to note that rest parameters should always be defined at the end of the parameters list, and only one rest parameter is allowed per function.


List the rules to follow for declaring Rest parameters? Give an example

Here are some rules to be followed for declaring Rest parameters

  • In a function, only 1 rest parameter is allowed.
  • Its type should be an array.
  • Should be the last parameter in the parameter list.

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Explain “as” syntax in TypeScript.

The "as" syntax in TypeScript is used to create a type assertion, which is a way to explicitly inform the compiler that you know the actual type of a value and want to treat it as a specific type. This can be useful when the TypeScript compiler cannot automatically infer the correct type or when you need to perform a type conversion during development.
Type assertions with as syntax do not change the runtime behavior of the underlying value, but they provide compile-time type information that can help catch type-related errors.

Here's an example of using the as syntax for a type assertion:

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In this example, someValue has an unknown type, which means the type of the value is unknown during development but can be any type during runtime. We use the as syntax to cast someValue to a string and then access the length property.


What do you know about Enum in TypeScript?

In TypeScript, an Enum (short for enumeration) is a custom data type that allows you to define a set of named numeric constants, making your code more self-explanatory and easier to maintain. Enums provide a robust and expressive way to handle sets of values that represent specific categories, states, or options in your program.

Here's an example of an Enum in TypeScript:

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Differentiate between Relative and Non-relative module imports.

In TypeScript, module imports can be either relative or non-relative, based on how the import path is specified. Here's a comparison between the two:

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Explain the anonymous function?

An anonymous function is a function without a name, typically defined using a lightweight syntax and commonly used as a one-time-use function or for passing as an argument to other functions. In JavaScript and TypeScript, anonymous functions can be used as callback functions, event listeners, or in scenarios where a simple function is required with no need to declare or reference it afterwards.


Explain the method overriding in TypeScript?

Method overriding in TypeScript is an object-oriented programming concept that allows a derived (subclass) to provide a new implementation for a method inherited from its base class (superclass), effectively replacing or customizing the behavior of the inherited method. This is useful when a derived class needs to maintain the same method signature as its base class but requires different logic or functionality.

Some rules for method overriding are as follows:

  • Method must have the same name as the parent class
  • Also, it must have the same parameter as in the parent class
  • IS-A relationship or inheritance should be present.


Explain Lambda/Arrow function?

Lambda, or arrow functions, are a more concise way to define functions in JavaScript and TypeScript. Introduced in ECMAScript 6 (ES2015), arrow functions simplify function expressions and provide a more compact syntax by using the fat arrow => to define the function. Arrow functions have two key features:

Concise Syntax: Arrow functions are shorter and more elegant compared to traditional function expressions.

Lexical this: Arrow functions do not have their own this binding, which means they inherit the this value from their enclosing scope. This behavior is particularly useful when dealing with callback functions or event listeners.


Explain conditional types in TypeScript.

Conditional types in TypeScript are a powerful feature introduced in TypeScript 2.8 to help model complex logic for the type system. They enable developers to define a type based on a condition, usually expressed using generic type parameters. A conditional type takes the form T extends U ? X : Y, where T and U are type variables, and X and Y represent the types that will be used depending on whether the condition is true or false, respectively.


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Here if T is a subtype of U, the conditional evaluates to X , else to Y.


How can you make read-only Arrays in TypeScript?

Using ReadonlyArray type, we can define Arrays to be read only. Thus any variable with reference to ReadonlyArray won't alter any element of the array.

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Explain Nullish Coalescing operator?

The nullish coalescing operator (introduced in TypeScript 3.7) is a logical operator that returns the right-hand side operand when the left-hand side operand is either null or undefined. Otherwise, it returns the left-hand side operand. In TypeScript and JavaScript, it is denoted by ‘??’. The nullish coalescing operator is useful for providing default values for variables or expressions that might be null or undefined, making your code more robust and less prone to errors.

Here's a simple example illustrating the use of the nullish coalescing operator:

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In this example, the result will be assigned the value of y (42) because x is null. If x had a non-null value, the result would be assigned the value of x.


Explain assertion functions?

In TypeScript, assertion functions are user-defined type guards that perform runtime checks and assertions to narrow down the types of variables further. They help ensure the correctness of data by checking the values' constraints and throwing errors when those constraints are not met. Assertion functions can help detect and prevent unexpected behavior or bugs early during runtime.

Here's an example of an assertion function:

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In this example, the assertIsString function takes two arguments: value, which is the variable to check, and errorMessage, which is the message to include in the error thrown if the provided value is not a string. The function uses the asserts keyword with the is keyword to specify the type that the value will be narrowed down to if the assertion succeeds.


Explain Triple-Slash Directives. List some of the triple-slash directives.

They are single line comments containing an XML tag to serve as compiler directives. Every directive indicates what to load at the time of the compilation process.

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Explain the Omit type ?

In TypeScript, the Omit utility type is used to create a new type from an existing type by excluding specific properties from it. It takes two type arguments: the first is the type you want to modify, and the second is a union of properties you want to omit from the original type. The Omit type helps create more specific types based on existing data structures without manually changing their properties.

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In this example, we have an User interface with three properties. By using the Omit type, we create a new UserWithoutEmail type that excludes the 'email' property from the original User interface. Now when you create the user object using the UserWithoutEmail type, you cannot include the 'email' property; doing so will cause a TypeScript error.


How can you compile TypeScript through Visual Studio Code?

  • In Visual Studio Code, you will find the TypeScript language support but not the TypeScript compiler.
  • For that, you need to install the TypeScript compiler to transpile the TypeScript source code to JavaScript.
  • You can install this through npm, the Node.js Package Manager
  • Further you can test your installation through version or help.


Can TypeScript files be supported from Node Modules? If yes, then how?

Yes, TypeScript files can be supported from Node Modules. To achieve this, you should follow these steps:

Step 1: Install TypeScript and the necessary declarations: You need to have TypeScript installed in your project. You can install it using npm or yarn:

npm install --save typescript

If you're using packages with TypeScript definitions in your project, add their respective '@types/' declarations using npm or yarn. For example, to add declarations for Node.js and Express:

npm install --save @types/node @types/express

Step 2: Configure your tsconfig.json: Create a tsconfig.json file in your project's root directory or modify an existing one, and set up the necessary configurations for your project.

Here's an example of a typical tsconfig.json configuration:

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This configuration sets the target ECMAScript version, module system, output directory for compiled files, and more.

Step 3: Run TypeScript with Node.js: To execute TypeScript files with Node.js, you need to compile them to JavaScript first. You can use the tsc command to do this:


This command will generate JavaScript files from your TypeScript files as specified in the tsconfig.json.

To automatically compile and run TypeScript files when changes are made, you can use tools like ts-node or nodemon with ts-node. First, install them in your project:

npm install --save ts-node nodemon

Next, modify your package.json file to add scripts for running your app with ts-node and nodemon:

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Now you can run your TypeScript Node.js app using npm start or npm run dev for auto-compilation and restart during development.


Explain Recursive Type Aliases?

Recursive type aliases are a feature in TypeScript that allows creating a type alias that refers to itself in its definition. This can be particularly useful when working with complex, hierarchical, or nested structures, which often arise when dealing with JSON data, tree-like structures or linked lists.


Explain “any” in TypeScript?

In TypeScript, any is a special type that represents any possible JavaScript value without constraints. It is used when you either don't know the type of a value, or you want to explicitly bypass the type-checking system.

When a variable is assigned the any type, TypeScript disables certain type-checking and allows the variable to store a value of any type. This means that you can assign, access, or use its properties and methods without TypeScript raising any type-related errors or warnings. However, this flexibility comes with a loss of type safety, making it easier to introduce runtime errors.

Here's an example using the any type in TypeScript:

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It's generally recommended to use the any type sparingly, and only when absolutely necessary, to maintain the benefits of TypeScript's type-checking system.


How are ‘let’, ‘var’, and ‘const’ different?

In TypeScript and JavaScript, let, var, and const are used to declare variables, but they have different scoping rules and behaviors:

var: Variables declared with var are function-scoped, meaning they are accessible throughout the entire function in which they are declared. If declared outside any function, they become globally-scoped. var also suffers from hoisting, which means that the variable declaration is moved to the top of the containing scope, causing uninitialized variables to be accessible (with the value undefined) before their actual declaration. This can lead to unexpected bugs and errors.

let: In contrast, let creates block-scoped variables, limiting their accessibility to the block, statement, or expression in which they are declared. This helps prevent unintentional access to variables outside their relevant scope. Variables declared with let are not hoisted, so you must declare them before trying to access them. Attempting to access a let-declared variable before its declaration results in a ReferenceError.

const: Similar to let, const is also block-scoped and isn't hoisted. However, variables declared using const must be assigned a value during their declaration and cannot be reassigned afterward. This makes const suitable for declaring constants or values that shouldn't be changed during program execution. Note that declaring an object with const does not make its properties immutable - only the reference to the object itself is constant.

To sum up, let and const are generally preferred over var due to their clearer, more predictable scoping and behavior. Use let for variables that may change over time and const for values that must remain constant throughout the program's execution.

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Advanced TypeScript interview questions and answers


Tell me about the ‘tsconfig.json’ file?

The tsconfig.json file is a configuration file used by TypeScript to control various aspects of the TypeScript compiler behavior when building (transpiling) TypeScript files into JavaScript. This file provides a way to define the options and settings that the TypeScript compiler (typically tsc CLI) should use during the compilation process.

Some of the settings you can configure in tsconfig.json are:

compilerOptions: This property is an object containing the configuration options for the TypeScript compiler, such as:

  1. target: Specifies the output language level, e.g., ES5, ES6, or a higher ECMAScript version.
  2. module: Sets the module system used in the output, e.g., CommonJS, AMD, UMD, or ES2015.
  3. outDir: Indicates the output directory for the compiled JavaScript files.
  4. sourceMap: Generates source map files for debugging purposes.
  5. strict: Enables stricter type checking and other compiler checks to ensure more robust code.
  6. watch: Enables watch mode, where the compiler will automatically recompile when there are changes in the TypeScript files.

exclude: An array that lists file patterns to be excluded from the compilation, usually for files that are dependencies or not required for the build process.

extends: Allows you to inherit settings from another configuration file, which promotes reuse and decreases duplication in multi-project scenarios.

files: An array that explicitly lists TypeScript files to be included in the compilation, typically used in smaller projects that have few source files.

These settings help to customize the TypeScript build process based on your project's requirements and conventions, resulting in consistent and maintainable code output. By configuring tsconfig.json, you can tailor the compilation process to cater to multiple environments, outputs, and module systems.


Tell me about “never” in TypeScript?

The never type in TypeScript represents a value that should never occur or be reached within the code. It is often used to narrow down the types of variables in specific control flow paths or to indicate that a function should always throw an error and never return a value. The never type is different from void, which represents the absence of a type, typically used for functions that do not explicitly return a value.

Here are a few use cases for the never type in TypeScript:

Function return type: When a function should always throw an exception and never return a value, you can use the never type as its return type.

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Unreachable states: In a control flow path where it is never possible to reach a certain state, the never type can be used for better type safety.

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In this example, exhaustiveCheck function expects the value parameter to be of type never. If we add more cases to the Fruit type but forget to handle one of them within the getFruitInfo function, TypeScript will raise a type error for incorrect handling of an unhandled case.


What are design patterns?

Design patterns are reusable, general-purpose solutions to common problems that arise during software design and development. They represent best practices formulated by experienced software developers to guide the software design process. Design patterns improve code structure, enhance maintainability, and ensure reliable functionality. They also help facilitate communication among developers by providing a shared vocabulary for discussing coding challenges and solutions.

Design patterns are categorized into three groups:

Creational Patterns: These patterns deal with the process of object creation, abstracting the object instantiation process and ensuring a more efficient and maintainable way to create objects. Examples include Singleton, Factory Method, Abstract Factory, Builder, and Prototype.

Structural Patterns: These patterns focus on building a flexible and efficient structure for combining objects or classes to form larger systems or subsystems while maintaining the ability to change or extend the composition. Examples include Proxy, Bridge, Decorator, Flyweight, among others.

Behavioral Patterns: These patterns define the ways in which objects interact and communicate, guiding how responsibilities are distributed among objects and ensuring flexibility and extensibility in the communication process. Examples include Interpreter, Iterator, Command, Observer, State, Mediator, Template Method, among others.


What is the Singleton Design Pattern?

The Singleton Design Pattern is a creational design pattern that ensures a class has only one instance throughout the lifetime of a program, and provides a global point of access to that instance. Singleton is particularly useful when a class must coordinate its behavior across multiple parts of an application, and it's essential to have a single, shared instance to maintain a centralized state or control resources.

The main principle behind the Singleton pattern is to make the constructor of the class private to restrict instantiation by external code, and instead provide a static method or property that manages the unique instance creation internally.


Explain Facade Design Pattern?

The Facade Design Pattern is a structural design pattern that provides a simplified interface for a complex subsystem or group of interfaces, making it easier for clients to interact with the subsystem. It involves creating a higher-level class (facade) that encapsulates the complexity and exposes simpler methods to the clients. The objective is to improve the usability of a system by masking internal complexities, reducing dependencies, and enhancing modularity.

A common analogy for the Facade Pattern is the use of a home theater system, which typically includes several components such as a TV, audio system, media player, and game console. Instead of manually configuring and managing each component independently, a user can use a remote control (facade) that provides a straightforward interface for controlling the entire system.


Explain “promise” in TypeScript.

A Promise in TypeScript represents an asynchronous operation that eventually produces a value. It is an ES6 feature which TypeScript fully supports, allowing developers to write concise and easily maintainable code when dealing with asynchronous processes like network requests, timers, or file operations.

A Promise can have three states:

pending: The initial state, where the promise neither has a resulting value nor represents a reason for rejection.

fulfilled: The successful completion state, where the promise has resulted in a value.

rejected: The failure state, where the promise has a reason for not completing successfully.


Explain how you can manage default parameters in a function in TypeScript?

In TypeScript, you can manage default parameters in a function by assigning a default value in the function signature. When a parameter is not provided explicitly by the caller, the default value will be used instead. Default parameters help improve the function's flexibility and ensure that it behaves predictably even when some parameters are not supplied by the caller. Here's a simple example to illustrate the concept:

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In this example, the greeting parameter has a default value set to "Hello". When the greet function is called without a second argument, the default value of "Hello" is used for the greeting parameter.


Explain the distributive conditional type in TypeScript and its usage?

In TypeScript, Distributive Conditional Types are a powerful feature that allows you to conditionally apply different types based on the input type. They work by applying conditionals at the individual union member level, effectively distributing the conditional across the input union type members. The syntax used for creating a distributive conditional type is:

T extends U ? X : Y

Here, if T extends U (i.e., T is a subtype of U), the type of expression is X; otherwise, it's Y. Now let's dive into an example to better understand distributive conditional types:

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In this example, we define a Flatten type, which takes an input type T. If T is an array type, it evaluates to the element type of the array; otherwise, it returns the original type T. When we use Flatten<string[]>, it matches the condition T extends any[], and TypeScript determines the type of ArrayType to be string, since string[] is an array type.

Distributive conditional types are commonly used for type manipulation, tailor-made typings for specific data structures, and to create more precise types in various scenarios, improving type safety, maintainability, and the overall quality of the code.


Explain the "type" keyword in TypeScript and its usage?

In TypeScript, the type keyword is used to define custom, reusable type aliases that can represent complex types or provide a more meaningful name for an existing type. A type alias can refer to primitive types, union types, intersection types, object types, function types, arrays, or any other valid TypeScript type. Type aliases improve code readability, maintainability, and allow developers to create more expressive type annotations over time.

Here's an example illustrating how to use the type keyword:

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In this example, we define three type aliases (Point, Coordinate, and LineSegment) to represent the structure and required data for geometric shapes. We then use these aliases to create a function that calculates the length of a given line segment. The type keyword enhances the readability of our code, making it easier to understand and maintain.


Explain the "import type" statement in TypeScript?

In TypeScript, the import type statement is used to import only the type information from a module, without importing the actual values or side-effects associated with the module. This feature is beneficial when working with type-only imports, as it helps clarify the intent of the import, prevents accidental value imports, and can have some performance improvements in certain bundlers.

When using import type, the imported types will be erased during the compilation process, as TypeScript only uses them for checking types and does not include them in the emitted JavaScript code. This ensures that no runtime overhead is introduced as a result of importing types.


List some disadvantages of TypeScript.

While TypeScript offers many advantages, such as improved scalability, better code organization, and strong typing, there are some disadvantages to consider:

Learning curve: Developers familiar with JavaScript may need to invest time in learning TypeScript's concepts, syntax, and tools before becoming proficient. This learning curve might feel more significant for those who are new to typed languages in general.

Compilation step: TypeScript needs to be compiled to JavaScript before running in a browser or other JavaScript environments. This additional step can add time to the development process, impact build times, and introduce the need for build automation tools like Grunt or Gulp.

Potential inconsistencies: When using third-party JavaScript libraries, TypeScript relies on accurate typing declaration files (.d.ts) to infer the types and properties correctly. Not all libraries have these files or keep them up to date, which can lead to inconsistencies and potential type errors. Developers may need to create or update the typings manually to fix discrepancies.

Limited backward compatibility: TypeScript usually attempts to align with the latest ECMAScript features. However, some advanced TypeScript features may not have equivalent support in older browsers or JavaScript engines. Transpilers like Babel can help mitigate this by compiling TypeScript code into a compatible JavaScript version, but this adds an extra layer of complexity to the process.

Verbosity: Writing TypeScript code can become verbose due to the necessity of type annotations and interfaces. This extra typing might be considered as additional boilerplate code by some developers, but it's essential to achieve the benefits of static typing.

Not applicable everywhere: TypeScript mainly targets the JavaScript ecosystem, making it less suitable for contexts where JavaScript itself is unsuitable. For example, it might not be the ideal choice for low-level or performance-critical projects where languages like Rust, C++, or WebAssembly are better options.

Despite these downsides, TypeScript's benefits often outweigh the drawbacks, especially in large-scale projects that demand robust, maintainable, and scalable codebases.


What rules to follow when declaring a variable in TypeScript?

When declaring a variable in TypeScript, you should follow these rules and best practices:

Use keywords: Declare variables using the const, let, or var keywords. Prefer const for immutable values and let for mutable values; avoid using var as it has less predictable scoping rules.

const PI = 3.14159;

let counter = 0;

Type annotation: Provide type information for variables by adding a type annotation to ensure type safety and catch potential errors during compile-time.

let message: string = "Hello, TypeScript!";

Initialize: Always initialize variables during declaration to avoid issues with uninitialized or undefined variables.

let userName: string = "John"; // Initialization with value

Use descriptive names: Choose meaningful and descriptive variable names that indicate the purpose or value they hold.

let userName: string = "Alice";

Follow naming conventions: Stick with common naming conventions, like camelCase for variable names and PascalCase for types and interfaces.

const defaultGreeting: string = "Hello";

type UserProfile = { name: string; age: number };

Use union types: For a variable that should accept multiple types, use union types.

let id: number | string = 42;

id = "abc123";

By adhering to these rules and best practices, you can create robust, type-safe, and maintainable TypeScript code.


When is an unknown type used?

In TypeScript, the unknown type is used when a value can potentially have any type but still requires type-checking before performing any operations on it. It is a safer alternative to the any type, which disables type-checking completely. By using the unknown type, you are signaling that a value's type is not predetermined, yet you still want to enforce type-checking before using it.

Using unknown is particularly useful when dealing with user input, values from external libraries or APIs, deserialized data, or any situation where the exact type of a value cannot be determined beforehand. When a value is of type unknown, TypeScript enforces a type guard or a type assertion to narrow down its type before using it in the code, making it a more type-safe approach.

Here's an example:

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In this example, the processInput function accepts a parameter of type unknown, and it uses a type check (typeof input === "string") to narrow down the type to string before operating on it. This ensures that the code is type-safe and any potential issues are caught during compile time.


Explain noImplicitAny?

noImplicitAny is a property available with the TypeScript project configuration file ( tsconfig.json). This file alters how the compiler manages the project’s implicit “any” types. One can also set the noImplicitAny flag for either true or false. While deciding on the flag value, it is necessary to see the changes when it is turned on or off.

When set to false, noImplicitAny, the compiler defaults the type to “any”. However, when the noImplicitAny flag is true, the compiler tries to decipher the type giving a compile-time error if it isn't able to infer the type.


Differentiate union and intersection types?

Union types and intersection types in TypeScript are used to combine multiple types; however, they serve different purposes and represent different relationships.

Union Types (A | B): Union types represent a type that can be any one of several types. If a value is a union type, it means that the value can be of any type included in the union. When using a union type, you can only access members common to all types in the union.


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Intersection Types (A & B): Intersection types represent a type that combines multiple types into one. If a value is an intersection type, it means that the value must have properties and behaviors consistent with all types included in the intersection. In other words, a value of an intersection type must satisfy all the types combined.


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Differentiate between extends and implements in TypeScript?

In TypeScript, extends and implements are used to establish relationships between classes and interfaces. They serve different purposes in terms of inheritance and structural contract enforcement:

extends: The extends keyword is used to establish a subclass relationship between classes and also between interfaces. When a class or interface extends another class or interface, it inherits all properties and methods from the parent entity.

Class extending a class: Child class inherits properties and methods from the parent class. A class can only extend one class (single inheritance).

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Interface extending an interface: Child interface inherits properties and methods from the parent interface. An interface can extend multiple interfaces.

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implements: The implements keyword is used to enforce that a class adheres to the structural contract defined by one or more interfaces. When a class implements an interface, it must provide definitions for all properties and methods declared in the implemented interface(s). A class can implement multiple interfaces.

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In summary, extends is used for inheritance between classes and interfaces. A class or interface can extend another class or interface, respectively, to inherit properties and methods. On the other hand, implements is used by a class to ensure it adheres to the contract(s) defined by one or more interfaces. A class must provide definitions for all properties and methods declared in the implemented interface(s).


Explain how optional chaining works in TypeScript.

It allows access to properties and methods within an object without the need to check the validity of every reference in the chain. It uses the question mark with a period as follows (?.). It analyzes each reference in the chain and does a null or undefined check before making an access to its children.


What are abstract classes?

Abstract classes in TypeScript are specialized classes that cannot be instantiated directly but serve as a base for other classes to extend. They are used to define common properties, methods, or a basic structure that must be shared among child classes.

Abstract classes can include a mix of fully implemented methods, empty methods (without any implementation), and properties. Methods and properties marked as abstract in an abstract class must be implemented by the child classes that extend the abstract class.

Here's an example to illustrate the concept of abstract classes in TypeScript:

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In this example, Animal is an abstract class with an abstract species property and an abstract makeSound() method. It also includes a non-abstract move() method. The Dog and Cat classes extend Animal and provide implementations for the abstract members (property species and method makeSound).


What are the two syntaxes for type assertions in TypeScript?

In TypeScript, type assertions are used to inform the compiler of the intended type when the inferred type might be too general, or when converting a more general type to a more specific one. It's important to note that type assertions do not perform type conversion or runtime checks; they act as a hint to the compiler during static type checking. There are two syntaxes for type assertions:

Angle-bracket syntax:

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as syntax:

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Both syntaxes have the same effect from a type-checking perspective, but certain environments, like in JSX code in React projects, only support the as syntax due to the conflict of angle brackets with JSX elements. It's recommended to be consistent with your choice across a project to maintain code readability.


Differentiate between type inference and contextual typing?

In TypeScript, both type inference and contextual typing are mechanisms used to determine the types of variables or expressions without explicit type annotations. While they share a common purpose, they are used in different scenarios:

Type Inference is the process by which TypeScript automatically deduces the type of a variable or expression based on the value it is assigned or initialized with. If you don't provide an explicit type annotation, TypeScript attempts to infer the type from the context. Type inference usually occurs when you declare and initialize a variable, assign a value to a variable, or set a function's return value.

Contextual Typing is the mechanism by which TypeScript deduces the type of an expression based on its location in the code or the context in which it is used. Contextual typing occurs when an expression is part of a larger expression, and the type can be inferred by analyzing the surrounding code. TypeScript uses contextual typing in situations like event handlers, function parameter types, or type guards.


Tell me about the process to declare a constant in TypeScript?

In TypeScript, declaring a constant is similar to declaring variables, but with a crucial difference: the value of a constant can't change once it's initialized. This immutable nature ensures that the value remains consistent throughout the application, increasing code safety and enabling easy reasoning about the behavior.

To declare a constant in TypeScript, use the const keyword followed by an identifier, an optional type annotation, an equal sign, and the constant's initial value. Here's an example:

const PI: number = 3.14159;

const WELCOME_MESSAGE: string = "Hello, World!";

In this example, two constants are declared: PI and WELCOME_MESSAGE. The PI constant is declared with a type annotation indicating it's a number, while WELCOME_MESSAGE is declared as a string. Note that the type annotations are optional, as TypeScript can often infer the correct type based on the initial value. However, providing an explicit type annotation can improve code readability and catch potential errors during compilation.

Remember that once a constant has been initialized, its value cannot be changed:

PI = 3.14; // Compilation error: Cannot assign to 'PI' because it is a constant.


How do you implement Mixins in TypeScript?

Mixins are a way of combining multiple classes or interfaces into a single class. TypeScript supports mixing classes through function-based implementation.


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How can you implement async/await in TypeScript?

The async/await feature can be used to simplify handling asynchronous operations in TypeScript. Functions with the async keyword return a Promise that can be awaited.


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How can you use mapped types in TypeScript?

Mapped Types allow you to create new types based on existing ones by iterating over their properties and modifying them.


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What is the keyof type operator in TypeScript?

The keyof type operator is used to create a new type that contains a set of keys of the provided type.


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How to use accessors (getters and setters) in TypeScript?

TypeScript allows the creation of getter and setter methods using get and set keywords.


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How can you create a custom decorator in TypeScript?

Decorator is a special type of declaration that can be attached to a class, method, or property to extend or modify its behavior. To create a custom decorator, define a function with the corresponding parameters.


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How can you handle enums in TypeScript?

Conditional types enable you to create types that are based on other types and can be refined based on some condition.


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How can you use mapped types with conditional types in TypeScript?

You can combine mapped and conditional types by using a mapped type as the extends clause of a conditional type.


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Wrapping up

In conclusion, the above 100 advanced TypeScript questions and answers provide an in-depth insight into various TypeScript features, concepts, and patterns. Familiarity with these topics will not only help developers enhance their programming skills and build maintainable, robust, and scalable applications but also equip them to tackle challenging interview questions with confidence. For hiring managers, these questions can serve as a reference to assess the proficiency of prospective TypeScript developers. By leveraging these questions, hiring managers can effectively gauge the candidate's ability to handle advanced TypeScript concepts and ensure they are well-suited for the role.

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