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TypeScript Use Cases

  • Large-scale projects:

    TypeScript's type system can help catch errors early in the development process, making it well-suited for large and complex projects.

  • JavaScript libraries and frameworks:

    TypeScript can be used to develop libraries and frameworks that are consumed by other JavaScript programs.

  • Tools that operate on or transpile to JavaScript:

    TypeScript's type information can be used to generate JavaScript that is safer or more optimized.

  • Teams with many developers:

    TypeScript's optional type annotations make it easier for developers to understand and reason about code written by others.

  • Java or C# developers:

    TypeScript's syntax and features are similar to those of Java and C#, making it an easy transition for developers familiar with those languages.

Top Skills to Look For in a TypeScript Developer

  • Strong understanding of JavaScript:

    TypeScript is a superset of JavaScript, so a strong understanding of JavaScript is crucial for working with TypeScript.

  • Familiarity with type annotations and type inference:

    TypeScript's type system is one of its key features, so a developer should be familiar with how to use type annotations and how the type system infers types.

  • Experience with object-oriented programming:

    TypeScript adds class-based object-oriented programming to JavaScript, so experience with concepts such as classes, interfaces, and inheritance is important.

  • Knowledge of ECMAScript standards:

    Understanding of ECMAScript standards such as ES6 and ES7 is useful, as TypeScript is built on top of these standards.

  • Familiarity with React, Angular or Vue:

    Many developers use TypeScript on frontend library such as React, Angular and Vue, so experience with those library can be an advantage

  • Experience with tooling and build process:

    Familiarity with tools such as Webpack and npm scripts, can be useful for integrating TypeScript into a development workflow.

  • Strong debugging skills:

    A developer should be able to troubleshoot and debug TypeScript code effectively.

  • Familiarity with testing frameworks and libraries:

    Experience with frameworks such as Jest, Mocha, or Jasmine can be useful for testing TypeScript code.

  • Ability to work with legacy codebase:

    Experience with migrating to TypeScript from javascript, or ability to work on legacy codebase that contains non-typed javascript is a good skill.

  • Continues learning mindset:

    TypeScript is an evolving language and the ecosystem is constantly updating, a developer who is willing to continuously learning new features, best practices and tools will be an advantage.

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TypeScript Interview Questions

What is TypeScript and why would you use it over JavaScript?

TypeScript is an open-source, statically-typed superset of JavaScript that adds optional static typing to the language. It is developed and maintained by Microsoft and widely used for web and Node.js development. TypeScript transpiles to plain JavaScript, which means that TypeScript code can run in any JavaScript environment.


Here are some key reasons why you might choose TypeScript over JavaScript:


  1. Static Typing: TypeScript allows you to specify types for variables, function parameters, and return values. This provides early error detection during development, as the TypeScript compiler can catch type-related issues before runtime. Static typing can improve code quality, reduce bugs, and enhance maintainability, especially in large and complex projects.


  1. IDE Support: TypeScript is well-supported by modern Integrated Development Environments (IDEs) like Visual Studio Code, WebStorm, and others. These IDEs can provide features like auto-completion, intelligent code navigation, and real-time error checking thanks to TypeScript’s type annotations.


  1. Enhanced Code Quality: The use of static types can make your code more self-documenting. It becomes easier for other developers to understand the purpose of variables and functions just by looking at their type annotations. This can lead to more maintainable and readable code.


  1. Large-Scale Projects: TypeScript excels in large and complex codebases. It helps maintain code integrity and prevents subtle bugs that might go unnoticed in JavaScript. Strong typing allows for better collaboration among team members.


  1. Better Tooling: TypeScript has a rich ecosystem of tools and libraries designed specifically for type-aware development. This includes third-party libraries, type definition files for existing JavaScript libraries, and a package manager (npm) that can manage TypeScript dependencies.


  1. Community and Adoption: TypeScript has gained significant traction in the web development community. Many popular frameworks and libraries, including Angular, React, and Vue.js, offer TypeScript support, and a large portion of the web development community uses TypeScript in their projects.


  1. Compatibility: TypeScript is backward-compatible with JavaScript, which means you can gradually adopt it in an existing JavaScript project. You can convert existing JavaScript files to TypeScript one by one, providing gradual type safety improvements.


  1. Stronger Tooling for Future ECMAScript Features: TypeScript often adopts proposed ECMAScript features before they become part of the JavaScript standard. This allows developers to use upcoming language features with type safety earlier than in plain JavaScript.


  1. Community Contributions: TypeScript is an open-source project with an active community that contributes to its development and maintains type definition files for various libraries. This helps ensure that TypeScript remains up-to-date with modern web development practices.


While TypeScript offers many advantages, it’s worth noting that it introduces an additional step in the development process (type checking and compilation). Some developers may prefer the flexibility of JavaScript, especially in smaller projects or when prototyping. The decision to use TypeScript or JavaScript should be based on the specific needs and goals of your project and team.

What are the basic types in TypeScript?

The basic types in TypeScript include “boolean”, “number”, “string”, “array”, “tuple”, “enum”, “unknown”, “any”, “void”, “null”, “undefined”, “never”, “object”, and “symbol”.

Explain type annotations and type inference in TypeScript

– Type Annotations: Explicitly specifying the type for a variable, function parameter, or function return value. E.g., “let name: string = ‘Alice’;”

– Type Inference: TypeScript automatically inferring the type of a variable from its value if not explicitly annotated. E.g., “let name = ‘Alice’;” TypeScript knows “name” is a “string”.

What are interfaces in TypeScript and how do they differ from classes?

Interfaces in TypeScript are used to define the shape of an object, ensuring it has the required properties with the appropriate types. Unlike classes, interfaces don’t have implementations and cannot be instantiated. They are purely for defining types.

How does TypeScript handle optional properties in interfaces?

In TypeScript, interfaces can have optional properties by marking them with a “?”. For instance, “interface Person { name: string; age?: number; }” Here, “name” is mandatory but “age” is optional.

Explain the difference between "any" and "unknown" types

Both are top types in TypeScript, but with a key difference:

– “any”: A type that can represent any JavaScript value with no type-checking constraints.

– “unknown”: A type-safe counterpart of “any”. Variables of type “unknown” need to be type-checked or cast to another type before they can be accessed or modified.

What are Union and Intersection types in TypeScript?

– Union Types (“|”): Allow a value to be one of several types. E.g., “let value: string | number;” means “value” can be a string or number.


– Intersection Types (“&”): Allow a value to have multiple types at once. E.g., if “TypeA” and “TypeB” are types, a variable of type “TypeA & TypeB” would have all properties of both types.

What are decorators in TypeScript?

Decorators are a special kind of declaration that can be attached to class declarations, methods, properties, or parameters. They are used for meta-programming and can be useful for scenarios like Angular’s @Component decorators.

Explain the purpose and usage of the "never" type in TypeScript

The “never” type represents a value that never occurs. It’s typically used to indicate that a function never returns (like when it throws an error) or to represent exhaustive type checks.

How do you declare a read-only property in TypeScript?

You can use the “readonly” modifier in front of a property declaration within a class or interface to make it read-only. E.g., “readonly name: string;”.

What is the difference between "enum" and "const enum" in TypeScript?

Both are used to define named constants. The key difference is that “const enums” are completely inlined at compile time. This means that, unlike regular “enums”, “const enums” won’t produce any JavaScript code when transpiled.

Explain the difference between type aliases and interfaces in TypeScript

While both can be used to describe object shapes and other types, key differences include:

– Type aliases can represent primitives, unions, tuples, and other types, whereas interfaces are limited to object shapes.

– Interfaces are more extensible through declaration merging.

– Type aliases can’t be re-opened to add new properties, unlike interfaces.

How do namespaces work in TypeScript, and why might you use them?

Namespaces in TypeScript are a way to group related code under a common name. They can help in organizing code and preventing global scope pollution. However, with the ES6 module system, the need for namespaces has diminished.

What is the key difference between "==" and "===" operators in TypeScript (and JavaScript)?

– “==”: Also known as the loose equality operator, it will try to perform type coercion if the types of the two variables compared are different.

– “===”: Also known as strict equality, it checks both value and type, without performing type coercion.

How can you create a custom type guard in TypeScript?

A custom type guard is a function whose return type is a type predicate. For instance:



function isFish(pet: Fish | Bird): pet is Fish {

    return (pet as Fish).swim !== undefined;



Here, “pet is Fish” is our type predicate.


These questions and answers should provide a comprehensive look at a candidate’s knowledge and understanding of TypeScript at a mid to senior level.

Can you explain the concept of declaration merging in TypeScript?

In TypeScript, declaration merging refers to the compiler’s ability to take two separate declarations and merge them into a single construct. This is a unique feature of TypeScript, and understanding it can be crucial when working with existing JavaScript libraries and augmenting types or adding types to untyped code.


Here are some key points to understand about declaration merging:


  1. Basic Idea:

If the TypeScript compiler finds multiple interface or namespace declarations with the same name in the same scope, it will treat them as a single interface or namespace.


  1. Merging Interfaces:

In TypeScript, interfaces with the same name will merge their members:



interface Box {

    height: number;

    width: number;



interface Box {

    scale: number;



let box: Box = {height: 5, width: 6, scale: 0.5};



In the above example, the two “Box” interfaces are merged as if they were written as a single interface.


  1. Function Members:

When function members in merged interfaces have the same name, they are treated as overloads:



interface Document {

    createElement(tagName: “div”): HTMLDivElement;



interface Document {

    createElement(tagName: “span”): HTMLSpanElement;



// The merged Document interface will have both overloads for createElement.



  1. Merging Namespaces:

Just as with interfaces, namespaces of the same name will merge their exported members:



namespace Animals {

    export class Zebra {}



namespace Animals {

    export interface Legged { numberOfLegs: number; }

    export class Dog {}



// Equivalent to:

// namespace Animals {

//     export interface Legged { numberOfLegs: number; }

//     export class Zebra {}

//     export class Dog {}

// }



  1. Merging Namespaces with Classes, Functions, and Enums:


It’s possible to merge namespaces with classes, functions, and enums:



class Album {

    label: Album.AlbumLabel;


namespace Album {

    export class AlbumLabel { /* … */ }




This pattern describes a pattern in JavaScript where a function has properties:



function buildLabel(name: string): string {

    return buildLabel.prefix + name + buildLabel.suffix;


namespace buildLabel {

    export let prefix = “Hello, “;

    export let suffix = “”;




This merging pattern doesn’t work for all declarations, only for functions, classes, and enums.


  1. Disallowed Merges:


You can’t merge different types of declarations. For instance, attempting to merge a function with a namespace that has a non-function member will raise an error:



function foo() { return “bar”; }

namespace foo {

    export let bar = 42;  // Error: Cannot merge namespace ‘foo’ with a function.




  1. Module Augmentation:


While declaration merging allows you to merge entities in the global scope, with ES6 modules, it’s common to augment types defined in modules (i.e., not in the global scope). For this, TypeScript has a feature called module augmentation. This allows you to add more members to an existing module.




Declaration merging is a powerful concept in TypeScript that allows developers to augment or combine types and other constructs. It provides flexibility, especially when working with external libraries or enhancing types in modular codebases. However, it’s essential to use this feature judiciously to avoid unintended consequences and ensure maintainability.

How do you handle legacy Javascript code and migration it to TypeScript?

Handling legacy JavaScript code and migrating it to TypeScript can be a complex task, depending on the size and structure of the codebase. However, here’s a step-by-step strategy to make the process smoother:


  1. Research & Backup:

   – Before initiating migration, understand the current codebase, its dependencies, and functionality.

   – Always create a backup or branch out the current version of the project to ensure that you can revert back if necessary.


  1. Initial TypeScript Setup:

   – Install the necessary TypeScript packages using npm or yarn: 


     npm install typescript –save-dev


   – Create a “tsconfig.json” file in your project root which will contain configurations for the TypeScript compiler.


  1. Incremental Migration:

   – TypeScript supports the concept of incremental adoption. You can set the “”allowJs”” option to “true” in “tsconfig.json”, which lets you transpile JavaScript files with TypeScript.

   – Rename a few .js files to .ts and fix any immediate issues. This allows you to gradually transition without having to migrate everything at once.


  1. Add Type Definitions:

   – Install type definitions for your dependencies using DefinitelyTyped (i.e., “@types/dependency-name”).

   – Initially, use the “any” type liberally to bypass strict type checking. Gradually replace “any” types with proper types as you understand the data structures and requirements better.


  1. Refactor Gradually:

   – Once files are renamed to .ts, start adding types to functions, variables, and classes. Start with commonly used utility functions or core modules.

   – Use interfaces for object shapes and create type aliases for commonly used custom types.


  1. Utilize TypeScript Features:

   – Implement features like Enums, Namespaces, or Decorators as they fit into your project.

   – Replace JSDoc type comments (if you have them) with actual TypeScript types.


  1. Strict Mode:

   – As you progress with the migration, turn on stricter options in “tsconfig.json” like “”strict”: true”, “”noImplicitAny”: true”, and others. This helps to enforce better type-checking and catch potential issues.


  1. Testing:

   – Ensure that tests are updated with the migrated code.

   – If you have unit tests, run them frequently to make sure your changes haven’t introduced regressions.

   – Consider setting up a CI (Continuous Integration) process that compiles the TypeScript code and runs tests to ensure everything is working as expected.


  1. Documentation & Training:

   – Update any project documentation to include TypeScript specifics.

   – Consider offering training sessions or workshops for your team, especially if they are new to TypeScript.


  1. Review & Iteration:

   – Periodically review the migration progress. Check for areas that may need refactoring or can benefit from more advanced TypeScript features.

   – Iterate on your types. As the application grows and evolves, you might find that some types can be refined further.


  1. Community & Tools:

   – Utilize tools like “ts-migrate” that help in automating some of the migration processes.

   – Stay updated with the TypeScript community. They often provide valuable insights, best practices, and solutions to common problems.


  1. Completion:

   – Once all files are migrated and you feel confident in the codebase’s stability, you can update the “tsconfig.json” to stop allowing JavaScript files.


Remember, the key to a successful migration is incremental progress. It’s okay to have a mix of JavaScript and TypeScript during the transition phase. Over time, as you address more of the codebase and become familiar with TypeScript’s features, you’ll find the process becomes faster and more intuitive.

Can you explain the difference between let, var, and const in TypeScript?

Certainly! In TypeScript (as well as in modern JavaScript ES6/ES2015), “let”, “var”, and “const” are all used to declare variables, but they have different scopes and characteristics. Here’s an overview of the differences:


  1. “var”:

   – Function Scope: Variables declared with “var” are function-scoped, meaning they are only accessible within the function they are declared in. If declared outside of any function, they are globally scoped.

   – Hoisting: Variables declared with “var” are hoisted to the top of their function or global scope. However, only the declaration is hoisted, not the initialization. 

   – Re-declaration: Within the same scope, you can re-declare a variable that was defined using “var” without getting any error.

   – Example:


     function exampleVar() {

         if (true) {

             var x = 10;


         console.log(x); // Outputs: 10




  1. “let”:

   – Block Scope: Variables declared with “let” are block-scoped, meaning they are only accessible within the nearest set of curly braces (function, if-else block, loop, etc.).

   – No Hoisting: While technically variables declared with “let” are hoisted, accessing them before their declaration in the code will result in a “ReferenceError”.

   – No Re-declaration: In the same scope, you can’t re-declare a variable that was defined using “let”.

   – Example:


     function exampleLet() {

         if (true) {

             let y = 20;


         console.log(y); // Error: y is not defined




  1. “const”:

   – Block Scope: Just like “let”, variables declared with “const” are block-scoped.

   – Value Immutability: Once you assign a value to a “const” variable, you can’t reassign a new value to it. This doesn’t mean the value itself is immutable, just that the variable cannot be reassigned.

   – Mandatory Initialization: When declaring a variable using “const”, you must provide an initial value.

   – No Re-declaration: In the same scope, you can’t re-declare a variable that was defined using “const”.

   – Example:


     const z = 30;

     z = 40; // Error: Assignment to constant variable.





– Prefer using “let” and “const” over “var” in modern TypeScript and JavaScript development due to their block-scoping characteristics and the clearer intent they provide (“const” for values that shouldn’t change and “let” for variables that will change). 

– “var” is mostly retained for backward compatibility with older versions of JavaScript.