TypeScript interview questions (that you need to master)

Without a shadow of a doubt, JavaScript is a widely-used, powerful and robust scripting language. Nevertheless, it has some limitations for constructing large-scale applications due to its complexity.

In response to that issue, Anders Hejlsberg from Microsoft (also a member of the C# team) developed TypeScript in 2012. Developed and maintained nowadays by Microsoft under the Apache 2 license, this superset of JavaScript with more features extends the original language with extra syntax, providing a robust editing interface.

Therefore, it can be said that TypeScript is an object-oriented, tightly typed programming language. Its code is ultimately transformed to JavaScript, so TypeScript can be used in any environment that supports JavaScript such as browsers (client-side), Node.js (server-side), and any applications based on it.

If you are looking forward to getting a job in web development related to this Javascript extension, we have selected the most commonly asked TypeScript interview questions and the respective answers, at both basic and advanced levels.

We also had included several TypeScript Code Challenges that will help you to be more prepared for your interview ahead. So when you had mastered our selection, you will be ready to excel in your TypeScript interview.

Multiple job opportunities.
One code challenge.

Get rid of repetitive hiring processes for all the positions you apply to, and access many job offers by taking a single real-world assessment.

Basic TypeScript questions

State the main differences between JavaScript and TypeScript

While JavaScript is a scripting language, TypeScript is defined as an Object-Oriented language. Therefore, it gives support for modules and optional parameter functions that JavaScript lacks.

TypeScript is a typed superset of JavaScript that compiles to plain JavaScript in the end, supporting object-oriented with classes, interfaces, and statically typed programming like C# or Java. Basically, TypeScript is the ES6 version of JavaScript with additional features.

Why a JavaScript developer should consider using TypeScript?

TypeScript makes the life of JavaScript developers easier, since:

It has better IDE Support: There is a wide range of IDEs with excellent support for TypeScript, like Atom, IntelliJ/WebStorm, Sublime, or Visual Studio & VS code.

It allows interoperability: Since TypeScript is closely related to JavaScript, it has great interoperability capabilities to work with JavaScript libraries.

It let us use new features of ECMAScript: TypeScript supports ECMAScript standards and transpile them to ECMAScript targets.

It provides Static Typing: While JavaScript is dynamically typed, TypeScript adds type inference. Even if we don’t explicitly type the types, they are still there to prevent run-time errors.

It provides strict null checking: Errors are common in JavaScript programming, but we can avoid them using TypeScript since we cannot use a variable that is not known to the compiler. Therefore, we can find errors while writing the code instead of having to run the script.

In a nutshell, TypeScript is fast, simple, and easy to learn for a JavaScript programmer. It runs on any browser or JavaScript engine, using the same syntax and semantics, so a backend developer may write front-end code faster, working with existing JavaScript frameworks and libraries.

Does TypeScript have any disadvantages?

Like any programming language, there are some disadvantages while using TypeScript. It is known that it takes a long time to compile the code, does not support abstract classes, and if a TypeScript application is run on the browser, it must be compiled first into JavaScript.

If you are a web developer that had mastered JavaScript for decades, TypeScript doesn’t bring anything really new to the table. Moreover, to use any third-party library we must create a definition file and the quality of type definition files could be an issue.

Mention the types of components in TypeScript

There are three types of components in TypeScript:

The Language: the syntax, keywords, and type annotations.

The TypeScript Compiler: the compiler (tsc) converts the instructions written in TypeScript to the JavaScript equivalent.

The TypeScript Language Service: it supports the common set of typical editor operations and exposes an additional layer around the core compiler pipeline, editor-like applications.

What are the types of TypeScript?

The Type System represents the different types of values supported by TypeScript, which checks the validity of the supplied values previously to do any manipulation or data storage by the program. Type can be classified as a) Built-in that includes boolean, null, number, string, undefined, void, and b) User-defined, which includes arrays, classes, enumerations (enums), interfaces, and tuple.

What are Variables in TypeScript?

Variables are named spaces in the memory, used to store values. The syntax for declaring a variable in TypeScript includes a colon (:) after the variable name, and then its type.

In a similar way to in JavaScript, we use the var keyword to declare a variable whose name must be an alphabet or numeric digits. We cannot start the name with digits and the name cannot contain spaces or special characters other than the underscore (_) or the dollar($) sign.

Mention the object-oriented terms supported by TypeScript

TypeScript supports the object-oriented terms Classes, Data Types, Inheritance, Interfaces, Member functions, and Modules.

What are Classes in TypeScript?

In order to take advantage of object-oriented techniques like encapsulation and abstraction, TypeScript introduced classes. Each class includes constructor, properties, and methods, and is compiled to plain JavaScript functions by the TypeScript compiler in order to work across browsers and platforms.

What are loops in TypeScript?

A loop allows repeating the execution of a statement or a series of statements. In TypeScript the following types of loops are implemented:

For Loop: it is a definite loop with a fixed number of iterations.

While Loop: when the supplied condition turns to be true, the while loop executes the given instructions.

Do..While Loop: this loop acts like to the while loop, except that the condition isn't evaluated the first time the loop runs.

How to create objects in Typescript?

Objects are collections of one-of-a-kind keys and values that resemble a dictionary. Keys resemble arrays and are referred to as associative arrays. An array, on the other hand, employs numbers to index the values.

Therefore, an object lets you use any type as the key. Therefore, an Object type in TypeScript refers to any value with properties and can be defined by specifying the properties and kinds of those properties.

What is contextual typing?

When one side of the equation has a type but the other does not, the TypeScript compiler determines the whole form for us. That way, we don’t need to spend time writing that part of the code, since TypeScript will figure it out for us.

Advanced TypeScript interview questions

What are Mixins In Javascript?

Mixin is a way of building up classes from reusable components and then building them by combining simpler partial classes. Instead of a class A extending class B to get its functionality, function B takes class A. Then, it returns a new class with this added functionality, where function B is a mixin.

Explain how TypeScript supports optional parameters in a function

TypeScript compiler flag an error when we try to invoke a function without providing the exact number and types of parameters, as declared in its function signature.  A way to surpass this difficulty is to use optional parameters, introduced by a question mark sign (‘?’). This tells the compiler that the parameters may or may not receive a value that can be appended as optional.

What is TypeScript Declare Keyword?

Since JavaScript libraries or frameworks don’t have TypeScript declaration files if we want to use them without compilation errors to declare keywords. The declare keyword states the ambient declarations and methods where we define a variable that may exist elsewhere.

Explain what are Generics in TypeScript

TypeScript Generics is a tool to create reusable components that can work with several data types, rather than a single data type. It also provides type safety without compromising the performance, or productivity. In generics, the type parameter is written between the open (<) and close (>) brackets.

Generics allow us to create generic classes, functions, methods, and interfaces.

What is Enum in TypeScript?

Enums stand for enumerations: a TypeScript data type that let us define a set of named constants, a collection of related values that can be numeric or string values. It makes it easier to document intent or create a set of distinct cases.

Explain the use of the Rest parameters

The rest parameter is used to pass zero or more values to a function. We declare it by prefixing the three-dot characters (‘…’) before the parameter and allowing the functions to have a variable number of arguments without using the arguments object. The rest parameter is useful where we have an undetermined number of parameters.

However, there are some rules to follow in the rest parameter. We can have only one rest parameter per function and it must be an array type, as well as it must be the last parameter in the parameter list.

What are Type assertions in TypeScript?

Type assertions are similar to typecasting in other languages but without type checking or restructuring of data like in C# or Java. Typecasting comes with runtime support, meanwhile, type assertion has no impact on runtime. However, type assertion can be used by the compiler to provide hints on how we want the code to be analyzed.

What are ambient declarations in TypeScripts?

The ambient declarations instruct the compiler about the source code that rests elsewhere, allowing us to use existing popular JavaScript libraries like jquery, angularjs, nodejs, etc.

If these source codes are absent at runtime and are invoked, the run will break without warning. Those ambient declarations files are like docs files, so if the source changes, the docs need to be updated or compiler errors will occur.

What is JSX in TypeScript?

JSX is an embeddable XML-like syntax that can be transformed into a valid JavaScript. It became popular with the React framework, and TypeScript fully supports embedding, type checking, and compiling JSX directly into JavaScript.

In order to use JSX in our code, we need to inject it through a file with a .tsx extension and enable the jsx option. Then, we can use one of the three JSX modes in TypeScript:

The preserve mode: it keeps the JSX as part of the output to be further used in another transform step. The output will have a .jsx file extension as well.

The react mode: it emits React.createElement, does not need to go through a JSX transformation to use it, and the output will have a .js file extension.

The react-native mode: is the equivalent of the preserve and keeps all JSX, but the output has a .js file extension.

How TypeScript files can be supported from Node Modules?

TypeScript has a series of declaration files to guarantee that TypeScript and JavaScript support works well. Those .d.ts files contain declarations of the APIs in the JavaScript language as well as the standard browser DOM APIs, fair defaults for any given code. However, we can configure the lib setting in the tsconfig.json to specify which declaration files should be used.

These can be done in TypeScript with a feature similar to @types/ support, to override a specific built-in lib. It will check for a scoped @typescript/lib-* package in node modules to select which lib files to include. Then, the package manager should be used to install the specific package to take over for our particular library.

What is a TypeScript Map file?

It is a source map file that contains information about the original files of a given project. The .map files are source map files that allow TypeScript tools to monitor the emitted JavaScript code against the TypeScript source files that created it. Using the TypeScript map files debuggers can debug the TypeScript file instead of the JavaScript file.

TypeScript Code Challenges

With these challenges, we have reached the end of this article on TypeScript Interview Questions. We hope that you feel now more confident and remember: make sure you study and practice as much as possible. Good luck with that interview ahead!

Multi-apply: How to apply to many job opportunities with one single Challenge

We have managed to get most of the companies we collaborate with to adopt our Rviewer challenges in their validation process as an official test.

And here's the bomb: This allows us to offer you, at last, Multi-Apply: with a single technical challenge you will be able to access more than one job offer. Yes, you heard it right: MANY job opportunities by taking a SINGLE code challenge.

Master coding interviews for growing tech companies

Discover here the ultimate guide to coding interview processes. Find out how they work and how you can optimize yourself as a developer to attract the attention of tech companies and get better results in their coding interviews.

Related articles

Stay tuned: receive the latest product updates

Subscribe to our newsletter to keep up with the latest trends!