Top 19 frequently asked TypeScript interview questions

Common interview questions that get asked about TypeScript

Introduction

TypeScript is everywhere on the web these days so if you're a front developer or even a full stack developer it's important to know the key concepts of TypeScript. Some of the below questions probably could be used for other OOP languages like c# or Java.

The below questions can be used for all levels. When interviewing I usually give the same test to all the different levels as this can give you a baseline to what level you think the developer is.

TypeScript interview questions

• What are some advantages of using TypeScript?

One or many of the below you should be looking for.

• Static typing
• New Features + Browser Compatibility
• Intellisense
• Type annotations
• Readability
• Fast refactoring
• What is the use of the tsconfig.json file?

The tsconfig.json file specifies the root files and the compiler options required to compile the project.

• How do you declare a variable in TypeScript?

You’re looking for of the below in the example.

Example:

let name = "Dave"; // TypeScript will automaically create this as a string;

let name: string = "Dave" // Using ":"

• Which keyword is used for inheritance in TypeScript?

extends is the keyword you should be looking for.

Example:

class dog extends IAnimal

• In TypeScript can you inherit off a class?

Yes, you can use the keyword extends

Example:

class Animal {
   numberOfLegs = 0;
}

class dog extends Animal

• Can you in inherit from more than one class?

No, you can’t in TypeScript.

• Is there anything that can help you with this?

mixins create partial classes which we can combine to form a single class that contains all the methods and properties from the partial classes.

• What is the super keyword in TypeScript?

Super is a TypeScript keyword which can be used by developers in expressions for base class constructor and base class properties reference.

Example:

class Dog extends Animal {
  constructor(name) {
    super();
    this.name = name;
    super.doSomething();
  }
}

• What is namespace in Typescript?

A namespace is simply a way to logically group related classes or interfaces in a wrapper.

Example:

namespace Transport { 
   export class Car {  }  
   export class Bike {  }  
}

• What is default visibility for properties/methods in Typescript classes?

public is the default

• What are all the other access modifiers that TypeScript supports?

public - All the members of the class, its child classes, and the instance of the class can access.

protected - All the members of the class and its child classes can access them. But the instance of the class can not access.

private - Only the members of the class can access them.

• How do you define an optional property in TypeScript?

You're looking for with ? or | undefined

Example :

// Example 1
interface ISomeInterface {
  foo?: string;
}

// Example 2
interface ISomeInterface {
   foo: string | undefined;
}

• What are decorators in TypeScript?

Decorators are simply functions that modify a class, property, method, or method parameter.

• How do you declare a decorator?

The syntax is an “@” symbol followed by a function.

Example:

@readonly
class foo {
    name?: string;
}

• What is function Overloading in TypeScript?

Function overloading is the ability to create multiple functions of the same name with different implementations.

Example:

class Account {
    Id: number;

    search(Id: number);
    search(name:string);
    search(value: any) {
    if (value && typeof value == "number") {
        //Do something
    }
    if (value && typeof value == "string") {
        //Do Something
    }
   }
}

• Can you access static methods in TypeScript?

Yes, you can!

Example

class Foo {
  
    static Hello() { 
    }
}

import { Foo } from "path/Foo";
Foo.Hello();

• What use is a constructor in TypeScript?

A constructor is responsible for initializing the properties/variable of the class.

Example

class Foo {
   name = "";
   id?: number;

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

• What is theas keyword used for in TypeScript?

The as is additional syntax for Type assertion in TypeScript.

Example:

let user = formData as User;

• What is generics in TypeScript?

Enables a developer to create a component that can work over a variety of types rather than a single one. This allows developers to consume these components and use their own types

A generic class/interface can be defined using <T>

Example:

function someType<T>(anything: T): T {     
    return anything; 
}

let myOutputString = someType<string>("myString");  // type of myOutputString variable will be 'string'

let myOutputNumber = someType<number>(12); // type of myOutputNumber variable will be 'number'

Conclusion

I think the questions above give a well-rounded TypeScript interview and allows the developer to show of their skills. With some questions being very specific on TypeScript and other questions focusing on OOP principles.

I think questions alone are not the best way to go about interviews. Other ideas to go along with questions:

• Coding challenges, like the Fizz Buzz test maybe not this exact one as I feel everyone has heard of this!
• Give them a task(s) beforehand to code and then explain to you what they have done in the interview
• Explain a project they have worked on as part of a team or individually

A note from In Plain English

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