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TypeScript Interview Questions

TypeScript Interview Questions

The word of development is rapidly growing, so as the programming languages associated with it. TypeScript is one of these increasingly popular programming languages. We will discuss several advance TypeScript interview questions while further continuing here for your practice. TypeScript is an object-oriented, open-source, strongly typed and compiled programming language. It’s a typed superset of JavaScript (compiled to JavaScript) and can be used for JavaScript application development supported to server-side and client-side execution. Simplifying it, TypeScript is an advanced version of JavaScript with additional features to help both programming language and new tool requirement for developers.

Development History

Developed at Microsoft by Anders Hejlsberg, the lead designer of C# and creator of Turbo Pascal and Delphi, TypeScript makes it first public appearance on October 2012 and currently running with a stable version of 3.4.5 released on April 2019.

With the conclusion of basic TypeScript knowledge, now let’s continue with our expert selected, crucial TypeScript interview questions with the suggested answer that may help you crack your next big interview.

Advantages

  • With TypeScript, production of pure object-oriented code is possible even with limited knowledge as this is completely object-oriented programming.
  • It can be used for both server side and client side development alike.
  • TypeScript comes with types which makes code easier to read and void major errors.
  • As it’s a package, TypeScript can be installed to projects via npm. This will make new features available and compile to all modern browsers.
  • TypeScript offers an API for DOM manipulation.
  • It also has the concept of the namespace by Module defining.
  • With IDE support here, developers will save a ton of valuable time here.
Last update: 31 Oct 2019, 32 Questions and Answers

Most Frequently Asked TypeScript Interview Questions And Answers With Examples:

TypeScript is an object-oriented, open source programming language maintained and developed by Microsoft. It is developed to overcome the flaws of JavaScript. As a superset of JavaScript, TypeScript can be used to develop large JavaScript applications with support for modules and classes, ES6 features, Type-checking, API definition, JavaScript packaging support, class library, and more.

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Steps to install TypeScript environment in Windows are given below.
  • First, download the latest version of TypeScript from its official website.
  • We can install TypeScript using Node.js, Visual Studio, or other supported editors, but the first two are the most common way to install it.
  • Download and run the .msi installer for Node support.
  • Then, we will use npm to install TypeScript using Node.js. Just running the below-mentioned command in the Windows command will install the latest version.
    npm install -g typescript
  • We can check the version of TypeScript by running the following command in the terminal.
    tsc -v
  • To get help for other options here, use the following command
    tsc -h
  • To install TypeScript using Visual Studio, download Visual Studio Code, and launch it.
  • Now by right clicking on the file → open in the command prompt, users may traverse to the file’s path.
  • Now, creating a TypeScript file is possible just by adding a new file with a .ts extension. All the features of TypeScript will be available out of the box
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TypeScript is strongly typed to syntactical benefits to this language while still allowing users to write normal JavaScript. It also ensures the production of predictable results with fewer errors and more secure.

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S.no TypeScript JavaScript
1. Typescript is an object-oriented language. JavaScript is a scripting language.
2. Static typing feature is available here. Doesn’t support static typing.
3. TypeScript supports modules. JavaScript doesn’t support modules.
4. It has an interface. Doesn’t have any interface.
5. The optional parameter function is supported here. JavaScript doesn’t support optional parameter function.
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Here is a list of benefits developers can enjoy with the use of TypeScript.

  • TypeScript supports optional static typing.
  • Incredible code scalability due to interface oriented development.
  • Compliance with ES-next.
  • Enhances code readability and quality with Types.
  • Use of advance compilers to check errors.
  • It includes dependency injection, which offers great testing and controller based APIs.
  • Extremely predictable and maintainable code-base.
  • Implementation of SOLID design patterns into a language is easy here.
  • Clean code generation due to static typing.
  • Refactoring is easy and fasts with TypeScript tools.
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In TypeScript, decorators are used to offering a way to add both meta-programming syntax and annotations for members and declarations.

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To enable decorators in TypeScript, developers first have to enable the option of experimentalDecorators via command line or tsconfig.json.

Here is the command line to use.
  • $tsc --target ES5 --experimentalDecorators
  • For tsconfig.json, use the following syntax.
    {
       "compilerOptions": {
          "target": "ES5",
          "experimentalDecorators": true
       }
    }
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In TypeScript, static typing means parameters, variables, and object members have types that compiler recognizes at the time of compile. This helps in detecting early errors faster than unit test runs. It also greatly helps IDEs with auto-completion and API exploration with statically typed DOM. Static typing is an important chapter which supports a number of TypeScript Interview Questions, so it’s crucial to practice this one.

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TypeScript developers can use the access modifiers to control the accessibility of the class members. There are two types of access modifiers in TypeScript; private and public modifiers. Class members are public by default, but users can add a private modifier if they wish.

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The prototype property in TypeScript allows users to include methods and properties to an object. It allows cloning objects (complex ones also) without coupling to their specific classes. Prototype objects can create full copies due to the accessibility of objects to each other’s private field.

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Super is a TypeScript keyword which can be used by developers in expressions for base class constructor and base class properties reference. This call is only allowed in constructors of derived classes. Call for this keyword consist the keyword super with an argument list closed in parentheses.

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In TypeScript, the callback function is used to schedule a call after some asynchronous processing is completed. It is passed to another function as a parameter that allows callback function to be called when the process of asynchronous processing is completed.

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Developers add tsconfig.json file or TypeScript configuration file to a project to guide the compiler as the JavaScript files are generated by it. This file also contains flags and options that are essential to run Angular applications.

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This one is a regular TypeScript interview question that almost every candidate faces. Here are the basic data types present in TypeScript described below.

  • Number Types: In TypeScript, number values are floating values which have a type of number. Users can assign any numeric value, including hexadecimal, binary, decimals, and octal literals also.
  • String Types: When a user desires to implement textual data, string types will be needed. It gets denoted by the keyword string.
  • Boolean Types: It is used for declaring variables and accompanied by the keyword boolean.
  • Enum Type: It can be used for numeric values with friendly names. It is declared with the keyword of “enum.”
  • Void Types: Used for the function that doesn’t return any value.
  • Null Types: Used to declare a variable type null. It can be assigned to boolean and number value.
  • Undefined Types: It is used to store the value undefined.
  • Any Types: It can be implemented to ensure data value to declare said values.
  • Never Types: It represents the data type of values that never occur.
  • Array Type: Used to declare data arrays.
  • Tuple Types: This data type allows users to create an array where the fixed number type elements are known but not the same.
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Mixin is a class present in TypeScript that implements a distinct aspect to functionality. It offers a code reuse form based on composing behavior. It can be used to declare a class that extends constructor, adds new members to class, and returns the class itself.

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Users can test both null and undefined at once in TypeScript using the following syntax.

if (x == null) {

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A module in TypeScript is a file containing functions, classes, or values. Users can make them public (to be visible from other modules) just by exporting them. Non exported modules stay private to other modules. These TypeScript interview questions are extremely career-promoting for both freshers as well as experienced professionals when practiced properly.

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The export keyword is used to export any declaration such as function, variables, interface, or type alias. The export * from "module" can be used by a module to wrap one or more modules and combine together their exporting. Whereas, the import keyword is used to export declaration.

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The Arrow function in TypeScript is used to eliminate the repetitive typing of function. It also lexically captures the meaning of both this and arguments.

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The two significant benefits that Arrow function in TypeScript offers:

  • They are less verbose compared to traditional functions.
  • This function takes the “this” from its surroundings.
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S.no TypeScript ES6
1. Extremely comfortable to eradicate the development error. ES6 is more comfortable in development deployment.
2. It’s an open source programming language. It’s a scripting language.
3. It supports all primitive data types. ES6 doesn’t support primitive data types.
4. TypeScript has 3 scopes: Global, Class, and Local. It has only two scopes: Global and Local.
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In TypeScript, Getter is the method to obtain the value of a specific property, whereas the setter method is used to set the value of the specific property.

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Developers can use the optional parameter to declare parameters in function optional so that the requirement to pass the value to optional parameters gets eliminated. We have to use “?” at the end of a parameter to mark it optional as just shown below.

function functionName(par1: number, par2?: number) {
}

Here par2 is an optional parameter.

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TypeScript uses the keyword ‘extends’ to support inheritance. The syntax will be as following:

class child_class_name extends parent_class_name

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