TypeScript Interview Questions and Answers-Prepare for your TypeScript interview by reviewing these TypeScript interview questions - and get some practical tips along the way!
TypeScript is a language that aims at easing development of large scale applications written in JavaScript. TypeScript adds common concepts such as classes, modules, interfaces, generics and (optional) static typing to JavaScript. It is a superset of JavaScript: all JavaScript code is valid TypeScript code so it can be added seamlessly to any project. The TypeScript compiler emits JavaScript.
TypeScript is a superset of the JavaScript language. It differentiates itself from competitors like CoffeeScript and Dart in that plain JavaScript code can be intermixed with TypeScript. Therefore JavaScript is TypeScript. But TypeScript has to be compiled into JavaScript before it can run in any JavaScript engine (web browser or node.js). This means you cannot embed TypeScript into a web page directly using <script> tags, but TypeScript (in .TS files) can get compiled into JavaScript (in .JS files) for usage.
TypeScript supports definition files that can contain type information of existing JavaScript libraries, much like C++ header files can describe the structure of existing object files. This enables other programs to use the values defined in the files as if they were statically typed TypeScript entities. There are third-party header files for popular libraries such as jQuery, MongoDB, and D3.js. TypeScript headers for the Node.js basic modules are also available, allowing development of Node.js programs within TypeScript.
First public version released in 2012 (TypeScript 0.8)
TypeScript 0.9 released in 2013, adding support for generics
TypeScript 1.0 released in 2014, including support baked into Visual Studio
TypeScript 2.1 released in 2017
Current version is Typescript 2.6.2 - November, 2017
What is TypeScript?
Typescript is a strongly typed, object oriented, compiled language.  Typescript is a superset of JavaScript that brings users optional static types and solid tooling. Using TypeScript can help avoid painful bugs people commonly run into when writing JavaScript by type checking the code. It was designed by Anders Hejlsberg (designer of C#) at Microsoft. TypeScript is both a language and a set of tools. TypeScript is a typed superset of JavaScript compiled to JavaScript. In other words, TypeScript is JavaScript plus some additional features.

What are the features of TypeScript?
TypeScript is a JavaScript: TypeScript starts with JavaScript and ends with JavaScript. Typescript adopts the basic building blocks of your program from JavaScript. Hence, you only need to know JavaScript to use TypeScript. All TypeScript code is converted into its JavaScript equivalent for the purpose of execution.
TypeScript supports other JS libraries: Compiled TypeScript can be consumed from any JavaScript code. TypeScript-generated JavaScript can reuse all of the existing JavaScript frameworks, tools, and libraries.
JavaScript is TypeScript: This means that any valid .js file can be renamed to .ts and compiled with other TypeScript files. JavaScript is not suitable for developing large applications because of the complexity of large applications. Typescript allows the developers to develop large applications by using object oriented principles.
TypeScript is portable. TypeScript is portable across browsers, devices, and operating systems. It can run on any environment that JavaScript runs on. Unlike its counterparts, TypeScript doesn’t need a dedicated VM or a specific runtime environment to execute.
TypeScript and ECMAScript: TypeScript language features like Modules and class-based orientation are in line with the EcmaScript 6 specification.

What are the components of typescript?
There are mainly 3 components of TypeScript:
TypeScript Language: The most important part for developers is the new language. The language consists of new syntax, keywords and allows you to write TypeScript.
TypeScript Compiler: The TypeScript compiler is open source, cross-platform and open specification, and is written in TypeScript. Compiler will compile your TypeScript into JavaScript. And it will also emit error, if any. It can also help in concatenating different files to a single output file and in generating source maps.
TypeScript Language Service: The "Language Service" exposes an additional layer around the core compiler pipeline that are editor-like applications. The language service supports the common set of a typical editor operations like statement completions, signature help, code formatting and outlining, colorization, etc. TypeScript language service which powers the interactive TypeScript experience in Visual Studio, VS Code, Sublime, the TypeScript playground and other editor.

What are the main benefits of TypeScript?
TypeScript has following benefits:
  • It helps in code structuring
  • Intelligence
  • It has an API for DOM manipulation.
  • Use class based object oriented programming
  • Impose coding guidelines
  • Offers type checking
  • Compile time error checking
  • ES6 features support
Why use TypeScript?
TypeScript is pure object oriented like Java. TypeScript can help programmers to write object-oriented programs and have them compiled to JavaScript, both on server side and client side.
Provide an optional type system for JavaScript.
Provide planned features from future JavaScript editions to current JavaScript engines.
TypeScript supports Object Oriented Programming concepts like classes, interfaces, inheritance, etc.

What is TypeScript Declare Keyword?
It’s quite possible that JavaScript libraries/frameworks don’t have TypeScript definition files and yet you want to use them without any errors. The solution is to use the declare keyword. The declare keyword is used for ambient declarations where you want to define a variable that may not have originated from a TypeScript file.

Does TypeScript Support All Object Oriented Principles?
 YES. There are 4 main principles to Object Oriented Programming: Encapsulation, Inheritance, Abstraction, and Polymorphism. TypeScript can implement all four of them with its smaller and cleaner syntax.

What is inheritance in TypeScript?
Inheritance means that a subclass or derived class implicitly contains all non-overridden member of the base class.

What is a Module?
The module system is an interesting feature of TypeScript, the statically typed superset of JavaScript. Modules provide the possibility to group related logic, encapsulate it, structure your code and prevent pollution of the global namespace.

What are Modules in Typescript?
Modules in Typescripts are very useful. Modules in Typescript helps in organizing the code. There are two types of Modules:
Internal Modules: Internal Modules are now replaceable by using Typescript’s namespace.
External Modules: External Modules used to specify and load dependencies between multiple external js files. If there is only one js file used, then external modules are not relevant.

What is overriding in TypeScript?
Method Overriding, in TypeScript, is a language feature that allows a derived class to provide a specific implementation of a method that is already provided by one of it’s or base classes. A member in a derived class is said to override a member in a base class when the derived class member has the same name and kind instance, or is static, as the base class member. A derived class will inherit all public members from its base class. The implementation in the derived class replaces the implementation in the base by providing a method that has the same signature, the same name or parameter and the same return type as the method in the base class.

What are the built-in types in typescript?
These are also called the primitive types in typescript. These are of 5 types: –
Number type: It is used to represent number type values and represents double precision floating point values.
Syntax:var variable_name: number;
String type: It represents a sequence of characters stored as Unicode UTF-16 code. It is the same as JavaScript primitive type.
Syntax: var variable_name: string;
Boolean type: In typescript, it is used to represent a logical value. When we use the Boolean type, we get output only in true or false. It is also the same as JavaScript primitive type.
Syntax: var variable_name: bool;
Null type: it represents a null literal and it is not possible to directly reference the null type value itself.
Syntax: var variable_name:number=null;
Undefined type: it is the type of undefined literal. This type of built-in type is the sub-type of all the types.
Syntax: var variable_name:number=undefined;

What is variable in TypeScript?
The variable is simply a name of storage location and all variables must be identified with unique names and these unique names are called identifiers.

What are Classes in TypeScript?
The concept of classes is very similar to .NET/Java. A Class can have constructor, member variables, properties and methods. TypeScript also allows access modifiers “private” and “public” for member variables and functions.

Is it possible to debug any TypeScript file?
 Yes, it is possible. To debug it, you need .js source map file. If you are new to source map, read more here. So compile the .ts file with the --source map flag to generate a source map file.

What is the extension supported by TypeScript?
TypeScript supports .ts extension.

What is scope variable?  And what are different types of scope variable?
The scope is set of objects, variables and function and the JavaScript can have global scope variable and local scope variable. There are three types of scope variable
Global Scope Variable: The global scope is a window object and it’s used out of function and within the functions.
Local Scope Variable: The local scope is a function object and it’s used within the functions.
Class Scope Variable: These variables are also called fields. Fields or class variables are declared within the class but outside the methods. These variables can be accessed using the object of the class. Fields can also be static. Static fields can be accessed using the class name.

What is TypeScript Definition Manager (TSD)?
TypeScript Definition Manager (TSD) is a package manager to search and install TypeScript definition files directly from the community driven Definitely Typed repository.

What is tsconfig.json File?
The presence of a tsconfig.json file in a directory indicates that the directory is the root of a TypeScript project. The tsconfig.json file specifies the root files and the compiler options required to compile the project. And using this file, we can streamline building TypeScript project.
What is Duck-typing in TypeScript?
Duck-typing: It is used to verify the presence of certain properties in the objects to check their suitability. Here, two objects are considered to be of the same type if both share the same set of properties.

How can you get TypeScript and install it?
TypeScript can be installed and managed via npm, the Node.js package manager. To install TypeScript, first ensure the npm is installed properly. And then run the following command to install TypeScript globally on your system.
Installing
For the latest stable version:
npm install -g typescript
For our nightly builds:
npm install -g typescript@next

How to compile a typescript file?
A typescript file is used to write more manageable and complex code in AppBuilder but before that, they need to be compiled to JavaScript. Here are the steps which are followed while compiling a typescript file to JavaScript: –
It should be firstly verified that the typescript engine has been enabled or not. In the title bar, click your user name and select options.
In the project navigator, select and right-click the TS files that are to be compiled.
Choose compile to JavaScript.
Add a <script> reference for the JS file in the HTML code if needed.

What is the use of pipe symbol in TypeScript?
It is used to combine two or more data types to denote a Union Type.

What is the syntax to declare Nested Namespace in TypeScript?
The syntax to declare Nested Namespace is:
namespace firstnamespace {  
   export namespace secondnamespace { 
      export class classname {    }  
   }  

What is Static vs Dynamic Typing?
Typing has always been about the variables the program. If I create a static typing language
I don’t have to immediately tell the compiler that this particular integer and will always be an integer. It will be exactly what I stored in the program even If I forget while coding and changed its value by passing it to another class or something. A number of good language examples are C, C++, and Java. Dynamic typing, on the other hand, is the direct opposite of what the above is about. Ruby, Perl and of course our poster boy. JavaScript. This doesn’t mean anyone of these languages is inherently weak or bad. And that should be the focus.
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