Top 91 TypeScript Interview Questions And Answers

Question 1

Explain _generics_ in TypeScript

Accepted Answer

Generics are able to create a component or function to work over a variety of types rather than a single one. ```js /** A class definition with a generic parameter */ class Queue { private data = []; push = (item: T) => this.data.push(item); pop = (): T => this.data.shift(); } const queue = new Queue(); queue.push(0); queue.push("1"); // ERROR : cannot push a string. Only numbers allowed ```

Question 2

What are the benefits of TypeScript?

Accepted Answer

TypeScript has following benefits.

*   It helps in code structuring.
*   Use class based object oriented programming.
*   Impose coding guidelines.
*   Offers type checking.
*   Compile time error checking.
*   Intellisense.

Question 3

Do we need to compile TypeScript files and why?

Accepted Answer

Yes we do. Typescript is just a language Extension browsers can't interpret it. Converting from TypeScript to JavaScript is called compiling. Compiling doesn't mean binary code is created in this case. For this kind of translation, also the term transpilation is used instead of compilation.

Question 4

List the built-in types in Typescript

Accepted Answer

These are also called the primitive types in TypeScript:
* **Number** type: it is used to represent number type values and represents double precision floating point values.
```js
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.
```js
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.
```js
var variable_name: bool;
```
* **Null** type: it represents a null literal and it is not possible to directly reference the null type value itself.
```js
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.
```js
var variable_name:number = undefined;
```

Question 5

What is TypeScript and why one should use it?

Accepted Answer

TypeScript is a free and open-source programming language developed and maintained by Microsoft. It is a strict syntactical superset of JavaScript, and adds optional static typing and class-based object-oriented programming to the language.

Question 6

How to call base class constructor from child class in TypeScript?

Accepted Answer

We can call base class constructor using `super()`.

Question 7

How to perform _string interpolation_ in TypeScript?

Accepted Answer

* You can use _template iterals_,
* _Template literals_ are literals delimited with backticks``(`)``.

For example:

```ts
let value = 100;
console.log(`The size is ${value}`);
```

Question 8

What is TypeScript and why do we need it?

Accepted Answer

JavaScript is the only client side language universally supported by all browsers. But JavaScript is not the best designed language. It’s not a class-based object-oriented language, doesn’t support class based inheritance, unreliable dynamic typing and lacks in compile time error checking. And TypeScript addresses all these problems. In other words, TypeScript is an attempt to “fix” JavaScript problems.

TypeScript is a free and open source programming language developed and maintained by Microsoft. It is a strict superset of JavaScript, and adds **optional static typing** and **class-based object-oriented programming** to the language. TypeScript is quite easy to learn and use for developers familiar with C#, Java and all strong typed languages. At the end of day “TypeScript is a language that generates plain JavaScript files.”

As stated on [Typescript official website](http://www.typescriptlang.org/), “TypeScript lets you write JavaScript the way you really want to. TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. Any browser. Any host. Any OS. Open Source.” Where “**typed**” means that it considers the types of variables, parameters and functions.

Question 9

What are _Modules_ in Typescript?

Accepted Answer

Modules in Typescript helps in organizing the code. There are 2 types of Modules — Internal and External

* **Internal Modules** are now replaceable by using Typescript’s namespace.

* **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.

Question 10

What is TypeScript and why would I use it in place of JavaScript?

Accepted Answer

**TypeScript** is a superset of JavaScript which primarily provides optional static typing, classes and interfaces. One of the big benefits is to enable IDEs to provide a richer environment for spotting common errors as *you type the code*. For a large JavaScript project, adopting TypeScript might result in more robust software, while still being deployable where a regular JavaScript application would run.

In details:
* TypeScript supports new ECMAScript standards and compiles them to (older) ECMAScript targets of your choosing. This means that you can use features of ES2015 and beyond, like modules, lambda functions, classes, the spread operator, destructuring, today. 
* JavaScript code is valid TypeScript code; TypeScript is a superset of JavaScript. 
* TypeScript adds type support to JavaScript. The type system of TypeScript is relatively rich and includes: interfaces, enums, hybrid types, generics, union and intersection types, access modifiers and much more. TypeScript makes typing a bit easier and a lot less explicit by the usage of type inference.
* The development experience with TypeScript is a great improvement over JavaScript. The IDE is informed in real-time by the TypeScript compiler on its rich type information. 
* With strict null checks enabled (`--strictNullChecks` compiler flag) the TypeScript compiler will not allow undefined to be assigned to a variable unless you explicitly declare it to be of nullable type. 
* To use TypeScript you need a build process to compile to JavaScript code. The TypeScript compiler can inline source map information in the generated .js files or create separate .map files. This makes it possible for you to set breakpoints and inspect variables during runtime directly on your TypeScript code. 
* TypeScript is open source (Apache 2 licensed, see github) and backed by Microsoft. *Anders Hejlsberg*, the lead architect of C# is spearheading the project.

Top 91 TypeScript Interview Questions

What is the difference between .ts and .tsx extensions in TypeScript?

List the built-in types in Typescript

How to call base class constructor from child class in TypeScript?

Do we need to compile TypeScript files and why?

What is TypeScript and why do we need it?

What is TypeScript and why one should use it?

How to perform string interpolation in TypeScript?

What are Modules in Typescript?

What is TypeScript and why would I use it in place of JavaScript?

Explain generics in TypeScript

What are the benefits of TypeScript?

What is the difference between types String and string in TypeScript?

What is Type Erasure in TypeScript?

How could you check null and undefined in TypeScript?

Which access modifiers are implied when not specified?

Which object oriented terms are supported by TypeScript?

What are the difference beetween Typescript and JavaScript?

What is a TypeScript Map file?

Could we use TypeScript on backend and how?

What are assertion functions?

What is the purpose of Nullish Coalescing operator?

How do we create an enum with string values?

What does the pipe, | mean in TypeScript?

What is Optional Chaining in TypeScript?

Describe what are conditional types in TypeScript?

How to make Arrays that can only be read, TypeScript?

How can we use optional chaining in TypeScript?

What is Decorators in TypeScript?

When to use interfaces and when to use classes in TypeScript?

What is Interface in TypeScript?

How to implement class constants in TypeScript?

Does TypeScript support all object oriented principles?

What is getters/setters in TypeScript?

What is the difference between Classes and Interfaces in Typescript?

How to choose between never, unknown, and any in TypeScript?

What are the use cases for a const assertion?

What is Typings in Typescript?

Explain Project References and its benefits

What is the default access modifier for members of a class in TypeScript?

Explain how and why we could use property decorators in TS?

How to make a readonly tuple type in TypeScript?

What is the unique symbol is used for?

What is the fundamental difference between Optional Chaining (?.) and Non-null assertion operator (!) in TypeScript?

What does Short-Circuiting mean in TypeScript?

List a few rules of private fields in TypeScript

What are some use cases of template literal types in TypeScript?

How to check the type of a variable or constant in TypeScript?

How to add types to an interface from another interface or extend types in TypeScript?

What is the difference between type and interface in TypeScript?

What is the difference between enum and const enums?

Does TypeScript supports function overloading?

What are different components of TypeScript?

How can you allow classes defined in a module to be accessible outside of the module?

What is the difference between Private and Protected variables in TypeScript?

Why do we need to use abstract keyword for classes and their methods in TypeScript?

What is Structural Typing?

How TypeScript is optionally statically typed language?

What is Mixin Class in TypeScript?

How to use external plain JavaScript libraries in TypeScript?

How does override keyword works in TypeScript?

Explain what is never datatype in TypeScript?

Explain what is Currying in TypeScript?

How to define a TypeScript class which has an index signature?

Why we need Index Signature in TypeScript?

What is Mixin Constructor Type?

What is dynamic import expression?

What is the difference between unknown and any type?

How to exclude property from type in TypeScript?

Why is the infer keyword needed in TypeScript?

What is the difference between interface vs type statements?

Explain the difference between declare enum vs declare const enum

What is the need of --incremental flag in TypeScript?

Explain when to use declare keyword in TypeScript

What does the tsconfig option lib do?

How to make a union type from a type alias or interface properties in TypeScript?

Is it possible to generate TypeScript declaration files from JS library?

What is the benefit of import assertions features in TypeScript?

What are Ambients in TypeScripts and when to use them?

What are the differences between the private keyword and private fields in TypeScript?

What is the difference between `.ts` and `.tsx` extensions in TypeScript?

What is the difference between types `String` and `string` in TypeScript?

How could you check `null` and `undefined` in TypeScript?

How do we create an `enum` with `string` values?

What does the pipe, `|` mean in TypeScript?

How to choose between `never`, `unknown`, and `any` in TypeScript?

What are the use cases for a `const` assertion?

How to make a `readonly` tuple type in TypeScript?

What is the `unique symbol` is used for?

What is the fundamental difference between Optional Chaining (`?.`) and Non-null assertion operator (`!`) in TypeScript?

List a few rules of `private` fields in TypeScript

What is the difference between `type` and `interface` in TypeScript?

What is the difference between `enum` and `const enum`s?

What is the difference between `Private` and `Protected` variables in TypeScript?

Why do we need to use `abstract` keyword for classes and their methods in TypeScript?

How does `override` keyword works in TypeScript?

Explain what is `never` datatype in TypeScript?

What is dynamic `import` expression?

What is the difference between `unknown` and `any` type?

Why is the `infer` keyword needed in TypeScript?

Explain the difference between `declare enum` vs `declare const enum`

What is the need of `--incremental` flag in TypeScript?

Explain when to use `declare` keyword in TypeScript

What does the `tsconfig` option `lib` do?

How to make a `union` type from a type alias or interface properties in TypeScript?

What is the benefit of `import` assertions features in TypeScript?

What are the differences between the `private` keyword and `private` fields in TypeScript?

How the `never` datatype can be useful?

Is there a way to check for both `null` and `undefined` in TypeScript?

In `a?.b.c`, if `a.b` is `null`, then `a.b.c` will evaluate to `undefined`, right?

What does the `const` assertion mean in TypeScript?