Gain insights into TypeScript's core concepts, from basics to advanced type checking and iterators. Learn about static typing and JS library support through practical, hands-on experience.

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If you’re looking to move beyond vanilla JavaScript and take your skills to the next level, then you’ve come to the right place. TypeScript is an in-demand language that sits on top of JavaScript. This means you can do everything you can in JavaScript with TypeScript, but also enjoy countless other perks including support for JS libraries, NPM, static typing, and much more.

In this course, you’ll start from the beginning and work your way up to more advanced concepts like type checking, iterators, and manipulating objects and arrays. You'll get a chance to get your hands dirty with plenty of real practice along the way.

For JavaScript developers, TypeScript is a must-know language, not only for today but also for years to come. Don't miss out.

Learn TypeScript: The Complete Course for Beginners

# Creating a "NonNullable" type

The first custom type shows how to use `never` to tell TypeScript to not compile if a custom map is not respected. The code map is a generic variable that is neither `undefined` nor `null.` In the case that the value is either one, TypeScript does not compile.

type NoNullValue<T> = T extends null | undefined
  ? never
  : T;

function print<T>(p: NoNullValue<T>): void {
    console.log(p);  
}

print("Test"); // Compile
// print(null); // Does not compile

Creating a custom mapping requires using the keyword `extends` which acts like an `if` statement. The `T` is checked against what is written after the `extends`. If true, it does after the `?`. In that example, the value is `never` on **line 2**. When TypeScript returns `never`, it knows that it should never be in that state and stops compilation. Otherwise, it selects the value after the `:` which is the type `T` itself.

This lesson explains how to create your own mapped type.

Introduction

The Basics of Variables

Comment

Enum

Generic Type

Functions

Mapped Type

Objects

Index Signature

Variables Advanced

Exception

Alias

Type Checking

Iterators

Manipulating Objects and Array

Sharing Code

Conclusion

Custom Mapped Type

Creating a “NonNullable” type