Introduction

In this chapter, you will learn about TypeScript declaration files. Declaration files give you the ability to give TypeScript more information about how a function or class is structured.

Why is it important to understand how declaration files work? Technically, declaration files speak directly to the core motivations for why TypeScript is becoming so popular. One of the common rationales for using TypeScript is because it guides developers through the application process. Let's walk through a real-world example as a case study.

In pure JavaScript, if we start working with a code library that we've never used before that formats dates, such as Moment JS, we would have to start by looking through the documentation in order to know what type of data we can pass to the Moment JS functions. When working with a new library, it is tedious work to figure out requirements, such as how many function arguments are required for each function and what data type each argument needs to be.

With the declaration files, however, TypeScript informs the text editor of the requirements for every function that a library has. So, instead of having to rely solely on documentation and Google searches, the text editor itself informs the developer how to work with each function. For example, the text editor, with the help of TypeScript, would inform us that the Moment JS format function takes in zero to one arguments, and the optional argument needs to be a string. And declaration files make all of this possible.