What is a “.d.ts” file in TypeScript?
TLDR
.d.ts files are called type declaration files. They exist for one purpose only: to describe the shape of an existing module and they only contain type information used for type checking.
Introduction
Upon learning the basics of TypeScript, you unlock superpowers.
At least that’s how I felt.
You automagically get warnings on potential errors, you get auto-completion out of the box in your code editor.
While seemingly magical, nothing with computers are.
So, what’s the trick here, TypeScript?
In a clearer language, how does TypeScript know so much? How does it decide what API is correct or not? What methods are available on a certain object or class, and which aren’t?
The answer is less magical.
TypeScript relies on types.
Occasionally, you do not write these types, but they exist.
They exist in files called declaration files.
These are files with a .d.ts ending.
A simple example
Consider the following TypeScript code:
// valid
const amount = Math.ceil(14.99)
// error…