What is it

Some JS libraries include typing file so they work with Typescript project without additional work.

But in case a library does not have a declaration file, you need to install the type package separately.

yarn add --dev @types/package-name

Ambient Declaration

Ambient declaration is a way to define types, variables, functions, or other entities without providing their actual implementation.

• These declarations describe the shape of code, allowing TypeScript to know about it for type-checking and tooling purposes.

--dev

--dev is indicating a dev dependency installation.

Dev dependency are things that are not direct dependencies for your project.

• will not be included in the bundle.

This is a good place to install typescript types declarations because they're only used in the build process.

Declaration File (.d.ts)

.d.ts file is a declaration file.

• Defines type information for JS code.
• Enables JS libraries to be used in TypeScript projects with proper type checking and autocompletion.

// js file: util.js
function sayHello(name) {
  console.log("Hello, " + name);
}

// d.ts file: util.d.ts
declare function sayHello(name: string): void;