Typescript - ( Declaration | Definition ) file (.d.ts file)

declaration files describe the library - .d.ts files

They are ambient.

They provide code completion for Javascript.

By default all visible “@types” packages are included in your compilation. Packages in node_modules/@types of any enclosing folder are considered visible; specifically, that means packages within ./node_modules/@types/, ../node_modules/@types/, ../../node_modules/@types/, and so on.

The definitions can be found at TypeSearch.

Other location:

• https://github.com/microsoft/TypeScript/tree/master/lib

• See DefinitelyTyped for the repository containing this files for each library.

Example with lodash

yarn add @types/lodash --dev
// or
npm install -S @types/lodash

by example

/** The number of widgets present */
declare var foo: number;
declare const foo: number; // Read only
declare let foo: number; // Block scoped

• declare function declare functions

declare function greet(greeting: string): void;

• declare namespace to declare to describe types or values accessed by dotted notation

declare namespace myLib {
    function makeGreeting(s: string): string;
    let numberOfGreetings: number;
}

Function - Overloaded Function (Overloaded)

declare function getWidget(n: number): Widget;
declare function getWidget(s: string): Widget[];

with Typescript - tsc (official compiler)

tsc --declaration ./js/index.ts
# or 
tsc --declaration -p tsconfig.json

where:

• -d or –declaration generates a '.d.ts' file.

• https://devblogs.microsoft.com/typescript/the-future-of-declaration-files-2/