wiki/content/20201001105545-lib_d_ts.md

---
date: 2020-10-01
id: afc1ca85-af24-4ef6-bcd8-91193c4fbd32
title: lib.d.ts
---

# Introduction

This is where TypeScript "cheats" and ships with a file called
[lib.d.ts](https://github.com/microsoft/TypeScript/blob/master/lib/lib.d.ts).
This file contains ambient declarations for a whole bunch of common
[JavaScript](20200613170905-javascript) constructs present in JavaScript
runtimes and the DOM.

-   This file is automatically included in the compilation context of a
    TypeScript project.
-   The objective of this file is to make it easy for you to start
    writing type checked JavaScript code.

# Modifying Native Types

If you want to be a clever clogs you can add stuff to the interfaces in
lib.d.ts by creating a global module (ie: `global.d.ts`)

# Example

## lib.d.ts magic

The following code is only possible because of lib.d.ts. If it's used
with the `nolib` option, TypeScript doesn't know that all JavaScript
objects have a `toString` function.

    var foo = 123;
    var bar = foo.toString();

## Example of what's in the lib.d.ts

``` typescript
declare var window: Window;

interface Window extends EventTarget, WindowTimers, WindowSessionStorage, WindowLocalStorage, WindowConsole, GlobalEventHandlers, IDBEnvironment, WindowBase64 {
    animationStartTime: number;
    applicationCache: ApplicationCache;
    clientInformation: Navigator;
    closed: boolean;
    crypto: Crypto;
    // so on and so forth...
}
```