wiki/content/20200930105954-ambient_declarations.md

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---
date: 20200930
id: e1048c95-3baa-445e-a943-bff7b86a0f29
title: Ambient Declarations
---
# Introduction
Ambient declarations allow you to safely use existing popular JavaScript
libraries and incrementally migrate your
JavaScript/CoffeeScript/Other-Compile-To-Js-Language project to
TypeScript.
# Declaration Files
Syntax is simple:
``` typescript
declare var foo: any
```
Declarations can be put in a `.ts` or `.d.ts` file, ie: `global.d.ts` or
`vendor.d.ts`
# Best Practices
- Use `.d.ts`
- Use [interfaces](20200929162220-interfaces) where possible