# Introduction

Deno is a JavaScript/TypeScript runtime with secure defaults and a great
developer experience.

It's built on V8, Rust, and Tokio.

## Feature Highlights

- Secure by default. No file, network, or environment access (unless explicitly
  enabled).
- Supports TypeScript out of the box.
- Ships a single executable (`deno`).
- Has built-in utilities like a dependency inspector (`deno info`) and a code
  formatter (`deno fmt`).
- Has
  [a set of reviewed (audited) standard modules](https://github.com/denoland/deno/tree/master/std)
  that are guaranteed to work with Deno.
- Scripts can be bundled into a single JavaScript file.

## Philosophy

Deno aims to be a productive and secure scripting environment for the modern
programmer.

Deno will always be distributed as a single executable. Given a URL to a Deno
program, it is runnable with nothing more than
[the ~15 megabyte zipped executable](https://github.com/denoland/deno/releases).
Deno explicitly takes on the role of both runtime and package manager. It uses a
standard browser-compatible protocol for loading modules: URLs.

Among other things, Deno is a great replacement for utility scripts that may
have been historically written with bash or python.

## Goals

- Only ship a single executable (`deno`).
- Provide Secure Defaults
  - Unless specifically allowed, scripts can't access files, the environment, or
    the network.
- Browser compatible: The subset of Deno programs which are written completely
  in JavaScript and do not use the global `Deno` namespace (or feature test for
  it), ought to also be able to be run in a modern web browser without change.
- Provide built-in tooling like unit testing, code formatting, and linting to
  improve developer experience.
- Does not leak V8 concepts into user land.
- Be able to serve HTTP efficiently

## Comparison to Node.js

- Deno does not use `npm`
  - It uses modules referenced as URLs or file paths
- Deno does not use `package.json` in its module resolution algorithm.
- All async actions in Deno return a promise. Thus Deno provides different APIs
  than Node.
- Deno requires explicit permissions for file, network, and environment access.
- Deno always dies on uncaught errors.
- Uses "ES Modules" and does not support `require()`. Third party modules are
  imported via URLs:

  ```javascript
  import * as log from "https://deno.land/std/log/mod.ts";
  ```

## Other key behaviors

- Remote code is fetched and cached on first execution, and never updated until
  the code is run with the `--reload` flag. (So, this will still work on an
  airplane.)
- Modules/files loaded from remote URLs are intended to be immutable and
  cacheable.