# Deno Manual

[toc]

## Disclaimer

A word of caution: Deno is very much under development. We encourage brave early
adopters, but expect bugs large and small. The API is subject to change without
notice.

[Bug reports](https://github.com/denoland/deno/issues) do help!

## Introduction

### Philosophy

Deno aims to be a useful multitool tool for the modern programmer.

It will always be distributed as a single executable - and that executable will
be sufficient software to run any deno program. Given a URL to a deno program,
you should be able to execute it with nothing more than <a
href="https://deno.land/benchmarks.html#size">the 50M deno executable</a>.

Deno explicitly takes on the role of both runtime and package manager. It uses a
standard browser-compatible protocol for loading modules: URLs.

Deno provides security guarantees about how programs can access your system with
the default being the most restrictive secure sandbox.

Deno provides <a href="https://github.com/denoland/deno_std">a set of reviewed
(audited) standard modules</a> that are guaranteed to work with Deno.

Deno is opinionated and defines <a
href="https://github.com/denoland/deno_std#style-guide">style guides</a> and has
<a href="https://github.com/denoland/deno_std/tree/master/prettier">automated
formatters</a>.

### Design goals

- Support TypeScript out of the box.

- No `package.json`. No npm. Not explicitly compatible with Node.

- Like the browser, allows imports from URLs:

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

- 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. See `~/.deno/src` for details on the cache.)

- Uses "ES Modules" and does not support `require()`.

- File system and network access can be controlled in order to run sandboxed
  code. Access between V8 (unprivileged) and Rust (privileged) is only done via
  serialized messages defined in this
  [flatbuffer](https://github.com/denoland/deno/blob/master/src/msg.fbs). This
  makes it easy to audit. For example, to enable write access use the flag
  `--allow-write` or for network access `--allow-net`.

- Only ship a single executable.

- Always dies on uncaught errors.

- [Aims to support top-level `await`.](https://github.com/denoland/deno/issues/471)

### Browser compatibility

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.

## Setup

### Binary Install

Deno works on OSX, Linux, and Windows. Deno is a single binary executable. It
has no external dependencies.

[deno_install](https://github.com/denoland/deno_install) provides convenience
scripts to download and install the binary.

Using Shell:

```
curl -fL https://deno.land/x/install/install.sh | sh
```

Or using PowerShell:

```powershell
iex (iwr https://deno.land/x/install/install.ps1)
```

_Note: Depending on your security settings, you may have to run
`Set-ExecutionPolicy RemoteSigned -Scope CurrentUser` first to allow downloaded
scripts to be executed._

With [Scoop](https://scoop.sh/):

```
scoop install deno
```

Deno can also be installed manually, by downloading a tarball or zip file at
[github.com/denoland/deno/releases](https://github.com/denoland/deno/releases).
These packages contain just a single executable file. You will have to set the
executable bit on Mac and Linux.

Once it's installed and in your \$PATH, try it:

```
deno https://deno.land/welcome.js
```

### Build from source

```
# Fetch deps.
git clone --recurse-submodules https://github.com/denoland/deno.git
cd deno
./tools/setup.py

# You may need to ensure that sccache is running.
# (TODO it's unclear if this is necessary or not.)
# prebuilt/mac/sccache --start-server

# Build.
./tools/build.py

# Run.
./target/debug/deno tests/002_hello.ts

# Test.
./tools/test.py

# Format code.
./tools/format.py
```

#### Prerequisites

To ensure reproducible builds, deno has most of its dependencies in a git
submodule. However, you need to install separately:

1. [Rust](https://www.rust-lang.org/en-US/install.html) >= 1.31.1
2. [Node](https://nodejs.org/)
3. Python 2.
   [Not 3](https://github.com/denoland/deno/issues/464#issuecomment-411795578).

Extra steps for Mac users:

1. [XCode](https://developer.apple.com/xcode/)
2. Openssl 1.1: `brew install openssl@1.1` (TODO: shouldn't be necessary)

Extra steps for Windows users:

1. Add `python.exe` to `PATH` (e.g. `set PATH=%PATH%;C:\Python27\python.exe`)
2. Get [VS Community 2017](https://www.visualstudio.com/downloads/) with
   `Desktop development with C++` toolkit and make sure to select the following
   required tools listed below along with all C++ tools.
   - Windows 10 SDK >= 10.0.17134
   - Visual C++ ATL for x86 and x64
   - Visual C++ MFC for x86 and x64
   - C++ profiling tools
3. Enable `Debugging Tools for Windows`. Go to `Control Panel` → `Programs` →
   `Programs and Features` → Select
   `Windows Software Development Kit - Windows 10` → `Change` → `Change` → Check
   `Debugging Tools For Windows` → `Change` -> `Finish`.

#### Other useful commands

```
# Call ninja manually.
./third_party/depot_tools/ninja -C target/debug

# Build a release binary.
DENO_BUILD_MODE=release ./tools/build.py :deno

# List executable targets.
./third_party/depot_tools/gn ls target/debug //:* --as=output --type=executable

# List build configuration.
./third_party/depot_tools/gn args target/debug/ --list

# Edit build configuration.
./third_party/depot_tools/gn args target/debug/

# Describe a target.
./third_party/depot_tools/gn desc target/debug/ :deno
./third_party/depot_tools/gn help

# Update third_party modules
git submodule update
```

Environment variables: `DENO_BUILD_MODE`, `DENO_BUILD_PATH`, `DENO_BUILD_ARGS`,
`DENO_DIR`.

## API reference

### deno --types

To get an exact reference of deno's runtime API, run the following in the
command line:

```
deno --types
```

[This is what the output looks like.](https://gist.github.com/ry/46da4724168cdefa763e13207d27ede5)

### Reference websites

[TypeScript Deno API](https://deno.land/typedoc/index.html).

If you are embedding deno in a Rust program, see
[Rust Deno API](https://deno.land/rustdoc/deno/index.html).

## Examples

### An implementation of the unix "cat" program

In this program each command-line argument is assumed to be a filename, the file
is opened, and printed to stdout.

```ts
(async () => {
  for (let i = 1; i < Deno.args.length; i++) {
    let filename = Deno.args[i];
    let file = await Deno.open(filename);
    await Deno.copy(Deno.stdout, file);
    file.close();
  }
})();
```

The `copy()` function here actually makes no more than the necessary kernel ->
userspace -> kernel copies. That is, the same memory from which data is read
from the file, is written to stdout. This illustrates a general design goal for
I/O streams in Deno.

Try the program:

```
> deno https://deno.land/x/examples/cat.ts /etc/passwd
```

### TCP echo server

This is an example of a simple server which accepts connections on port 8080,
and returns to the client anything it sends.

```ts
const { listen, copy } = Deno;

(async () => {
  const addr = "0.0.0.0:8080";
  const listener = listen("tcp", addr);
  console.log("listening on", addr);
  while (true) {
    const conn = await listener.accept();
    copy(conn, conn);
  }
})();
```

When this program is started, the user is prompted for permission to listen on
the network:

```
> deno https://deno.land/x/examples/echo_server.ts
⚠️  Deno requests network access to "listen". Grant? [yN] y
listening on 0.0.0.0:8080
```

For security reasons, deno does not allow programs to access the network without
explicit permission. To avoid the console prompt, use a command-line flag:

```
> deno https://deno.land/x/examples/echo_server.ts --allow-net
```

To test it, try sending a HTTP request to it by using curl. The request gets
written directly back to the client.

```
> curl http://localhost:8080/
GET / HTTP/1.1
Host: localhost:8080
User-Agent: curl/7.54.0
Accept: */*
```

It's worth noting that like the `cat.ts` example, the `copy()` function here
also does not make unnecessary memory copies. It receives a packet from the
kernel and sends back, without further complexity.

### File server

This one serves a local directory in HTTP.

```
alias file_server="deno  --allow-net --allow-read \
  https://deno.land/x/http/file_server.ts"
```

Run it:

```
% file_server .
Downloading https://deno.land/x/http/file_server.ts...
[...]
HTTP server listening on http://0.0.0.0:4500/
```

And if you ever want to upgrade to the latest published version:

```
file_server --reload
```

### Linking to third party code

In the above examples, we saw that Deno could execute scripts from URLs. Like
browser JavaScript, Deno can import libraries directly from URLs. This example
uses a URL to import a test runner library:

```ts
import { test, assertEqual } from "https://deno.land/x/testing/mod.ts";

test(function t1() {
  assertEqual("hello", "hello");
});

test(function t2() {
  assertEqual("world", "world");
});
```

Try running this:

```
> deno https://deno.land/x/examples/example_test.ts
Compiling /Users/rld/src/deno_examples/example_test.ts
Downloading https://deno.land/x/testing/mod.ts
Compiling https://deno.land/x/testing/mod.ts
running 2 tests
test t1
... ok
test t2
... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```

Note that we did not have to provide the `--allow-net` flag for this program,
and yet it accessed the network. The runtime has special access to download
imports and cache them to disk.

Deno caches remote imports in a special directory specified by the `$DENO_DIR`
environmental variable. It default to `$HOME/.deno` if `$DENO_DIR` is not
specified. The next time you run the program, no downloads will be made. If the
program hasn't changed, it won't be recompiled either.

**But what if `https://deno.land/` goes down?** Relying on external servers is
convenient for development but brittle in production. Production software should
always bundle its dependencies. In Deno this is done by checking the `$DENO_DIR`
into your source control system, and specifying that path as the `$DENO_DIR`
environmental variable at runtime.

**How do you import to a specific version?** Simply specify the version in the
URL. For example, this URL fully specifies the code being run:
`https://unpkg.com/liltest@0.0.5/dist/liltest.js`. Combined with the
aforementioned technique of setting `$DENO_DIR` in production to stored code,
one can fully specify the exact code being run, and execute the code without
network access.

**It seems unwieldy to import URLs everywhere. What if one of the URLs links to
a subtly different version of a library? Isn't it error prone to maintain URLs
everywhere in a large project?** The solution is to import and re-export your
external libraries in a central `package.ts` file (which serves the same purpose
as Node's `package.json` file). For example, let's say you were using the above
testing library across a large project. Rather than importing
`"https://deno.land/x/testing/mod.ts"` everywhere, you could create a
`package.ts` file the exports the third-party code:

```ts
export { test, assertEqual } from "https://deno.land/x/testing/mod.ts";
```

And throughout project one can import from the `package.ts` and avoid having
many references to the same URL:

```ts
import { test, assertEqual } from "./package.ts";
```

This design circumvents a plethora of complexity spawned by package management
software, centralized code repositories, and superfluous file formats.

## Command line interface

### Flags

```
> deno -h
Usage: deno script.ts

Options:
        --allow-read    Allow file system read access.
        --allow-write   Allow file system write access.
        --allow-net     Allow network access.
        --allow-env     Allow environment access.
        --allow-run     Allow running subprocesses.
    -A, --allow-all     Allow all permissions.
        --recompile     Force recompilation of TypeScript code.
    -h, --help          Print this message.
    -D, --log-debug     Log debug output.
    -v, --version       Print the version.
    -r, --reload        Reload cached remote resources.
        --v8-options    Print V8 command line options.
        --types         Print runtime TypeScript declarations.
        --prefetch      Prefetch the dependencies.
        --info          Show source file related info
        --fmt           Format code.

Environment variables:
        DENO_DIR        Set deno's base directory.
```

### Environmental variables

There are several env vars that control how Deno behaves:

`DENO_DIR` defaults to `$HOME/.deno` but can be set to any path to control where
generated and cached source code is written and read to.

`NO_COLOR` will turn off color output if set. See https://no-color.org/. User
code can test if `NO_COLOR` was set without having `--allow-env` by using the
boolean constant `deno.noColor`.

### V8 flags

V8 has many many internal command-line flags, that you can see with
`--v8-options`.
[It looks like this.](https://gist.github.com/ry/1c5b080dcbdc6367e5612392049c9ee7)

Particularly useful ones:

```
--async-stack-trace
```

## Internal details

### Schematic diagram

<img src="schematic_v0.2.png">

### Profiling

To start profiling,

```sh
# Make sure we're only building release.
export DENO_BUILD_MODE=release
# Build deno and V8's d8.
./tools/build.py d8 deno
# Start the program we want to benchmark with --prof
./target/release/deno tests/http_bench.ts --allow-net --prof &
# Exercise it.
third_party/wrk/linux/wrk http://localhost:4500/
kill `pgrep deno`
```

V8 will write a file in the current directory that looks like this:
`isolate-0x7fad98242400-v8.log`. To examine this file:

```sh
D8_PATH=target/release/ ./third_party/v8/tools/linux-tick-processor
isolate-0x7fad98242400-v8.log > prof.log
# on macOS, use ./third_party/v8/tools/mac-tick-processor instead
```

`prof.log` will contain information about tick distribution of different calls.

To view the log with Web UI, generate JSON file of the log:

```sh
D8_PATH=target/release/ ./third_party/v8/tools/linux-tick-processor
isolate-0x7fad98242400-v8.log --preprocess > prof.json
```

Open `third_party/v8/tools/profview/index.html` in your brower, and select
`prof.json` to view the distribution graphically.

To learn more about `d8` and profiling, check out the following links:

- [https://v8.dev/docs/d8](https://v8.dev/docs/d8)
- [https://v8.dev/docs/profile](https://v8.dev/docs/profile)

### Debugging with LLDB

We can use LLDB to debug deno.

```sh
lldb -- target/debug/deno tests/worker.js
> run
> bt
> up
> up
> l
```

To debug Rust code, we can use `rust-lldb`. It should come with `rustc` and is a
wrapper around LLDB.

```sh
rust-lldb -- ./target/debug/deno tests/http_bench.ts --allow-net
# On macOS, you might get warnings like
# `ImportError: cannot import name _remove_dead_weakref`
# In that case, use system python by setting PATH, e.g.
# PATH=/System/Library/Frameworks/Python.framework/Versions/2.7/bin:$PATH
(lldb) command script import "/Users/kevinqian/.rustup/toolchains/1.30.0-x86_64-apple-darwin/lib/rustlib/etc/lldb_rust_formatters.py"
(lldb) type summary add --no-value --python-function lldb_rust_formatters.print_val -x ".*" --category Rust
(lldb) type category enable Rust
(lldb) target create "../deno/target/debug/deno"
Current executable set to '../deno/target/debug/deno' (x86_64).
(lldb) settings set -- target.run-args  "tests/http_bench.ts" "--allow-net"
(lldb) b op_start
(lldb) r
```

### libdeno

deno's privileged side will primarily be programmed in Rust. However there will
be a small C API that wraps V8 to 1) define the low-level message passing
semantics, 2) provide a low-level test target, 3) provide an ANSI C API binding
interface for Rust. V8 plus this C API is called "libdeno" and the important
bits of the API is specified here:
[deno.h](https://github.com/denoland/deno/blob/master/libdeno/deno.h)
[libdeno.ts](https://github.com/denoland/deno/blob/master/js/libdeno.ts)

### Flatbuffers

We use Flatbuffers to define common structs and enums between TypeScript and
Rust. These common data structures are defined in
[msg.fbs](https://github.com/denoland/deno/blob/master/src/msg.fbs)

### Updating prebuilt binaries

```
./third_party/depot_tools/upload_to_google_storage.py -b denoland  \
  -e ~/.config/gcloud/legacy_credentials/ry@tinyclouds.org/.boto `which sccache`
mv `which sccache`.sha1 prebuilt/linux64/
gsutil acl ch -u AllUsers:R gs://denoland/608be47bf01004aa11d4ed06955414e93934516e
```

### Continuous Benchmarks

https://deno.land/benchmarks.html

The benchmark chart supposes `//website/data.json` has the type
`BenchmarkData[]` where `BenchmarkData` is defined like the below:

```typescript
interface ExecTimeData {
  mean: number;
  stddev: number;
  user: number;
  system: number;
  min: number;
  max: number;
}

interface BenchmarkData {
  created_at: string;
  sha1: string;
  benchmark: {
    [key: string]: ExecTimeData;
  };
  binarySizeData: {
    [key: string]: number;
  };
  threadCountData: {
    [key: string]: number;
  };
  syscallCountData: {
    [key: string]: number;
  };
}
```

## Contributing

[Style Guide](style_guide.html)

Progress towards future releases is tracked
[here](https://github.com/denoland/deno/milestones).

Please don't make [the benchmarks](https://deno.land/benchmarks.html) worse.

Ask for help in the [community chat room](https://gitter.im/denolife/Lobby).

### Submitting a pull request

Before submitting, please make sure the following is done:

1. There are tests that cover the changes.
2. Ensure `./tools/test.py` passes.
3. Format your code with `tools/format.py`
4. Make sure `./tools/lint.py` passes.

### Changes to `third_party`

[`deno_third_party`](https://github.com/denoland/deno_third_party) contains most
of the external code that Deno depends on, so that we know exactly what we are
executing at any given time. It is carefully mantained with a mixture of manual
labor and private scripts. It's likely you will need help from @ry or
@piscisaureus to make changes.

### Adding Ops (aka bindings)

We are very concerned about making mistakes when adding new APIs. When adding an
Op to Deno, the counterpart interfaces on other platforms should be researched.
Please list how this functionality is done in Go, Node, Rust, and Python.

As an example, see how `deno.rename()` was proposed and added in
[PR #671](https://github.com/denoland/deno/pull/671).

### Documenting APIs

It is important to document public APIs and we want to do that inline with the
code. This helps ensure that code and documentation are tightly coupled
together.

#### Utilize JSDoc

All publicly exposed APIs and types, both via the `deno` module as well as the
global/`window` namespace should have JSDoc documentation. This documentation is
parsed and available to the TypeScript compiler, and therefore easy to provide
further downstream. JSDoc blocks come just prior to the statement they apply to
and are denoted by a leading `/**` before terminating with a `*/`. For example:

```ts
/** A simple JSDoc comment */
export const FOO = "foo";
```