denoland-deno/std/manual.md

Deno Manual

Table of Contents

Project Status / 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 do help!

We are actively working towards 1.0, but there is no date guarantee.

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 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 10 megabyte zipped executable. 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:

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.

Built-in Deno Utilities / Commands

• dependency inspector (deno info)
• code formatter (deno fmt)
• bundling (deno bundle)
• runtime type info (deno types)
• test runner (deno test)
• command-line debugger (--debug) coming soon
• linter (deno lint) coming soon

Setup

Binary Install

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

deno_install provides convenience scripts to download and install the binary.

Using Shell:

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

Using PowerShell:

iwr https://deno.land/x/install/install.ps1 -useb | iex

Using Scoop (windows):

scoop install deno

Using Chocolatey (windows):

choco install deno

Using Homebrew (mac):

brew install deno

Using Cargo:

cargo install deno_cli

Deno binaries can also be installed manually, by downloading a tarball or zip file at 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/std/examples/welcome.ts

Build from source

Clone on Linux or Mac:

git clone --recurse-submodules https://github.com/denoland/deno.git

On Windows, a couple extra steps are required to clone because we use symlinks in the repository. First enable "Developer Mode" (otherwise symlinks would require administrator privileges). Then you must set core.symlinks=true before the checkout is started.

git config --global core.symlinks true
git clone --recurse-submodules https://github.com/denoland/deno.git

Now we can start the build:

# Build.
cargo build -vv

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

# Test.
cargo test

# 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 >= 1.36.0
2. Python 2. Not 3.

Extra steps for Mac users: install XCode :(

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 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".
4. Make sure you are using git version 2.19.2.windows.1 or newer.

Other useful commands

# Call ninja manually.
ninja -C target/debug

# Build a release binary.
cargo build --release

# List executable targets.
gn --root=core/libdeno ls target/debug "//:*" --as=output --type=executable

# List build configuration.
gn --root=core/libdeno args target/debug/ --list

# Edit build configuration.
gn --root=core/libdeno args target/debug/

# Describe a target.
gn --root=core/libdeno desc target/debug/ :deno
gn help

# Update third_party modules
git submodule update

# Skip downloading binary build tools and point the build
# to the system provided ones (for packagers of deno ...).
export DENO_BUILD_ARGS="clang_base_path=/usr clang_use_chrome_plugins=false"
DENO_NO_BINARY_DOWNLOAD=1 DENO_GN_PATH=/usr/bin/gn cargo build

Environment variables: DENO_BUILD_MODE, DENO_BUILD_PATH, DENO_BUILD_ARGS, DENO_DIR, DENO_GN_PATH, DENO_NO_BINARY_DOWNLOAD.

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.

Reference websites

TypeScript Deno API.

If you are embedding deno in a Rust program, see Rust Deno API.

The Deno crate is hosted on crates.io.

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.

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 --allow-read https://deno.land/std/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.

const listener = Deno.listen({ port: 8080 });
console.log("listening on 0.0.0.0:8080");
for await (const conn of listener) {
  Deno.copy(conn, conn);
}

When this program is started, it throws PermissionDenied error.

$ deno https://deno.land/std/examples/echo_server.ts
error: Uncaught PermissionDenied: run again with the --allow-net flag
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    ...

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

$ deno --allow-net https://deno.land/std/examples/echo_server.ts

To test it, try sending data to it with netcat:

$ nc localhost 8080
hello world
hello world

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.

Inspecting and revoking permissions

Sometimes a program may want to revoke previously granted permissions. When a program, at a later stage, needs those permissions, it will fail.

// lookup a permission
const status = await Deno.permissions.query({ name: "write" });
if (status.state !== "granted") {
  throw new Error("need write permission");
}

const log = await Deno.open("request.log", "a+");

// revoke some permissions
await Deno.permissions.revoke({ name: "read" });
await Deno.permissions.revoke({ name: "write" });

// use the log file
const encoder = new TextEncoder();
await log.write(encoder.encode("hello\n"));

// this will fail.
await Deno.remove("request.log");

File server

This one serves a local directory in HTTP.

deno install file_server https://deno.land/std/http/file_server.ts --allow-net --allow-read

Run it:

$ file_server .
Downloading https://deno.land/std/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

Reload specific modules

Sometimes we want to upgrade only some modules. You can control it by passing an argument to a --reload flag.

To reload everything

--reload

To reload all standard modules

--reload=https://deno.land/std

To reload specific modules (in this example - colors and file system utils) use a comma to separate URLs

--reload=https://deno.land/std/fs/utils.ts,https://deno.land/std/fmt/colors.ts

Permissions whitelist

Deno also provides permissions whitelist.

This is an example to restrict file system access by whitelist.

$ deno --allow-read=/usr https://deno.land/std/examples/cat.ts /etc/passwd
error: Uncaught PermissionDenied: run again with the --allow-read flag
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    ...

You can grant read permission under /etc dir

$ deno --allow-read=/etc https://deno.land/std/examples/cat.ts /etc/passwd

--allow-write works same as --allow-read.

This is an example to restrict host.

const result = await fetch("https://deno.land/");

$ deno --allow-net=deno.land https://deno.land/std/examples/curl.ts https://deno.land/

Run subprocess

API Reference

Example:

// create subprocess
const p = Deno.run({
  args: ["echo", "hello"]
});

// await its completion
await p.status();

Run it:

$ deno --allow-run ./subprocess_simple.ts
hello

Here a function is assigned to window.onload. This function is called after the main script is loaded. This is the same as onload of the browsers, and it can be used as the main entrypoint.

By default when you use Deno.run() subprocess inherits stdin, stdout and stderr of parent process. If you want to communicate with started subprocess you can use "piped" option.

const fileNames = Deno.args.slice(1);

const p = Deno.run({
  args: [
    "deno",
    "run",
    "--allow-read",
    "https://deno.land/std/examples/cat.ts",
    ...fileNames
  ],
  stdout: "piped",
  stderr: "piped"
});

const { code } = await p.status();

if (code === 0) {
  const rawOutput = await p.output();
  await Deno.stdout.write(rawOutput);
} else {
  const rawError = await p.stderrOutput();
  const errorString = new TextDecoder().decode(rawError);
  console.log(errorString);
}

Deno.exit(code);

When you run it:

$ deno run --allow-run ./subprocess.ts <somefile>
[file content]

$ deno run --allow-run ./subprocess.ts non_existent_file.md

Uncaught NotFound: No such file or directory (os error 2)
    at DenoError (deno/js/errors.ts:22:5)
    at maybeError (deno/js/errors.ts:41:12)
    at handleAsyncMsgFromRust (deno/js/dispatch.ts:27:17)

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:

import { test, runIfMain } from "https://deno.land/std/testing/mod.ts";
import { assertEquals } from "https://deno.land/std/testing/asserts.ts";

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

test(function t2() {
  assertEquals("world", "world");
});

runIfMain(import.meta);

Try running this:

$ deno run test.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 defaults to the system's cache directory 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. The default directory is:

• On Linux/Redox: $XDG_CACHE_HOME/deno or $HOME/.cache/deno
• On Windows: %LOCALAPPDATA%/deno (%LOCALAPPDATA% = FOLDERID_LocalAppData)
• On macOS: $HOME/Library/Caches/deno
• If something fails, it falls back to $HOME/.deno

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 can I trust a URL that may change By using a lock file (using the --lock command line flag) you can ensure you're running the code you expect to be.

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 deps.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/std/testing/mod.ts" and "https://deno.land/std/testing/asserts.ts" everywhere, you could create a deps.ts file that exports the third-party code:

export { runTests, test } from "https://deno.land/std/testing/mod.ts";
export { assertEquals } from "https://deno.land/std/testing/asserts.ts";

And throughout the same project, you can import from the deps.ts and avoid having many references to the same URL:

import { assertEquals, runTests, test } from "./deps.ts";

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

Using external type definitions

Deno supports both JavaScript and TypeScript as first class languages at runtime. This means it requires fully qualified module names, including the extension (or a server providing the correct media type). In addition, Deno has no "magical" module resolution.

The out of the box TypeScript compiler though relies on both extension-less modules and the Node.js module resolution logic to apply types to JavaScript modules.

In order to bridge this gap, Deno supports compiler hints that inform Deno the location of .d.ts files and the JavaScript code they relate to. A compiler hint looks like this:

// @deno-types="./foo.d.ts"
import * as foo from "./foo.js";

Where the hint affects the next import statement (or export ... from statement) where the value of the @deno-types will be substituted at compile time instead of the specified module. Like in the above example, the Deno compiler will load ./foo.d.ts instead of ./foo.js. Deno will still load ./foo.js when it runs the program.

Not all type definitions are supported.

Deno will use the compiler hint to load the indicated .d.ts files, but some .d.ts files contain unsupported features. Specifically, some .d.ts files expect to be able to load or reference type definitions from other packages using the module resolution logic. For example a type reference directive to include node, expecting to resolve to some path like ./node_modules/@types/node/index.d.ts. Since this depends on non-relative "magical" resolution, Deno cannot resolve this.

Why not use the triple-slash type reference?

The TypeScript compiler supports triple-slash directives, including a type reference directive. If Deno used this, it would interfere with the behavior of the TypeScript compiler.

Testing if current file is the main program

To test if the current script has been executed as the main input to the program check import.meta.main.

if (import.meta.main) {
  console.log("main");
}

Command line interface

Flags

Use deno help to see the help text.

deno
A secure JavaScript and TypeScript runtime

Docs: https://deno.land/manual.html
Modules: https://deno.land/x/
Bugs: https://github.com/denoland/deno/issues

To run the REPL:

  deno

To execute a sandboxed script:

  deno https://deno.land/std/examples/welcome.ts

To evaluate code from the command line:

  deno eval "console.log(30933 + 404)"

To get help on the another subcommands (run in this case):

  deno help run

USAGE:
    deno [OPTIONS] [SUBCOMMAND]

OPTIONS:
    -A, --allow-all                    Allow all permissions
        --allow-env                    Allow environment access
        --allow-hrtime                 Allow high resolution time measurement
        --allow-net=<allow-net>        Allow network access
        --allow-read=<allow-read>      Allow file system read access
        --allow-run                    Allow running subprocesses
        --allow-write=<allow-write>    Allow file system write access
    -c, --config <FILE>                Load compiler configuration file
        --current-thread               Use tokio::runtime::current_thread
    -h, --help                         Prints help information
        --importmap <FILE>             Load import map file
        --lock <FILE>                  Check the specified lock file
        --lock-write                   Write lock file. Use with --lock.
    -L, --log-level <log-level>        Set log level [possible values: debug, info]
        --no-fetch                     Do not download remote modules
    -r, --reload=<CACHE_BLACKLIST>     Reload source code cache (recompile TypeScript)
        --seed <NUMBER>                Seed Math.random()
        --v8-flags=<v8-flags>          Set V8 command line options
        --v8-options                   Print V8 command line options
    -v, --version                      Print the version

SUBCOMMANDS:
    [SCRIPT]       Script to run
    bundle         Bundle module and dependencies into single file
    completions    Generate shell completions
    eval           Eval script
    fetch          Fetch the dependencies
    fmt            Format files
    help           Prints this message or the help of the given subcommand(s)
    info           Show info about cache or info related to source file
    install        Install script as executable
    run            Run a program given a filename or url to the source code
    test           Run tests
    types          Print runtime TypeScript declarations
    version        Print the version
    xeval          Eval a script on text segments from stdin

ENVIRONMENT VARIABLES:
    DENO_DIR        Set deno's base directory
    NO_COLOR        Set to disable color
    HTTP_PROXY      Set proxy address for HTTP requests (module downloads, fetch)
    HTTPS_PROXY     Set proxy address for HTTPS requests (module downloads, fetch)

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.

Shell completion

You can generate completion script for your shell using the deno completions <shell> command. The command outputs to stdout so you should redirect it to an appropriate file.

The supported shells are:

• zsh
• bash
• fish
• powershell
• elvish

Example:

deno completions bash > /usr/local/etc/bash_completion.d/deno.bash
source /usr/local/etc/bash_completion.d/deno.bash

V8 flags

V8 has many many internal command-line flags, that you can see with --v8-options. It looks like this.

Particularly useful ones:

--async-stack-trace

Bundling

deno bundle [URL] will output a single JavaScript file, which includes all dependencies of the specified input. For example:

> deno bundle https://deno.land/std/examples/colors.ts
Bundling "colors.bundle.js"
Emitting bundle to "colors.bundle.js"
9.2 kB emitted.

The bundle can just be run as any other module in Deno would:

deno colors.bundle.js

Bundles can also be loaded in the web browser. For example:

<script src="website.bundle.js"></script>

Bundles, whether loaded in the web browser, or in Deno, would run the root module which is specified on the command line when creating the bundle, so put any initiation logic in that module.

Installing executable scripts

Deno provides ability to easily install and distribute executable code via deno install command.

deno install [EXE_NAME] [URL] [FLAGS...] will install script available at URL with name EXE_NAME.

This command is a thin wrapper that creates executable shell scripts which invoke deno with specified permissions and CLI flags.

Example:

$ deno install file_server https://deno.land/std/http/file_server.ts --allow-net --allow-read
[1/1] Compiling https://deno.land/std/http/file_server.ts

✅ Successfully installed file_server.
/Users/deno/.deno/bin/file_server

By default scripts are installed at $HOME/.deno/bin and that directory must be added to the path manually.

$ echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.bashrc

Installation directory can be changed using -d/--dir flag:

$ deno install --dir /usr/local/bin prettier https://deno.land/std/prettier/main.ts --allow-write --allow-read

When installing a script you can specify permissions that will be used to run the script. They are placed after the script URL and can be mixed with any additional CLI flags you want to pass to the script.

Example:

$ deno install format_check https://deno.land/std/prettier/main.ts --allow-write --allow-read --check --print-width 88 --tab-width 2

Above command creates an executable called format_check that runs prettier with write and read permissions. When you run format_check deno will run prettier in check mode and configured to use 88 column width with 2 column tabs.

It is a good practice to use import.meta.main idiom for an entry point for executable file. See Testing if current file is the main program section.

Example:

// https://example.com/awesome/cli.ts
async function myAwesomeCli(): Promise<void> {
  -- snip --
}

if (import.meta.main) {
  myAwesomeCli();
}

When you create executable script make sure to let users know by adding example installation command to your repository:

# Install using deno install

$ deno install awesome_cli https://example.com/awesome/cli.ts

Proxies

Deno supports proxies for module downloads and fetch API.

Proxy configuration is read from environmental variables: HTTP_PROXY and HTTPS_PROXY.

In case of Windows if environmental variables are not found Deno falls back to reading proxies from registry.

Lock file

Deno can store and check module subresource integrity for modules using a small JSON file. Use the --lock=lock.json to enable and specify lock file checking. To update or create a lock use --lock=lock.json --lock-write.

Import maps

Deno supports import maps.

One can use import map with --importmap=<FILE> CLI flag.

Current limitations:

• single import map
• no fallback URLs
• Deno does not support std: namespace
• Does supports only file:, http: and https: schemes

Example:

// import_map.json

{
   "imports": {
      "http/": "https://deno.land/std/http/"
   }
}

// hello_server.ts

import { serve } from "http/server.ts";

const body = new TextEncoder().encode("Hello World\n");
for await (const req of serve(":8000")) {
  req.respond({ body });
}

$ deno run --importmap=import_map.json hello_server.ts

WASM support

Deno can execute wasm binaries.

const wasmCode = new Uint8Array([
  0, 97, 115, 109, 1, 0, 0, 0, 1, 133, 128, 128, 128, 0, 1, 96, 0, 1, 127,
  3, 130, 128, 128, 128, 0, 1, 0, 4, 132, 128, 128, 128, 0, 1, 112, 0, 0,
  5, 131, 128, 128, 128, 0, 1, 0, 1, 6, 129, 128, 128, 128, 0, 0, 7, 145,
  128, 128, 128, 0, 2, 6, 109, 101, 109, 111, 114, 121, 2, 0, 4, 109, 97,
  105, 110, 0, 0, 10, 138, 128, 128, 128, 0, 1, 132, 128, 128, 128, 0, 0,
  65, 42, 11
]);
const wasmModule = new WebAssembly.Module(wasmCode);
const wasmInstance = new WebAssembly.Instance(wasmModule);
console.log(wasmInstance.exports.main().toString());

WASM files can also be loaded using imports:

import { fib } from "./fib.wasm";
console.log(fib(20));

Program lifecycle

Deno supports browser compatible lifecycle events: load and unload. You can use these event to provide setup and cleanup code in your program.

load event listener supports asynchronous functions and will await these functions. unload event listener supports only synchronous code. Both events are not cancellable.

Example:

// main.ts
import "./imported.ts";

const handler = (e: Event): void => {
  console.log(`got ${e.type} event in event handler (main)`);
};

window.addEventListener("load", handler);

window.addEventListener("unload", handler);

window.onload = (e: Event): void => {
  console.log(`got ${e.type} event in onload function (main)`);
};

window.onunload = (e: Event): void => {
  console.log(`got ${e.type} event in onunload function (main)`);
};

// imported.ts
const handler = (e: Event): void => {
  console.log(`got ${e.type} event in event handler (imported)`);
};

window.addEventListener("load", handler);
window.addEventListener("unload", handler);

window.onload = (e: Event): void => {
  console.log(`got ${e.type} event in onload function (imported)`);
};

window.onunload = (e: Event): void => {
  console.log(`got ${e.type} event in onunload function (imported)`);
};

console.log("log from imported script");

Note that you can use both window.addEventListener and window.onload/window.onunload to define handlers for events. There is a major difference between them, let's run example:

$ deno main.ts
log from imported script
log from main script
got load event in onload function (main)
got load event in event handler (imported)
got load event in event handler (main)
got unload event in onunload function (main)
got unload event in event handler (imported)
got unload event in event handler (main)

All listeners added using window.addEventListener were run, but window.onload and window.onunload defined in main.ts overridden handlers defined in imported.ts.

Internal details

Deno and Linux analogy

Linux	Deno
Processes	Web Workers
Syscalls	Ops
File descriptors (fd)	Resource ids (rid)
Scheduler	Tokio
Userland: libc++ / glib / boost	https://deno.land/std/
/proc/$$/stat	Deno.metrics()
man pages	deno types

Resources

Resources (AKA rid) are Deno's version of file descriptors. They are integer values used to refer to open files, sockets, and other concepts. For testing it would be good to be able to query the system for how many open resources there are.

const { resources, close } = Deno;
console.log(resources());
// { 0: "stdin", 1: "stdout", 2: "stderr" }
close(0);
console.log(resources());
// { "stdout", 2: "stderr" }

Metrics

Metrics is Deno's internal counters for various statics.

> console.table(Deno.metrics())
┌──────────────────┬────────┐
│     (index)      │ Values │
├──────────────────┼────────┤
│  opsDispatched   │   9    │
│   opsCompleted   │   9    │
│ bytesSentControl │  504   │
│  bytesSentData   │   0    │
│  bytesReceived   │  856   │
└──────────────────┴────────┘

Schematic diagram

Profiling

To start profiling,

# Make sure we're only building release.
# Build deno and V8's d8.
ninja -C target/release d8

# Start the program we want to benchmark with --prof
./target/release/deno tests/http_bench.ts --allow-net --v8-flags=--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:

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:

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 browser, and select prof.json to view the distribution graphically.

Useful V8 flags during profiling:

• --prof
• --log-internal-timer-events
• --log-timer-events
• --track-gc
• --log-source-code
• --track-gc-object-stats

Note that you might need to run Deno with --current-thread flag to capture full V8 profiling output.

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

• https://v8.dev/docs/d8
• https://v8.dev/docs/profile

Debugging with LLDB

We can use LLDB to debug Deno.

$ lldb -- target/debug/deno run 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.

$ rust-lldb -- ./target/debug/deno run --allow-net tests/http_bench.ts
# 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/1.36.0-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

Deno Core

The core binding layer for Deno. It is released as a standalone crate. Inside of core is V8 itself, with a binding API called "libdeno". See the crate documentation for more details.

Continuous Benchmarks

See our benchmarks over here

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

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;
  };
}

Logos

These Deno logos, like the Deno software, are distributed under the MIT license (public domain and free for use)

• A hand drawn one by @ry

• An animated one by @hashrock

• A high resolution SVG one by @kevinkassimo

• A pixelated animation one by @tanakaworld

Contributing

Style Guide

Progress towards future releases is tracked here.

Please don't make the benchmarks worse.

Ask for help in the community chat room.

If you are going to work on an issue, mention so in the issue comments before you start working on the issue.

Submitting a pull request

Before submitting, please make sure the following is done:

1. That there is a related issue and it is referenced in the PR text.
2. There are tests that cover the changes.
3. Ensure cargo test passes.
4. Format your code with tools/format.py
5. Make sure ./tools/lint.py passes.

Changes to third_party

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 maintained 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.

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:

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