denoland-deno/website/manual.md

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 do help!

Introduction

A secure JavaScript/TypeScript runtime built with V8, Rust, and Tokio

Philosophy

Deno aims to be a productive and secure scripting environment 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 the 50 megabyte deno executable.

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 set of reviewed (audited) standard modules that are guaranteed to work with Deno.

Goals

• Support TypeScript out of the box.

• Like the browser, allows imports from URLs:

  import * as log from "https://deno.land/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. 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.

• 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.

• Aims to support top-level await.

• Be able to serve HTTP efficiently. (Currently it is relatively slow.)

• Provide useful tooling out of the box:

  • command-line debugger not yet
  • linter not yet
  • dependency inspector (deno info)
  • code formatter (deno fmt),

Non-goals

• No package.json.

• No npm.

• Not explicitly compatible with Node.

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 | iex

Using Scoop (windows):

scoop install deno

Using Homebrew (mac):

brew install deno

Deno 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 run https://deno.land/welcome.ts

Build from source

Clone on Unix/MacOs

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

Clone on Windows (with Admin privileges)

git clone -c core.symlinks=true --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 run 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 >= 1.34.1
2. Node
3. 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.
./third_party/depot_tools/ninja -C target/debug

# Build a release binary.
./tools/build.py --release 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

# Skip downloading binary build tools and point the build
# to the system provided ones (for packagers of deno ...).
./tools/setup.py --no-binary-download
export DENO_BUILD_ARGS="clang_base_path=/usr clang_use_chrome_plugins=false"
DENO_GN_PATH=/usr/bin/gn DENO_NINJA_PATH=/usr/bin/ninja ./tools/build.py

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

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.

(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 run --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 { 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 run https://deno.land/std/examples/echo_server.ts
⚠️  Deno requests network access to "listen". Grant? [a/y/n/d (a = allow always, y = allow once, n = deny once, d = deny always)]

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

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.

Inspecting and revoking permissions

Sometimes a program may want to revoke previously granted permissions. When a program, at a later stage, needs those permissions, a new prompt will be presented to the user.

const { permissions, revokePermission, open, remove } = Deno;

(async () => {
  // lookup a permission
  if (!permissions().write) {
    throw new Error("need write permission");
  }

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

  // revoke some permissions
  revokePermission("read");
  revokePermission("write");

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

  // this will prompt for the write permission or fail.
  await remove("request.log");
})();

File server

This one serves a local directory in HTTP.

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

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

Permissions whitelist

deno also provides permissions whitelist.

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

$ deno run --allow-read=/usr https://deno.land/std/examples/cat.ts /etc/passwd
⚠️  Deno requests read access to "/etc/passwd". Grant? [a/y/n/d (a = allow always, y = allow once, n = deny once, d = deny always)]

You can grant read permission under /etc dir

$ deno run --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.

(async () => {
  const result = await fetch("https://deno.land/std/examples/echo_server.ts");
})();

$ deno run --allow-net=deno.land allow-net-whitelist-example.ts

Run subprocess

API Reference

Example:

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

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

main();

Run it:

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

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.

async function main() {
  const decoder = new TextDecoder();

  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 = decoder.decode(rawError);
    console.log(errorString);
  }

  Deno.exit(code);
}

main();

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 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" everywhere, you could create a deps.ts file the exports the third-party code:

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

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

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

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

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.

USAGE:
    deno [FLAGS] [OPTIONS] [SUBCOMMAND]

FLAGS:
    -A, --allow-all       Allow all permissions
        --allow-env       Allow environment access
        --allow-hrtime    Allow high resolution time measurement
        --allow-run       Allow running subprocesses
    -h, --help            Prints help information
        --no-prompt       Do not use prompts
    -r, --reload          Reload source code cache (recompile TypeScript)
        --v8-options      Print V8 command line options

OPTIONS:
        --allow-net=<allow-net>        Allow network access
        --allow-read=<allow-read>      Allow file system read access
        --allow-write=<allow-write>    Allow file system write access
    -c, --config <FILE>                Load compiler configuration file
        --importmap <FILE>             Load import map file
    -L, --log-level <log-level>        Set log level [possible values: debug, info]
        --seed <NUMBER>                Seed Math.random()
        --v8-flags=<v8-flags>          Set V8 command line options

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 source file related info
    install        Install script as executable
    run            Run a program given a filename or url to the source code
    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

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, using AMD, which includes all dependencies of the specified input.

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

To run then bundle in Deno use

deno https://deno.land/std/bundle/run.ts colors.bundle.js

Bundles can also be loaded in the web browser with the assistance of RequireJS. Suppose we have a bundle called website.bundle.js, then the following HTML should be able to load it:

<script src="https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.6/require.min.js"></script>
<script src="website.bundle.js"></script>
<script>
  requirejs(["website"], website => website.main());
</script>

Here we assume there's an exported function main() from website.ts.

// website.ts
export main() {
  console.log("hello from the web browser");
}

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

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";

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

main();

$ deno run --importmap=import_map.json hello_server.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	deno_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());
// output like: { 0: "stdin", 1: "stdout", 2: "stderr", 3: "repl" }

// close resource by rid
close(3);

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.
./tools/build.py --release 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:

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.

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/1.34.1-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.

Flatbuffers

We use Flatbuffers to define common structs and enums between TypeScript and Rust. These common data structures are defined in 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

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 ./tools/test.py 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";