Deno standard library
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Deno Manual

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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.


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.


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. 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.


  • 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 "";

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)
  • linter (deno lint) coming soon


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

Download and Install

deno_install provides convenience scripts to download and install the binary.

Using Shell:

curl -fsSL | sh

Using PowerShell:

iwr -useb | iex

Using Scoop (Windows):

scoop install deno

Using Chocolatey (Windows):

choco install deno

Using Homebrew (macOS):

brew install deno

Using Cargo:

cargo install deno

Deno binaries can also be installed manually, by downloading a tarball or zip file at These packages contain just a single executable file. You will have to set the executable bit on macOS and Linux.

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


Build from Source

Follow the build instruction for contributors.

API reference

deno types

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

$ deno types

The output is the concatenation of three library files that are built into Deno:

Reference websites

TypeScript Deno API.

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

The Deno crate is hosted on


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 = 0; i < Deno.args.length; i++) {
  let filename = Deno.args[i];
  let file = await;
  await Deno.copy(Deno.stdout, file);

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 /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");
for await (const conn of listener) {
  Deno.copy(conn, conn);

When this program is started, it throws PermissionDenied error.

$ deno
error: Uncaught PermissionDenied: network access to "", 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

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"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 --allow-net --allow-read file_server

Run it:

$ file_server .
HTTP server listening on

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


To reload all standard modules


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


Permissions whitelist

Deno also provides permissions whitelist.

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

$ deno --allow-read=/usr /etc/passwd
error: Uncaught PermissionDenied: read access to "/etc/passwd", 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 /etc/passwd

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

This is an example to restrict host.

const result = await fetch("");
$ deno

Run subprocess

API Reference


// create subprocess
const p ={
  cmd: ["echo", "hello"],

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

Run it:

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

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 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;

const p ={
  cmd: [
  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);


When you run it:

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

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

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)

Handle OS Signals

API Reference

You can use Deno.signal() function for handling OS signals.

for await (const _ of Deno.signal(Deno.Signal.SIGINT)) {

Deno.signal() also works as a promise.

await Deno.signal(Deno.Singal.SIGINT);

If you want to stop watching the signal, you can use dispose() method of the signal object.

const sig = Deno.signal(Deno.Signal.SIGINT);
setTimeout(() => { sig.dispose(); }, 5000);

for await (const _ of sig) {

The above for-await loop exits after 5 seconds when sig.dispose() is called.

File system events

To poll for file system events:

const iter = Deno.fsEvents("/");
for await (const event of iter) {
  console.log(">>>> event", event);
  // { kind: "create", paths: [ "/foo.txt" ] }

Note that the exact ordering of the events can vary between operating systems. This feature uses different syscalls depending on the platform:

Linux: inotify macOS: FSEvents Windows: ReadDirectoryChangesW

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 an assertion library:

import { assertEquals } from "";

Deno.test(function t1() {
  assertEquals("hello", "hello");

Deno.test(function t2() {
  assertEquals("world", "world");

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 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: 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 assertion library across a large project. Rather than importing "" everywhere, you could create a deps.ts file that exports the third-party code:

export {
} from "";

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 three ways of referencing type definition files without having to resort to "magic" resolution.

Compiler hint

If you are importing a JavaScript module, and you know where the type definition for that module is located, you can specify the type definition at import. This takes the form of a compiler hint. Compiler hints inform Deno the location of .d.ts files and the JavaScript code that is imported that they relate to. The hint is @deno-types and when specified the value will be used in the compiler instead of the JavaScript module. For example, if you had foo.js, but you know that along side of it was foo.d.ts which was the types for the file, the code would look like this:

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

The value follows the same resolution logic as importing a module, meaning the file needs to have an extension and is relative to the current module. Remote specifiers are also allowed.

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.

Triple-slash reference directive in JavaScript files

If you are hosting modules which you want to be consumed by Deno, and you want to inform Deno about the location of the type definitions, you can utilise a triple-slash directive in the actual code. For example, if you have a JavaScript module and you would like to provide Deno with the location of the type definitions which happen to be alongside that file, your JavaScript module named foo.js might look like this:

/// <reference types="./foo.d.ts" />
export const foo = "foo";

Deno will see this, and the compiler will use foo.d.ts when type checking the file, though foo.js will be loaded at runtime. The resolution of the value of the directive follows the same resolution logic as importing a module, meaning the file needs to have an extension and is relative to the current file. Remote specifiers are also allowed.

X-TypeScript-Types custom header

If you are hosting modules which you want to be consumed by Deno, and you want to inform Deno the location of the type definitions, you can use a custom HTTP header of X-TypeScript-Types to inform Deno of the location of that file.

The header works in the same way as the triple-slash reference mentioned above, it just means that the content of the JavaScript file itself does not need to be modified, and the location of the type definitions can be determined by the server itself.

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 in TypeScript files?

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. Deno only looks for the directive in JavaScript (and JSX) files.

Referencing TypeScript library files

When you use deno run, or other Deno commands which type check TypeScript, that code is evaluated against custom libraries which describe the environment that Deno supports. By default, the compiler runtime APIs which type check TypeScript also use these libraries (Deno.compile() and Deno.bundle()).

But if you want to compile or bundle TypeScript for some other runtime, you may want to override the default libraries. To do this, the runtime APIs support the lib property in the compiler options. For example, if you had TypeScript code that is destined for the browser, you would want to use the TypeScript "dom" library:

const [errors, emitted] = await Deno.compile(
    "main.ts": `document.getElementById("foo");\n`,
    lib: ["dom", "esnext"],

For a list of all the libraries that TypeScript supports, see the lib compiler option documentation.

Don't forget to include the JavaScript library

Just like tsc, when you supply a lib compiler option, it overrides the default ones, which means that the basic JavaScript library won't be included and you should include the one that best represents your target runtime (e.g. es5, es2015, es2016, es2017, es2018, es2019, es2020 or esnext).

Including the Deno namespace

In addition to the libraries that are provided by TypeScript, there are four libraries that are built into Deno that can be referenced:

  • deno.ns - Provides the Deno namespace.
  • deno.shared_globals - Provides global interfaces and variables which Deno supports at runtime that are then exposed by the final runtime library.
  • deno.window - Exposes the global variables plus the Deno namespace that are available in the Deno main worker and is the default for the runtime compiler APIs.
  • deno.worker - Exposes the global variables that are available in workers under Deno.

So to add the Deno namespace to a compilation, you would include the deno.ns lib in the array. For example:

const [errors, emitted] = await Deno.compile(
    "main.ts": `document.getElementById("foo");\n`,
    lib: ["dom", "esnext", "deno.ns"],

Note that the Deno namespace expects a runtime environment that is at least ES2018 or later. This means if you use a lib "lower" than ES2018 you will get errors logged as part of the compilation.

Using the triple slash reference

You do not have to specify the lib in the compiler options. Deno also supports the triple-slash reference to a lib. which can be embedded in the contents of the file. For example, if you have a main.ts like:

/// <reference lib="dom" />


It would compile without errors like this:

const [errors, emitted] = await Deno.compile("./main.ts", undefined, {
  lib: ["esnext"],

Note that the dom library conflicts with some of the default globals that are defined in the default type library for Deno. To avoid this, you need to specify a lib option in the compiler options to the runtime compiler APIs.

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) {

Command line interface


Use deno help to see help text documenting Deno's flags and usage. Use deno help <subcommand> for subcommand-specific flags.

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 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


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.

# list available v8 flags
$ deno --v8-flags=--help

#  example for applying multiple flags
$ deno --v8-flags=--expose-gc,--use-strict

Particularly useful ones:



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

> deno bundle colors.bundle.js
Bundling "colors.bundle.js"
Emitting bundle to "colors.bundle.js"
9.2 kB emitted.

If you omit the out file, the bundle will be sent to stdout.

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

deno colors.bundle.js

The output is a self contained ES Module, where any exports from the main module supplied on the command line will be available. For example, if the main module looked something like this:

export { foo } from "./foo.js";

export const bar = "bar";

It could be imported like this:

import { foo, bar } from "./lib.bundle.js";

Bundles can also be loaded in the web browser. The bundle is a self-contained ES module, and so the attribute of type must be set to "module". For example:

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

Or you could import it into another ES module to consume:

<script type="module">
  import * as website from "website.bundle.js";

Installing executable scripts

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

deno install [FLAGS...] [EXE_NAME] [URL] [SCRIPT_ARGS...] will install the script available at URL under the name EXE_NAME.

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


$ deno install --allow-net --allow-read file_server
[1/1] Compiling

✅ Successfully installed file_server.

By default scripts are installed at $HOME/.deno/bin or $USERPROFILE/.deno/bin and one of that directories 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 --allow-net --allow-read --dir /usr/local/bin file_server

When installing a script you can specify permissions that will be used to run the script.


$ deno install --allow-net --allow-read file_server 8080

Above command creates an executable called file_server that runs with write and read permissions and binds to port 8080.

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.


async function myAwesomeCli(): Promise<void> {
  -- snip --

if (import.meta.main) {

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

# Install using deno install

$ deno install awesome_cli


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.

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

Current limitations:

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


// import_map.json

   "imports": {
      "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);

WASM files can also be loaded using imports:

import { fib } from "./fib.wasm";

Compiler API

Deno supports runtime access to the built-in TypeScript compiler. There are three methods in the Deno namespace that provide this access.


This works similar to deno cache in that it can fetch and cache the code, compile it, but not run it. It takes up to three arguments, the rootName, optionally sources, and optionally options. The rootName is the root module which will be used to generate the resulting program. This is like the module name you would pass on the command line in deno --reload run example.ts. The sources is a hash where the key is the fully qualified module name, and the value is the text source of the module. If sources is passed, Deno will resolve all the modules from within that hash and not attempt to resolve them outside of Deno. If sources are not provided, Deno will resolve modules as if the root module had been passed on the command line. Deno will also cache any of these resources. The options argument is a set of options of type Deno.CompilerOptions, which is a subset of the TypeScript compiler options containing the ones supported by Deno.

The method resolves with a tuple. The first argument contains any diagnostics (syntax or type errors) related to the code. The second argument is a map where the keys are the output filenames and the values are the content.

An example of providing sources:

const [diagnostics, emitMap] = await Deno.compile("/foo.ts", {
  "/foo.ts": `import * as bar from "./bar.ts";\nconsole.log(bar);\n`,
  "/bar.ts": `export const bar = "bar";\n`,

assert(diagnostics == null); // ensuring no diagnostics are returned

We would expect map to contain 4 "files", named /, /foo.js, /, and /bar.js.

When not supplying resources, you can use local or remote modules, just like you could do on the command line. So you could do something like this:

const [diagnostics, emitMap] = await Deno.compile(

In this case emitMap will contain a simple console.log() statement.


This works a lot like deno bundle does on the command line. It is also like Deno.compile(), except instead of returning a map of files, it returns a single string, which is a self-contained JavaScript ES module which will include all of the code that was provided or resolved as well as exports of all the exports of the root module that was provided. It takes up to three arguments, the rootName, optionally sources, and optionally options. The rootName is the root module which will be used to generate the resulting program. This is like module name you would pass on the command line in deno bundle example.ts. The sources is a hash where the key is the fully qualified module name, and the value is the text source of the module. If sources is passed, Deno will resolve all the modules from within that hash and not attempt to resolve them outside of Deno. If sources are not provided, Deno will resolve modules as if the root module had been passed on the command line. Deno will also cache any of these resources. The options argument is a set of options of type Deno.CompilerOptions, which is a subset of the TypeScript compiler options containing the ones supported by Deno.

An example of providing sources:

const [diagnostics, emit] = await Deno.bundle("/foo.ts", {
  "/foo.ts": `import * as bar from "./bar.ts";\nconsole.log(bar);\n`,
  "/bar.ts": `export const bar = "bar";\n`,

assert(diagnostics == null); // ensuring no diagnostics are returned

We would expect emit to be the text for an ES module, which would contain the output sources for both modules.

When not supplying resources, you can use local or remote modules, just like you could do on the command line. So you could do something like this:

const [diagnostics, emit] = await Deno.bundle(

In this case emit will be a self contained JavaScript ES module with all of its dependencies resolved and exporting the same exports as the source module.


This is based off of the TypeScript function transpileModule(). All this does is "erase" any types from the modules and emit JavaScript. There is no type checking and no resolution of dependencies. It accepts up to two arguments, the first is a hash where the key is the module name and the value is the content. The only purpose of the module name is when putting information into a source map, of what the source file name was. The second argument contains optional options of the type Deno.CompilerOptions. The function resolves with a map where the key is the source module name supplied, and the value is an object with a property of source and optionally map. The first is the output contents of the module. The map property is the source map. Source maps are provided by default, but can be turned off via the options argument.

An example:

const result = await Deno.transpileOnly({
  "/foo.ts": `enum Foo { Foo, Bar, Baz };\n`,


We would expect the enum would be rewritten to an IIFE which constructs the enumerable, and the map to be defined.

TypeScript Compiler Options

In the Deno ecosystem, all strict flags are enabled in order to comply with TypeScript's ideal of being strict by default. However, in order to provide a way to support customization a configuration file such as tsconfig.json might be provided to Deno on program execution.

You need to explicitly tell Deno where to look for this configuration by setting the -c argument when executing your application.

deno -c tsconfig.json mod.ts

Following are the currently allowed settings and their default values in Deno:

  "compilerOptions": {
    "allowJs": false,
    "allowUmdGlobalAccess": false,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "alwaysStrict": true,
    "assumeChangesOnlyAffectDirectDependencies": false,
    "checkJs": false,
    "disableSizeLimit": false,
    "generateCpuProfile": "profile.cpuprofile",
    "jsx": "react",
    "jsxFactory": "React.createElement",
    "lib": [],
    "noFallthroughCasesInSwitch": false,
    "noImplicitAny": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "noImplicitUseStrict": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "preserveConstEnums": false,
    "removeComments": false,
    "resolveJsonModule": true,
    "strict": true,
    "strictBindCallApply": true,
    "strictFunctionTypes": true,
    "strictNullChecks": true,
    "strictPropertyInitialization": true,
    "suppressExcessPropertyErrors": false,
    "suppressImplicitAnyIndexErrors": false,
    "useDefineForClassFields": false

For documentation on allowed values and use cases please visit the typescript docs.

Note: Any options not listed above are either not supported by Deno or are listed as deprecated/experimental in the TypeScript documentation.

Program lifecycle

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

Listener for load events can be asynchronous and will be awaited. Listener for unload events need to be synchronous. Both events cannot be cancelled.


// 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
/proc/$$/stat Deno.metrics()
man pages deno types


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;
// { 0: "stdin", 1: "stdout", 2: "stderr" }
// { 1: "stdout", 2: "stderr" }


Metrics is Deno's internal counter for various statistics.

> console.table(Deno.metrics())
│     (index)      │ Values │
│  opsDispatched   │   9    │
│   opsCompleted   │   9    │
│ bytesSentControl │  504   │
│  bytesSentData   │   0    │
│  bytesReceived   │  856   │

Schematic diagram


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

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

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) 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 --  "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;


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


  • Read the 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.


Cloning the Repository

Clone on Linux or Mac:

git clone --recurse-submodules

Extra steps for Windows users:

  1. Enable "Developer Mode" (otherwise symlinks would require administrator privileges).
  2. Make sure you are using git version or newer.
  3. Set core.symlinks=true before the checkout:
    git config --global core.symlinks true
    git clone --recurse-submodules


The easiest way to build Deno is by using a precompiled version of V8:

cargo build -vv

However if you want to build Deno and V8 from source code:

V8_FROM_SOURCE=1 cargo build -vv

When building V8 from source, there are more dependencies:

Python 2. Ensure that a suffix-less python/python.exe exists in your PATH and it refers to Python 2, not 3.

For Linux users glib-2.0 development files must also be installed. (On Ubuntu, run apt install libglib2.0-dev.)

Mac users must have XCode installed.

For Windows users:

  1. Get VS Community 2019 with "Desktop development with C++" toolkit and make sure to select the following required tools listed below along with all C++ tools.

    • Visual C++ tools for CMake
    • Windows 10 SDK (10.0.17763.0)
    • Testing tools core features - Build Tools
    • Visual C++ ATL for x86 and x64
    • Visual C++ MFC for x86 and x64
    • C++/CLI support
    • VC++ 2015.3 v14.00 (v140) toolset for desktop
  2. 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". Or use: Debugging Tools for Windows (Notice: it will download the files, you should install X64 Debuggers And Tools-x64_en-us.msi file manually.)

See rusty_v8's README for more details about the V8 build.


Build with Cargo:

# Build:
cargo build -vv

# Build errors?  Ensure you have latest master and try building again, or if that doesn't work try:
cargo clean && cargo build -vv

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

Testing and Tools

Test deno:

# Run the whole suite:
cargo test

# Only test cli/js/:
cargo test js_unit_tests

Test std/:

cargo test std_tests

Lint the code:


Format the code:


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/
  5. Make sure ./tools/ 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";