How to Use the Deno Built-in Tools

How to Use the Deno Built-in Tools

Learn to use the Deno built-in tools including a linter, test runner, script tools, and many others. We introduce each inclusion and provide usage advice.

Learn to use the Deno built-in tools including a linter, test runner, script tools, and many others. We introduce each inclusion and provide usage advice.

One surprising difference between Deno and Node.js is the number of tools built into the runtime. Other than a Read-Eval-Print Loop (REPL) console, Node.js requires third-party modules to handle most indirect coding activities such as testing and linting. The Deno built-in tools provide almost everything you need out of the box.

Before we begin, a note. Deno is new! Use these tools with caution. Some may be unstable. Few have configuration options. Others may have undesirable side effects such as recursively processing every file in every subdirectory. It’s best to test tools from a dedicated project directory.

Install Deno

Install Deno on macOS or Linux using the following terminal command:

curl -fsSL | sh

Or from Windows Powershell:

iwr -useb | iex

Further installation options are provided in the Deno manual.

Enter deno --version to check installation has been successful. The version numbers for the V8 JavaScript engine, TypeScript compiler, and Deno itself are displayed.

Upgrade Deno

Upgrade Deno to the latest version with:

deno upgrade

Or upgrade to specific release such as v1.3.0:

deno upgrade --version 1.30.0

Most of the tools below are available in all versions, but later editions may have more features and bug fixes.

Deno Help

A list of tools and options can be viewed by entering:

deno help

Read-Eval-Print Loop (REPL)

Like Node.js, a REPL expression evaluation console can be accessed by entering deno in your terminal. Each expression you enter returns a result or undefined:

$ deno

Deno 1.3.0
exit using ctrl+d or close()
> const w = 'World';
> w
> console.log(`Hello ${w}!`);
Hello World!
> close()


Previously entered expressions can be re-entered by using the cursor keys to navigate through the expression history.

Dependency Inspector

A tree of all module dependencies can be viewed by entering deno info <module> where <module> is the path/URL to an entry script.

Consider the following lib.js library code with exported hello and sum functions:

// general library: lib.js

 * return "Hello <name>!" string
 * @module lib
 * @param {string} name
 * @returns {string} Hello <name>!
export function hello(name = 'Anonymous') {

  return `Hello ${ name.trim() }!`;


 * Returns total of all arguments
 * @module lib
 * @param {...*} args
 * @returns {*} total
export function sum(...args) {

  return [...args].reduce((a, b) => a + b);


These can be used from a main entry script, index.js, in the same directory:

// main entry script: index.js

// import lib.js modules
import { hello, sum } from './lib.js';

  spr = sum('Site', 'Point', '.com', ' ', 'reader'),
  add = sum(1, 2, 3);

// output
console.log( hello(spr) );
console.log( 'total:', add );

The result of running deno run ./index.js:

$ deno run ./index.js

Hello reader!
total: 6

The dependencies used by index.js can be examined with deno info ./index.js:

$ deno info ./index.js

local: /home/deno/testing/index.js
type: JavaScript
  └── file:///home/deno/testing/lib.js

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