Deno 2.0 — Complete Guide and Features

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

# Run TypeScript directly
deno run --allow-read app.ts

# Format code
deno fmt
deno lint

import { serve } from "https://deno.land/std/http/server.ts";

const handler = (req: Request): Response => {
  const url = new URL(req.url);

  if (url.pathname === "/") {
    return new Response("Hello from Deno!");
  }

  if (url.pathname === "/api/users") {
    return Response.json([{ id: 1, name: "Alice" }]);
  }

  return new Response("404 Not Found", { status: 404 });
};

serve(handler, { port: 8000 });
console.log("Server running on http://localhost:8000");

# Allow file system access
deno run --allow-read app.ts
deno run --allow-write app.ts
deno run --allow-net app.ts

# Allow all
deno run --allow-all app.ts

# Specific directory
deno run --allow-read=/path/to/dir app.ts

// Code requires permission to run
const text = await Deno.readTextFile("config.txt");
// Error if --allow-read not provided

const response = await fetch("https://api.example.com");
// Error if --allow-net not provided

// Read file
const content = await Deno.readTextFile("file.txt");

// Write file
await Deno.writeTextFile("output.txt", "Hello");

// List directory
for await (const entry of Deno.readDir(".")) {
  console.log(entry.name);
}

// File stats
const stat = await Deno.stat("file.txt");
console.log(`Size: ${stat.size} bytes`);

// HTTP
import { serve } from "https://deno.land/std/http/server.ts";

// Testing
import { assertEquals } from "https://deno.land/std/assert/mod.ts";

// Collections
import { groupBy } from "https://deno.land/std/collections/mod.ts";

// Encoding
import { encodeBase64 } from "https://deno.land/std/encoding/base64.ts";

// UUID
import { v4 } from "https://deno.land/std/uuid/mod.ts";
const id = v4.generate();

import { assertEquals, assertThrows } from "https://deno.land/std/assert/mod.ts";

Deno.test("addition", () => {
  assertEquals(1 + 1, 2);
});

Deno.test("async operation", async () => {
  const result = await someAsyncFunction();
  assertEquals(result, "expected");
});

Deno.test("should throw", () => {
  assertThrows(() => {
    throw new Error("Oops");
  });
});

deno test
deno test --coverage=./coverage

// From deno.land
import { serve } from "https://deno.land/std/http/server.ts";

// From npm via esm.sh
import express from "https://esm.sh/express@4.18.0";

// From npm via jsr (Deno ecosystem)
import { assertEquals } from "jsr:@std/assert";

// Lock file for versions
// deno.lock (auto-generated)

// Read environment variable
const apiKey = Deno.env.get("API_KEY");
if (!apiKey) {
  throw new Error("API_KEY not set");
}

// Load from .env file
import { load } from "https://deno.land/std/dotenv/mod.ts";
const env = await load();
const databaseUrl = env.DATABASE_URL;

# Set variables
export API_KEY=secret
export DATABASE_URL=postgresql://localhost/db

deno run --allow-env app.ts

// cli.ts
import { parse } from "https://deno.land/std/flags/mod.ts";

const args = parse(Deno.args, {
  string: ["name"],
  boolean: ["help"],
});

if (args.help) {
  console.log("Usage: deno run cli.ts --name=Alice");
  Deno.exit(0);
}

console.log(`Hello, ${args.name || "World"}!`);

deno run cli.ts --name=Alice
# Hello, Alice!

// oak is a popular Deno web framework
import { Application, Router } from "https://deno.land/x/oak/mod.ts";

const app = new Application();
const router = new Router();

router.get("/users", (ctx) => {
  ctx.response.body = [{ id: 1, name: "Alice" }];
});

app.use(router.routes());

await app.listen({ port: 8000 });