Migrating a Node.js project to Deno

Introduction

Moving a Node.js project to Deno helps you use a modern runtime with built-in security and simpler module management.

You want to try a runtime that has built-in TypeScript support without extra setup.

You want better security by default, controlling file and network access.

You want to avoid using package managers like npm and use URL imports instead.

You want to use modern JavaScript and TypeScript features easily.

You want to simplify your project setup with a single executable.

Syntax

Deno

import { serve } from "https://deno.land/std@0.201.0/http/server.ts";

serve((_req) => new Response("Hello from Deno!"), { port: 8000 });

Deno uses URL imports instead of package.json and node_modules.

Permissions must be explicitly granted when running Deno scripts.

Examples

Importing a file system function from Deno's standard library using a URL.

Deno

import { readFileStr } from "https://deno.land/std/fs/mod.ts";

const data = await readFileStr("./file.txt");
console.log(data);

Using Deno's built-in API to read a file asynchronously.

Deno

const decoder = new TextDecoder("utf-8");
const data = await Deno.readFile("./file.txt");
console.log(decoder.decode(data));

Using the global fetch API in Deno just like in browsers.

Deno

const response = await fetch("https://api.example.com/data");
const json = await response.json();
console.log(json);

Sample Program

This simple Deno server listens on port 8080 and responds with a greeting message.

Deno

import { serve } from "https://deno.land/std@0.201.0/http/server.ts";

serve((_req) => new Response("Hello from Deno!"), { port: 8080 });

Important Notes

Node.js uses CommonJS modules, but Deno uses ES modules with URL imports.

Deno requires explicit permission flags like --allow-read or --allow-net when running scripts.

Some Node.js built-in modules are not available in Deno; you may need to find Deno-compatible alternatives.

Summary

Deno uses URL imports instead of npm packages.

You must grant permissions explicitly in Deno.

Code changes may be needed to replace Node.js APIs with Deno APIs.

Practice

1. Which of the following is a key difference when migrating a Node.js project to Deno?

easy

A. Deno uses CommonJS modules by default.

B. Deno uses URL imports instead of npm packages.

C. Deno requires no code changes from Node.js.

D. Deno automatically grants all permissions to the code.

Solution

1. Step 1: Understand module import differences

   Node.js uses npm packages imported by name, while Deno uses direct URL imports.

2. Step 2: Check permission handling and module system

   Deno requires explicit permission grants and uses ES modules, not CommonJS.

3. Final Answer:

   Deno uses URL imports instead of npm packages. -> Option B

4. Quick Check:

   Module import style = D [OK]

Hint: Remember: Deno imports modules via URLs, not npm names. [OK]

Common Mistakes:

• Assuming Deno uses npm packages directly
• Thinking Deno grants permissions automatically
• Believing Node.js code runs unchanged in Deno

2. Which syntax correctly imports the lodash library in Deno?

easy

A. import _ from 'https://cdn.skypack.dev/lodash';

B. const _ = require('lodash');

C. import _ from 'lodash';

D. import _ from 'npm:lodash';

Solution

1. Step 1: Identify Deno import style

   Deno imports modules using URLs, not package names or require().

2. Step 2: Check each option

   import _ from 'https://cdn.skypack.dev/lodash'; uses a valid URL import from a CDN, which is correct for Deno.

3. Final Answer:

   import _ from 'https://cdn.skypack.dev/lodash'; -> Option A

4. Quick Check:

   URL import syntax = C [OK]

Hint: Use full URLs for imports in Deno, not package names. [OK]

Common Mistakes:

• Using require() which is Node.js only
• Importing by package name without URL
• Using npm: prefix which Deno does not support

3. What will be the output when running this Deno script?

import { readFileStr } from 'https://deno.land/std/fs/mod.ts';
const data = await readFileStr('./file.txt');
console.log(data.trim());

Assuming file.txt contains " Hello Deno \n".

medium

A. Error: readFileStr is not a function

B. Hello Deno

C. undefined

D. oneD olleH

Solution

1. Step 1: Understand readFileStr usage

   The function readFileStr was deprecated and removed in recent versions of Deno std library. Using it will cause an error.

2. Step 2: Analyze output

   Since readFileStr is not a function, the script will throw an error.

3. Final Answer:

   Error: readFileStr is not a function -> Option A

4. Quick Check:

   Function availability = D [OK]

Hint: Use Deno.readTextFile or Deno.readTextFileSync instead of deprecated readFileStr. [OK]

Common Mistakes:

• Expecting original spaces without trim()
• Confusing readFileStr with non-existent function
• Ignoring await causing undefined

4. You migrated a Node.js script to Deno but get this error: PermissionDenied: read access to './data.json' is not allowed. How do you fix it?

medium

A. Install the missing npm package for file access.

B. Change the file path to an absolute path.

C. Run the script with deno run --allow-read flag.

D. Use require() instead of import.

Solution

1. Step 1: Understand Deno's permission model

   Deno requires explicit permission flags to access files for security.

2. Step 2: Apply correct permission flag

   Using --allow-read grants read access to files, fixing the error.

3. Final Answer:

   Run the script with deno run --allow-read flag. -> Option C

4. Quick Check:

   Permission error fix = A [OK]

Hint: Add --allow-read flag to grant file read permission. [OK]

Common Mistakes:

• Ignoring permission flags in Deno
• Trying to fix path instead of permissions
• Using Node.js require() which Deno disallows

5. You have this Node.js code snippet using the 'fs' module:

const fs = require('fs');
const data = fs.readFileSync('./info.txt', 'utf8');
console.log(data);

How should you rewrite it in Deno to read the file synchronously?

hard

A. const data = Deno.readFile('./info.txt'); console.log(data);

B. const data = await Deno.readTextFile('./info.txt'); console.log(data);

C. import fs from 'https://deno.land/std/fs/mod.ts'; const data = fs.readFileSync('./info.txt');

D. const data = Deno.readTextFileSync('./info.txt'); console.log(data);

Solution

1. Step 1: Identify synchronous file read in Deno

   Deno provides readTextFileSync() for synchronous text file reading.

2. Step 2: Compare options

   const data = Deno.readTextFileSync('./info.txt'); console.log(data); correctly uses readTextFileSync and logs the data synchronously.

3. Final Answer:

   const data = Deno.readTextFileSync('./info.txt'); console.log(data); -> Option D

4. Quick Check:

   Synchronous file read in Deno = A [OK]

Hint: Use Deno.readTextFileSync for sync file reading like Node's readFileSync. [OK]

Common Mistakes:

• Using await with synchronous functions
• Trying to import Node.js fs module in Deno
• Using readFile which returns Uint8Array, not string