Migrating a Node.js project to Deno - Performance & Optimization

Performance: Migrating a Node.js project to Deno

MEDIUM IMPACT

This affects the initial load speed and runtime performance by changing how modules are loaded and executed in the browser and server environment.

Loading external modules in a Node.js project versus Deno

Deno

import { serve } from 'https://deno.land/std@0.200.0/http/server.ts';
serve(() => new Response('Hello from Deno'), { port: 3000 });

Deno uses native ES modules loaded directly via URLs, reducing bundle size and eliminating bundling overhead.

📈 Performance Gainsaves 100kb+ bundle size, non-blocking module loading

Loading external modules in a Node.js project versus Deno

Deno

const express = require('express');
const app = express();
app.listen(3000);

Node.js uses CommonJS modules and requires bundling or transpiling for browser compatibility, increasing bundle size and load time.

📉 Performance Costadds 100kb+ to bundle, blocks rendering during module resolution

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Node.js with CommonJS and bundling	High due to large bundles	Multiple reflows if UI waits on data	High paint cost from blocking scripts	[X] Bad
Deno with native ES modules	Low, modules loaded on demand	Single reflow after async data load	Lower paint cost due to non-blocking scripts	[OK] Good

Rendering Pipeline

Migrating to Deno changes how modules are loaded and executed, affecting the browser's critical rendering path by reducing blocking scripts and improving asynchronous IO.

→Script Parsing

→Layout

→Paint

⚠️ BottleneckScript Parsing and Execution

Core Web Vital Affected

LCP

This affects the initial load speed and runtime performance by changing how modules are loaded and executed in the browser and server environment.

Optimization Tips

1Use native ES modules to reduce blocking script loads.

2Prefer async APIs to avoid blocking the main thread.

3Import third-party libraries on demand to minimize bundle size.

Performance Quiz - 3 Questions

How does using native ES modules in Deno improve performance compared to Node.js CommonJS modules?

A They load asynchronously and reduce blocking during startup.

B They bundle all code into one file to reduce requests.

C They use synchronous loading to speed up execution.

D They cache modules on the server to avoid downloads.

DevTools: Performance

How to check: Record a page load and interaction timeline; look for long scripting tasks and blocking times.

What to look for: Shorter scripting tasks and faster time to first contentful paint indicate better performance with Deno.

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