Why Node.js compatibility enables migration in Deno - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import a Node.js module in Deno.

Deno

import express from '[1]';

Drag options to blanks, or click blank then click option'

Aexpress

Bhttps://deno.land/std/http/server.ts

Cnode_modules/express

Dnpm:express

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to import Node.js modules without the npm prefix.

Using local paths instead of npm package names.

In Deno, to import a Node.js package, you use the npm: prefix.

2fill in blank

medium

Complete the code to run a Node.js script in Deno with compatibility.

Deno

deno run --compat --unstable [1]

Drag options to blanks, or click blank then click option'

Aapp.js

Bapp.ts

Cserver.py

Dindex.html

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to run TypeScript files without compiling.

Using non-JavaScript files.

To run a Node.js JavaScript file with compatibility, use deno run --compat --unstable followed by the JavaScript file.

3fill in blank

hard

Fix the error in the import statement to enable Node.js compatibility in Deno.

Deno

import fs from '[1]';

Drag options to blanks, or click blank then click option'

Anpm:fs

Bfs

Cnode:fs

Dhttps://deno.land/std/fs/mod.ts

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'fs' without prefix causes errors.

Using npm prefix for built-in Node.js modules.

Using node:fs tells Deno to use the Node.js built-in fs module with compatibility.

4fill in blank

hard

Fill both blanks to create a Deno script that imports a Node.js package and runs it with compatibility.

Deno

import http from '[1]';

await Deno.[2]();

Drag options to blanks, or click blank then click option'

Anpm:http

Brun

Cserve

Dnode:http

Attempts:

3 left

💡 Hint

Common Mistakes

Using npm prefix for built-in modules.

Calling a non-existent method on Deno.

Use node:http to import Node.js HTTP module and serve to start the server.

5fill in blank

hard

Fill all three blanks to migrate a Node.js Express app to Deno with compatibility.

Deno

import express from '[1]';

const app = express();
app.get('/', (req, [2]) => {
  [3].send('Hello from Deno with Node.js compatibility!');
});

Drag options to blanks, or click blank then click option'

Anpm:express

Bres

Cresponse

Dreq

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'req' instead of 'res' to send response.

Importing Express without npm prefix.

Import Express with npm:express. The second argument is the response object, commonly named res, used to send the response.

Practice

1. Why does Node.js compatibility make migrating to Deno easier?

easy

A. Because Node.js and Deno share the same package manager

B. Because Deno uses the same runtime as Node.js internally

C. Because Deno requires no internet connection to run Node.js code

D. Because you can reuse existing Node.js code and libraries without rewriting them

Solution

1. Step 1: Understand Node.js compatibility in Deno

   Deno supports many Node.js APIs and npm packages, allowing reuse of existing code.

2. Step 2: Connect compatibility to migration effort

   Reusing code means less rewriting and fewer errors, making migration easier and faster.

3. Final Answer:

   Because you can reuse existing Node.js code and libraries without rewriting them -> Option D

4. Quick Check:

   Reuse code = easier migration [OK]

Hint: Think: reusing code saves time and effort [OK]

Common Mistakes:

• Confusing runtime internals with compatibility
• Assuming Deno uses Node.js package manager
• Believing no internet is needed for Node.js code

2. Which of the following is the correct way to import a Node.js package in Deno?

easy

A. import express from 'npm:express';

B. import express from 'node_modules/express';

C. import express from 'http://express.com';

D. import express from './express.js';

Solution

1. Step 1: Recall Deno's npm package import syntax

   Deno uses the prefix 'npm:' to import Node.js packages directly from npm.

2. Step 2: Evaluate each import statement

   Only 'import express from "npm:express";' uses the correct Deno syntax for Node.js packages.

3. Final Answer:

   import express from 'npm:express'; -> Option A

4. Quick Check:

   npm prefix = correct import [OK]

Hint: Look for 'npm:' prefix in import statements [OK]

Common Mistakes:

• Using relative paths for npm packages
• Trying to import from URLs without npm prefix
• Confusing Node.js import style with Deno's

3. What will the following Deno code output if the 'lodash' package is imported using Node.js compatibility?

import _ from 'npm:lodash';
console.log(_.capitalize('deno migration'));

medium

A. 'Deno migration'

B. 'deno migration'

C. SyntaxError

D. ReferenceError: _ is not defined

Solution

1. Step 1: Understand lodash's capitalize function

   It capitalizes the first letter of the string, so 'deno migration' becomes 'Deno migration'.

2. Step 2: Confirm Node.js compatibility import works

   Using 'npm:lodash' imports lodash correctly in Deno, so the function runs without error.

3. Final Answer:

   'Deno migration' -> Option A

4. Quick Check:

   lodash.capitalize works = 'Deno migration' [OK]

Hint: Recall lodash.capitalize capitalizes first letter [OK]

Common Mistakes:

• Expecting original string without capitalization
• Thinking import fails causing ReferenceError
• Confusing syntax errors with runtime errors

4. Identify the error in this Deno code using Node.js compatibility:

import fs from 'fs';
const data = await fs.promises.readFile('./file.txt', 'utf-8');
console.log(data);

medium

A. readFile does not return a promise

B. The import path should use 'npm:fs' prefix

C. fs module is not available in Deno without compatibility flag

D. Missing await keyword before import

Solution

1. Step 1: Check how Node.js core modules are imported in Deno

   Deno does not support direct import of Node.js core modules like 'fs' without enabling compatibility.

2. Step 2: Understand Deno's compatibility requirements

   To use 'fs', Deno requires the --compat flag and proper import from 'node:fs' or 'npm:' packages.

3. Final Answer:

   fs module is not available in Deno without compatibility flag -> Option C

4. Quick Check:

   Compatibility flag needed for core modules [OK]

Hint: Core Node modules need compatibility flag in Deno [OK]

Common Mistakes:

• Assuming 'fs' imports like in Node.js
• Confusing promise behavior of readFile
• Thinking await is needed before import

5. You want to migrate a Node.js project using Express to Deno. Which approach best leverages Node.js compatibility to minimize migration effort?

hard

A. Rewrite all Express routes using Deno's native HTTP server APIs

B. Use Deno's compatibility layer to import Express via 'npm:express' and run with --compat flag

C. Convert Express code to use Deno's Oak framework manually

D. Run the Node.js project separately and call it from Deno via subprocess

Solution

1. Step 1: Evaluate migration options for Express in Deno

   Rewriting or converting code is time-consuming and error-prone.

2. Step 2: Use Node.js compatibility layer

   Deno's compatibility layer allows importing Express with 'npm:' prefix and running with --compat flag, enabling reuse of existing code.

3. Final Answer:

   Use Deno's compatibility layer to import Express via 'npm:express' and run with --compat flag -> Option B

4. Quick Check:

   Compatibility layer + npm import = minimal rewrite [OK]

Hint: Use compatibility layer and npm imports to reuse code [OK]

Common Mistakes:

• Thinking rewriting is always necessary
• Confusing Oak framework as direct Express replacement
• Ignoring compatibility flags needed