Node.js built-in module support in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Deno code using Node.js 'path' module?

Consider this Deno script that imports the Node.js 'path' module. What will it print?

Deno

import path from 'node:path';
console.log(path.basename('/foo/bar/baz.txt'));

A 'baz.txt'

B '/foo/bar/baz.txt'

C 'baz'

D SyntaxError: Cannot find module 'node:path'

💡 Hint

Think about what the basename function returns from a file path.

The basename function returns the last part of a path, which is the file name with extension.

📝 Syntax

intermediate

Which import statement correctly loads the Node.js 'fs' module in Deno?

Deno supports Node.js built-in modules with special import paths. Which option correctly imports the 'fs' module?

A const fs = require('fs');

B import fs from 'node:fs';

C import * as fs from 'node_fs';

D import fs from 'fs';

💡 Hint

Deno uses the 'node:' prefix for Node.js built-in modules.

Deno requires the 'node:' prefix to import Node.js built-in modules. The other options are invalid in Deno.

🔧 Debug

advanced

Why does this Deno code throw an error when importing 'node:crypto'?

This code throws an error when run in Deno. What is the cause?

Deno

import crypto from 'node:crypto';
console.log(crypto.randomUUID());

A The import path should be 'crypto' without the 'node:' prefix.

B The 'randomUUID' method does not exist in the 'crypto' module.

C Deno does not support the 'node:crypto' module by default without enabling compatibility flags.

D The code is missing an async function wrapper.

💡 Hint

Check Deno's Node.js compatibility support and flags.

Deno requires enabling Node.js compatibility or specific flags to use 'node:crypto'. Without them, the import fails.

❓ state_output

advanced

What is the output of this Deno script using 'node:events' module?

Analyze the output of this code snippet:

Deno

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.on('greet', (name) => console.log(`Hello, ${name}!`));
emitter.emit('greet', 'Alice');

A Hello, Alice!

B greet Alice

C Error: EventEmitter is not a constructor

D No output

💡 Hint

EventEmitter triggers the listener when 'emit' is called.

The listener logs 'Hello, Alice!' when the 'greet' event is emitted with 'Alice'.

🧠 Conceptual

expert

Which statement about Node.js built-in module support in Deno is TRUE?

Select the correct statement about how Deno supports Node.js built-in modules.

A Deno fully supports all Node.js built-in modules natively without any flags or compatibility layers.

B Deno does not support Node.js built-in modules at all.

C Deno requires rewriting Node.js built-in modules in TypeScript to use them.

D Deno supports Node.js built-in modules only when imported with the 'node:' prefix and with compatibility enabled.

💡 Hint

Think about Deno's approach to Node.js compatibility.

Deno supports Node.js built-in modules with the 'node:' prefix and enabling compatibility features or flags.

Practice

1. What is the correct way to import the Node.js built-in path module in Deno?

easy

A. import * as path from "deno_path";

B. import path from "path";

C. import { path } from "deno:path";

D. import path from "node:path";

Solution

1. Step 1: Understand Deno's Node.js module import syntax

   Deno uses the "node:" prefix to import Node.js built-in modules.

2. Step 2: Apply correct import statement for path

   The correct import is import path from "node:path"; to access Node.js path module in Deno.

3. Final Answer:

   import path from "node:path"; -> Option D

4. Quick Check:

   Node.js built-in modules use "node:" prefix in Deno [OK]

Hint: Use "node:" prefix to import Node.js built-ins in Deno [OK]

Common Mistakes:

• Omitting the "node:" prefix
• Using "deno:" prefix instead
• Trying to import from "path" without prefix
• Using incorrect module names

2. Which of the following is the correct syntax to import the Node.js fs module in Deno?

easy

A. import * as fs from "node:fs";

B. import { fs } from "node:fs";

C. import fs from "fs";

D. import fs from "deno:fs";

Solution

1. Step 1: Recall import style for Node.js modules in Deno

   Deno requires the "node:" prefix and supports namespace import for modules like fs.

2. Step 2: Choose correct import syntax for fs

   The correct syntax is import * as fs from "node:fs"; to import all fs exports as a namespace.

3. Final Answer:

   import * as fs from "node:fs"; -> Option A

4. Quick Check:

   Namespace import with "node:" prefix is correct [OK]

Hint: Use namespace import with "node:" prefix for modules like fs [OK]

Common Mistakes:

• Using default import for modules that export multiple members
• Omitting the "node:" prefix
• Trying to destructure import incorrectly
• Using "deno:" prefix

3. What will be the output of the following Deno code snippet?

import { join } from "node:path";
const fullPath = join("/home", "user", "docs");
console.log(fullPath);

medium

A. /home\user\docs

B. /home/user/docs

C. home/user/docs

D. Error: Module not found

Solution

1. Step 1: Understand join from Node.js path module

   The join function combines path segments using the correct separator for the OS.

2. Step 2: Analyze the given path segments

   Segments "/home", "user", "docs" joined on Unix-like OS produce "/home/user/docs".

3. Final Answer:

   /home/user/docs -> Option B

4. Quick Check:

   Node.js path.join joins segments with OS separator [OK]

Hint: Node.js path.join joins paths with OS-specific separator [OK]

Common Mistakes:

• Assuming Windows separator on Unix
• Expecting error due to import
• Confusing join with concat
• Ignoring leading slash in first segment

4. Identify the error in this Deno code importing Node.js buffer module:

import { Buffer } from "buffer";
const buf = Buffer.from("hello");
console.log(buf.toString());

medium

A. Missing "node:" prefix in import path

B. Buffer is not exported from Node.js buffer module

C. Buffer.from is not a function

D. toString() method does not exist on Buffer

Solution

1. Step 1: Check import path for Node.js built-in modules in Deno

   Deno requires the "node:" prefix to import Node.js built-in modules like buffer.

2. Step 2: Identify the missing prefix causing module resolution error

   Importing from "buffer" without "node:" prefix will fail in Deno.

3. Final Answer:

   Missing "node:" prefix in import path -> Option A

4. Quick Check:

   Always use "node:" prefix for Node.js built-ins in Deno [OK]

Hint: Always prefix Node.js modules with "node:" in Deno imports [OK]

Common Mistakes:

• Omitting the "node:" prefix
• Assuming default import works without prefix
• Confusing Deno modules with Node.js modules
• Misunderstanding Buffer API

5. You want to use the Node.js events module in Deno to create an event emitter. Which code snippet correctly imports and uses it?

hard

A. import EventEmitter from "node:events";
const emitter = new EventEmitter();

B. import { EventEmitter } from "node:events";
const emitter = new events.EventEmitter();

C. import * as events from "node:events";
const emitter = new events.EventEmitter();

D. import events from "events";
const emitter = new events.EventEmitter();

Solution

1. Step 1: Understand how Node.js events module exports EventEmitter

   The events module exports EventEmitter as a named export, so namespace import is common.

2. Step 2: Check correct import syntax with "node:" prefix and usage

   Using import * as events from "node:events"; then new events.EventEmitter() is correct in Deno.

3. Final Answer:

   import * as events from "node:events"; const emitter = new events.EventEmitter(); -> Option C

4. Quick Check:

   Namespace import with "node:" prefix for EventEmitter [OK]

Hint: Use namespace import for EventEmitter from "node:events" [OK]

Common Mistakes:

• Using default import for EventEmitter
• Omitting "node:" prefix
• Importing from "events" without prefix
• Trying to destructure EventEmitter incorrectly