Using npm packages in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main command to enable npm package support in Deno?

Use the --unstable flag with Deno commands to enable npm package support, for example: deno run --unstable.

beginner

How do you import an npm package in a Deno script?

You import npm packages using the <code>npm:</code> prefix in the import statement, like <code>import _ from "npm:lodash";</code>.

intermediate

Why does Deno require the --unstable flag for npm packages?

Because npm support is still experimental in Deno, the --unstable flag signals you accept using features that may change.

intermediate

What file does Deno create to track npm dependencies?

Deno creates a deno.lock file to track and lock npm dependencies.

beginner

Can you use npm packages without internet after installing them once in Deno?

Yes, Deno caches npm packages locally after the first install, so you can use them offline later.

Which prefix is used to import npm packages in Deno?

A deno:

B npm:

C node:

D pkg:

Deno uses the npm: prefix to import npm packages.

What flag must you add to run Deno scripts using npm packages?

A --unstable

B --allow-net

C --import-map

D --reload

The --unstable flag enables experimental npm package support in Deno.

After importing an npm package, where does Deno store it?

A In a local cache folder

B In the system temp folder

C In the project root

D It does not store it locally

Deno caches npm packages locally to avoid re-downloading them.

Which file does Deno use to lock npm package versions?

A lockfile.json

B package.json

C deno.json

D deno.lock

Deno uses deno.lock to lock npm package versions.

Can you import npm packages directly without any special flags in Deno?

A Only if you use import maps

B Yes, always

C No, you need the --unstable flag

D Only for certain packages

Npm package support is experimental and requires the --unstable flag.

Explain how to import and use an npm package in a Deno script.

Think about the command line flag and import syntax.

Describe why Deno requires special handling for npm packages and what files it uses to manage them.

Consider stability and dependency tracking.

Practice

1. What is the correct way to import an npm package named lodash in a Deno script?

easy

A. import _ from "npm:lodash";

B. import _ from "https://cdn.lodash.com/lodash.js";

C. import _ from "deno:lodash";

D. import _ from "node_modules/lodash";

Solution

1. Step 1: Understand Deno npm import syntax

   Deno uses the syntax import package from "npm:package-name" to import npm packages directly.

2. Step 2: Match the syntax to the lodash package

   Using import _ from "npm:lodash"; correctly imports lodash from npm in Deno.

3. Final Answer:

   import _ from "npm:lodash"; -> Option A

4. Quick Check:

   npm import syntax = import from "npm:package" [OK]

Hint: Use import from "npm:package-name" to add npm packages [OK]

Common Mistakes:

• Using URL imports instead of npm: prefix
• Trying to import from local node_modules folder
• Using incorrect prefix like deno: or https:

2. Which of the following is the correct syntax to import the axios package from npm in Deno?

easy

A. import axios from "npm:axios";

B. import axios from "npm axios";

C. import axios from "npm:/axios";

D. import axios from "npm::axios";

Solution

1. Step 1: Check correct npm import syntax

   The correct syntax uses a colon after npm, no spaces or extra characters: npm:package-name.

2. Step 2: Identify the valid option

   Only import axios from "npm:axios"; matches the correct syntax.

3. Final Answer:

   import axios from "npm:axios"; -> Option A

4. Quick Check:

   Correct npm import = "npm:package" [OK]

Hint: Remember no spaces or extra symbols in npm import path [OK]

Common Mistakes:

• Adding spaces between npm and package name
• Using slashes or double colons incorrectly
• Omitting the npm: prefix

3. Given this Deno code snippet, what will be the output?

import chalk from "npm:chalk";
console.log(chalk.green("Success!"));

medium

A. Runtime error: chalk is undefined.

B. A syntax error because chalk cannot be imported this way.

C. The text 'chalk.green("Success!")' printed literally.

D. The text 'Success!' printed in green color in the console.

Solution

1. Step 1: Understand chalk npm package usage

   Chalk colors console text. Importing via npm in Deno works if syntax is correct.

2. Step 2: Analyze the console.log statement

   Calling chalk.green("Success!") returns the string 'Success!' styled green in supported terminals.

3. Final Answer:

   The text 'Success!' printed in green color in the console. -> Option D

4. Quick Check:

   npm chalk import + chalk.green() = green text output [OK]

Hint: chalk colors text; npm import works if syntax is right [OK]

Common Mistakes:

• Assuming import fails without checking syntax
• Expecting literal string output instead of colored text
• Confusing runtime errors with import errors

4. What is wrong with this Deno code snippet?

import express from "npm:express";
const app = express();
app.listen(3000);
console.log("Server running on port 3000");

medium

A. The import statement syntax is incorrect.

B. Deno does not support importing express from npm.

C. Missing permission flag to allow network access.

D. express() is not a function in this context.

Solution

1. Step 1: Check import syntax and package support

   Importing express from npm is valid syntax in Deno 1.28+ and later.

2. Step 2: Identify runtime requirements

   Running a server requires network permissions. Without --allow-net, Deno blocks network calls causing failure.

3. Final Answer:

   Missing permission flag to allow network access. -> Option C

4. Quick Check:

   Network access needs --allow-net flag in Deno [OK]

Hint: Remember to add --allow-net for server network access [OK]

Common Mistakes:

• Assuming all npm packages work without permissions
• Confusing import syntax errors with permission errors
• Not running Deno with required flags

5. You want to use the npm package uuid in Deno to generate a version 4 UUID. Which code snippet correctly imports and uses it?

hard

A. import v4 from "npm:uuid";
console.log(v4());

B. import { v4 as uuidv4 } from "npm:uuid";
console.log(uuidv4());

C. import uuid from "npm:uuid";
console.log(uuid.v4());

D. import * as uuid from "npm:uuid";
console.log(uuid.v4);

Solution

1. Step 1: Understand uuid package export style

   The uuid package exports v4 as a named export, so it must be imported with curly braces or aliased.

2. Step 2: Check each import usage

   A: Default import fails--no default export in uuid.
   B: Correct named import with alias and proper call.
   C: Default import lacks v4 property.
   D: Namespace import ok, but missing () logs function, not UUID.

3. Final Answer:

   import { v4 as uuidv4 } from "npm:uuid"; console.log(uuidv4()); -> Option B

4. Quick Check:

   Named import with alias for v4 UUID = import { v4 as uuidv4 } from "npm:uuid"; console.log(uuidv4()); [OK]

Hint: Use named import with alias for uuid v4 function [OK]

Common Mistakes:

• Importing default when package exports named functions
• Calling v4 without importing it properly
• Omitting parentheses when calling uuid.v4