Importing from npm packages in Deno - Performance & Optimization

Performance: Importing from npm packages

MEDIUM IMPACT

This affects page load speed and initial script execution time by controlling how external code is fetched and cached.

Using npm packages in a Deno project

Deno

import { random } from 'npm:lodash-es';

console.log(random(1, 10));

Imports only the needed function from lodash-es, reducing bundle size and load time.

📈 Performance GainSaves ~60kb, faster initial load, non-blocking smaller import

Using npm packages in a Deno project

Deno

import _ from 'npm:lodash';

console.log(_.random(1, 10));

Imports the entire lodash package, increasing bundle size and network load.

📉 Performance CostAdds ~70kb to bundle, blocks rendering until full package loads

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Import full npm package	0 (no DOM changes)	0	0	[X] Bad
Import selective functions from npm package	0	0	0	[OK] Good

Rendering Pipeline

When importing npm packages, the browser or runtime fetches and parses the code before executing scripts, affecting the critical rendering path.

→Network Fetch

→Script Parsing

→Script Execution

→Rendering

⚠️ BottleneckNetwork Fetch and Script Parsing due to large package size

Core Web Vital Affected

LCP

This affects page load speed and initial script execution time by controlling how external code is fetched and cached.

Optimization Tips

1Avoid importing entire npm packages if only a few functions are needed.

2Prefer ES module versions or smaller package variants for faster load.

3Leverage Deno's caching to prevent repeated network fetches.

Performance Quiz - 3 Questions

What is a performance risk when importing entire npm packages in Deno?

A Faster script execution due to caching

B Increased bundle size and slower initial load

C Reduced network requests

D Improved rendering speed

DevTools: Network

How to check: Open DevTools, go to Network panel, filter by JS files, and observe the size and load time of imported packages.

What to look for: Large JS files from npm imports indicate slower load; smaller files mean better performance.

Practice

1. In Deno, how do you import a package from npm?

easy

A. Use import pkg from "deno:package-name";

B. Use import pkg from "https://npmjs.com/package-name";

C. Use import pkg from "npm:package-name";

D. Use require("package-name")

Solution

1. Step 1: Understand Deno's npm import syntax

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

2. Step 2: Identify the correct import statement

   The correct syntax is import pkg from "npm:package-name"; which tells Deno to fetch from npm.

3. Final Answer:

   Use import pkg from "npm:package-name"; -> Option C

4. Quick Check:

   npm imports use "npm:" prefix [OK]

Hint: Remember npm imports start with "npm:" prefix [OK]

Common Mistakes:

• Using URL instead of npm prefix
• Using require() which is Node.js syntax
• Using wrong prefix like deno:

2. Which of the following is the correct syntax to import only the join function from the npm package path in Deno?

easy

A. import { join } from "npm:path";

B. import join from "npm:path/join";

C. import * as join from "npm:path";

D. import join from "path";

Solution

1. Step 1: Understand named imports from npm packages in Deno

   You can import specific functions using curly braces from the package name with npm prefix.

2. Step 2: Match correct syntax

   The correct syntax is import { join } from "npm:path"; to import only the join function.

3. Final Answer:

   import { join } from "npm:path"; -> Option A

4. Quick Check:

   Named imports use curly braces with npm prefix [OK]

Hint: Use curly braces to import specific parts from npm packages [OK]

Common Mistakes:

• Trying to import default when function is named export
• Using wrong path like "npm:path/join"
• Omitting npm prefix

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

import { v4 } from "npm:uuid";
console.log(typeof v4);

medium

A. "string"

B. "object"

C. "undefined"

D. "function"

Solution

1. Step 1: Understand what v4 from uuid is

   The v4 export from the uuid package is a function that generates UUID strings.

2. Step 2: Check the type of v4

   Using typeof v4 returns "function" because v4 is a function.

3. Final Answer:

   "function" -> Option D

4. Quick Check:

   uuid v4 is a function [OK]

Hint: Remember uuid v4 is a function generating IDs [OK]

Common Mistakes:

• Thinking v4 is a string UUID directly
• Confusing type with object or undefined
• Forgetting to import with npm prefix

4. You wrote this import in Deno:

import express from "npm:express";

But running the code gives an error: SyntaxError: The requested module does not provide an export named 'default'. What is the likely fix?

medium

A. Change import to import * as express from "npm:express";

B. Change import to import express from "express";

C. Add --allow-net flag when running Deno

D. Use require("express") instead

Solution

1. Step 1: Understand the error about missing default export

   The error means express does not have a default export, so import express from ... fails.

2. Step 2: Use namespace import to get all exports

   Using import * as express from "npm:express"; imports all exports as an object, fixing the error.

3. Final Answer:

   Change import to import * as express from "npm:express"; -> Option A

4. Quick Check:

   No default export means use * as import [OK]

Hint: If default export missing, try namespace import * as [OK]

Common Mistakes:

• Removing npm prefix
• Using require() which is not supported in Deno
• Ignoring error and expecting code to run

5. You want to import the lodash package from npm in Deno and use its chunk function to split an array into smaller arrays of size 2. Which code snippet correctly does this?

hard

A. import { chunk } from "npm:lodash";
console.log(chunk([1,2,3,4,5], 2));

B. import _ from "npm:lodash";
console.log(_.chunk([1,2,3,4,5], 2));

C. import chunk from "npm:lodash/chunk";
console.log(chunk([1,2,3,4,5], 2));

D. import { chunk } from "lodash";
console.log(chunk([1,2,3,4,5], 2));

Solution

1. Step 1: Understand lodash exports and npm import in Deno

   Lodash exports a default object with functions like chunk. To use chunk, import the default as _.

2. Step 2: Check import syntax with npm prefix

   Use import _ from "npm:lodash"; to import lodash default export, then call _.chunk(...).

3. Final Answer:

   import _ from "npm:lodash"; console.log(_.chunk([1,2,3,4,5], 2)); -> Option B

4. Quick Check:

   Lodash default import then call chunk method [OK]

Hint: Lodash exports default object; import default and call method [OK]

Common Mistakes:

• Trying named import for chunk which lodash does not export separately
• Omitting npm prefix
• Importing from plain "lodash" without npm prefix