Relative vs absolute URL resolution in Node.js - Visual Side-by-Side Comparison

Concept Flow - Relative vs absolute URL resolution

Start with base URL

↓

Receive input URL

↓

Is input URL absolute?

Yes→Use input URL as is

No↓

Combine base URL with relative input

↓

Normalize combined URL

↓

Return resolved absolute URL

This flow shows how Node.js resolves a URL: it checks if the input is absolute, if not, it combines it with a base URL and normalizes the result.

Execution Sample

Node.js

import { URL } from 'url';
const base = new URL('https://example.com/path/');
const relative = 'sub/page.html';
const resolved = new URL(relative, base);
console.log(resolved.href);

This code resolves a relative URL against a base URL and prints the absolute URL.

Execution Table

Step	Input URL	Base URL	Is Absolute?	Action	Resulting URL
1	'sub/page.html'	'https://example.com/path/'	No	Combine base + relative	'https://example.com/path/sub/page.html'
2	'https://google.com/search'	'https://example.com/path/'	Yes	Use input as is	'https://google.com/search'
3	'../up.html'	'https://example.com/path/'	No	Combine and normalize	'https://example.com/up.html'
4	'/root.html'	'https://example.com/path/'	No	Combine with root path	'https://example.com/root.html'
5	'ftp://files.com/file.txt'	'https://example.com/path/'	Yes	Use input as is	'ftp://files.com/file.txt'

💡 Resolution ends after returning the absolute URL based on input type.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
resolved.href	undefined	https://example.com/path/sub/page.html	https://google.com/search	https://example.com/up.html	https://example.com/root.html	ftp://files.com/file.txt

Key Moments - 3 Insights

Why does '../up.html' resolve to 'https://example.com/up.html' instead of 'https://example.com/path/../up.html'?

Because the URL resolution normalizes the path by removing '..' and its preceding directory, as shown in step 3 of the execution_table.

What happens if the input URL is already absolute?

The input URL is used as is without combining with the base URL, as shown in steps 2 and 5.

How does a leading slash in the relative URL affect resolution?

A leading slash resets the path to the root of the base URL's domain, demonstrated in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the resolved URL at step 3?

A https://example.com/up.html

B https://example.com/path/up.html

C https://example.com/path/../up.html

D https://example.com/path/../path/up.html

💡 Hint

Check the 'Resulting URL' column for step 3 in the execution_table.

At which step does the input URL get used without combining with the base URL?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look for 'Is Absolute?' = Yes and 'Action' = Use input as is in the execution_table.

If the relative URL was changed from 'sub/page.html' to '/sub/page.html' in step 1, what would the resolved URL be?

A https://example.com/path/sub/page.html

B https://example.com/path//sub/page.html

C https://example.com/sub/page.html

D https://example.com/subpage.html

💡 Hint

Recall that a leading slash resets the path to the root, as shown in step 4.

Concept Snapshot

URL resolution in Node.js:
- If input URL is absolute, use it directly.
- If relative, combine with base URL.
- Normalize paths (remove '..' and '.')
- Leading slash resets to domain root
- Use new URL(relativeOrAbsolute, baseURL) to resolve

Full Transcript

This lesson shows how Node.js resolves URLs using the URL constructor. It starts with a base URL and an input URL. If the input is absolute, it uses it directly. If relative, it combines with the base and normalizes the path. Examples include relative paths like 'sub/page.html', parent directory references like '../up.html', and absolute URLs like 'https://google.com/search'. The process ensures the final URL is absolute and normalized for use in applications.

Practice

1. Which of the following is a correct description of a relative URL in Node.js?

easy

A. A URL that includes the protocol and domain name.

B. A URL that depends on a base URL to form a full address.

C. A URL that always starts with 'http://' or 'https://'.

D. A URL that cannot be resolved using the URL class.

Solution

1. Step 1: Understand relative URL meaning

   A relative URL does not include the full path and depends on a base URL to form a complete address.

2. Step 2: Compare options

   A URL that depends on a base URL to form a full address. correctly describes this. Options B and C describe absolute URLs. A URL that cannot be resolved using the URL class. is incorrect because Node.js URL class can resolve relative URLs with a base.

3. Final Answer:

   A URL that depends on a base URL to form a full address. -> Option B

4. Quick Check:

   Relative URL = depends on base URL [OK]

Hint: Relative URLs need a base URL to become complete [OK]

Common Mistakes:

• Confusing relative URLs with absolute URLs
• Thinking relative URLs include protocol
• Believing relative URLs cannot be resolved

2. Which of the following is the correct way to create a new URL object for a relative path '/images/photo.jpg' with base 'https://example.com/gallery/' in Node.js?

easy

A. new URL('/images/photo.jpg', 'https://example.com/gallery/')

B. new URL('https://example.com/gallery/' + '/images/photo.jpg')

C. new URL('images/photo.jpg')

D. new URL('https://example.com/gallery/images/photo.jpg')

Solution

1. Step 1: Understand URL constructor usage

   The URL constructor takes two arguments: the path and the base URL. For relative paths, the first argument is the relative path, and the second is the base.

2. Step 2: Check each option

   new URL('/images/photo.jpg', 'https://example.com/gallery/') correctly uses new URL(relativePath, baseURL). new URL('https://example.com/gallery/' + '/images/photo.jpg') concatenates strings but does not use the URL constructor properly. new URL('images/photo.jpg') misses the base URL. new URL('https://example.com/gallery/images/photo.jpg') uses an absolute URL, not relative.

3. Final Answer:

   new URL('/images/photo.jpg', 'https://example.com/gallery/') -> Option A

4. Quick Check:

   URL(relative, base) = correct syntax [OK]

Hint: Use new URL(relativePath, baseURL) for relative URLs [OK]

Common Mistakes:

• Concatenating strings instead of using URL constructor
• Omitting the base URL for relative paths
• Passing absolute URL as first argument when base is given

3. What will be the output of the following Node.js code?

const { URL } = require('url');
const base = 'https://example.com/folder/';
const relative = '../image.png';
const fullUrl = new URL(relative, base);
console.log(fullUrl.href);

medium

A. https://example.com/folder/../image.png

B. Error: Invalid URL

C. https://example.com/image.png

D. https://example.com/folder/image.png

Solution

1. Step 1: Understand relative path resolution

   The relative path '../image.png' means go one folder up from 'https://example.com/folder/' which leads to 'https://example.com/'.

2. Step 2: Resolve the full URL

   Combining base and relative path, the URL class normalizes the path to 'https://example.com/image.png'. https://example.com/image.png matches this.

3. Final Answer:

   https://example.com/image.png -> Option C

4. Quick Check:

   Relative '..' moves up one directory [OK]

Hint: Relative '..' moves up one folder in URL paths [OK]

Common Mistakes:

• Not normalizing '..' in URL paths
• Expecting '../' to stay in the URL string
• Confusing relative and absolute URL outputs

4. Identify the error in this Node.js code snippet that tries to resolve a relative URL:

const { URL } = require('url');
const base = 'https://example.com/path';
const relative = 'file.txt';
const url = new URL(relative, base);
console.log(url.href);

medium

A. Base URL is missing trailing slash, causing incorrect resolution.

B. Relative URL should start with a slash '/' to be valid.

C. URL constructor requires only one argument for relative URLs.

D. No error; code outputs 'https://example.com/path/file.txt' correctly.

Solution

1. Step 1: Check base URL format

   The base URL 'https://example.com/path' lacks a trailing slash, so it is treated as a file, not a folder.

2. Step 2: Understand URL resolution effect

   Appending 'file.txt' to a base treated as file replaces 'path' with 'file.txt', resulting in 'https://example.com/file.txt', not 'https://example.com/path/file.txt'.

3. Final Answer:

   Base URL is missing trailing slash, causing incorrect resolution. -> Option A

4. Quick Check:

   Base URL trailing slash affects relative URL resolution [OK]

Hint: Always add trailing slash to base URL if it's a folder [OK]

Common Mistakes:

• Omitting trailing slash on base URL
• Thinking relative URLs must start with '/'
• Passing wrong number of arguments to URL constructor

5. You want to combine a base URL 'https://example.com/api/v1/' with a relative URL '../../images/pic.jpg' in Node.js. What will be the resolved absolute URL?

hard

A. https://example.com/api/v1/images/pic.jpg

B. https://example.com/api/images/pic.jpg

C. https://example.com/api/v1/../../images/pic.jpg

D. https://example.com/images/pic.jpg

Solution

1. Step 1: Analyze relative path '../../images/pic.jpg'

   The '../../' means go up two folders from 'https://example.com/api/v1/'.

2. Step 2: Move up two levels from base URL

   Starting at 'https://example.com/api/v1/', going up one level is 'https://example.com/api/', and up another is 'https://example.com/'. Then append 'images/pic.jpg'.

3. Final Answer:

   https://example.com/images/pic.jpg -> Option D

4. Quick Check:

   Two '..' moves up two directories [OK]

Hint: Count '..' to move up folders in URL paths [OK]

Common Mistakes:

• Not moving up enough folders for multiple '..'
• Appending relative path without normalization
• Confusing base URL folder levels