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CONTRIBUTING.md

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Contributing Guide

This guide will help you add new image CDN providers to the library. It covers implementation details, utility functions, and best practices.

Overview

Each provider consists of:

  1. A TypeScript file containing the implementation (src/providers/[provider].ts) and types for provider-specific operations and options.
  2. A test file (src/providers/[provider].test.ts)
  3. Example URLs in src/demo/examples.json
  4. Detection domains or paths in data if appropriate
  5. Adding to the types and exports in:
    • src/providers/types.ts
    • src/types.ts
    • src/extract.ts
    • src/transform.ts
    • src/async.ts
    • deno.jsonc

Core Concepts and Utilities

URL Manipulation

The library provides several utilities for URL handling:

// Convert strings or URLs to URL objects (handles relative URLs)
const url = toUrl("https://example.com/image.jpg");

// Convert back to string, preserving relativeness
const urlString = toCanonicalUrlString(url);

// Path manipulation
const cleanPath = stripLeadingSlash("/path/to/image.jpg");
const formattedPath = addTrailingSlash("path/to/image");

Operations Handlers

The most important utility is createOperationsHandlers, which creates standardized parser and generator functions:

const { operationsGenerator, operationsParser } = createOperationsHandlers<
	ExampleCdnOperations
>({
	// Map standard operation names to provider-specific names
	keyMap: {
		width: "w",
		height: "h",
		quality: "q",
		format: "fmt",
	},
	// Set default values
	defaults: {
		quality: 80,
		format: "auto",
	},
	// Normalize format names
	formatMap: {
		jpg: "jpeg",
	},
	// Define parameter formatting
	kvSeparator: "=", // key=value
	paramSeparator: "&", // param1&param2
});

Step-by-Step Implementation

Let's create a complete example provider "example-cdn":

1. Define Operations Interface

// example-cdn.ts
import type {
	Operations,
	URLExtractor,
	URLGenerator,
	URLTransformer,
} from "../types.ts";
import {
	createExtractAndGenerate,
	createOperationsHandlers,
	toCanonicalUrlString,
	toUrl,
} from "../utils.ts";

// Only add NEW operations specific to your provider
export interface ExampleCdnOperations extends Operations {
	// Provider-specific operations
	specialCrop?: "smart" | "center";
	blur?: number;

	// DON'T include these - they're in base Operations
	// width?: number;      ❌
	// height?: number;     ❌
	// quality?: number;    ❌
	// format?: string;     ❌
}

// Optional provider-specific options
export interface ExampleCdnOptions {
	baseUrl?: string;
}

2. Configure Operations Handlers

// Different parameter formatting styles:

// Query parameters: ?width=100&height=200
const queryStyle = createOperationsHandlers<ExampleCdnOperations>({
	keyMap: {
		width: "w",
		height: "h",
		quality: "q",
		format: "fmt",
	},
	defaults: {
		quality: 80,
	},
	kvSeparator: "=",
	paramSeparator: "&",
});

// Path segments: /w_100/h_200/q_80
const pathStyle = createOperationsHandlers<ExampleCdnOperations>({
	keyMap: {
		width: "w",
		height: "h",
		quality: "q",
		format: "fmt",
	},
	defaults: {
		quality: 80,
	},
	kvSeparator: "_",
	paramSeparator: "/",
});

// You can also disable parameters:
const noHeightStyle = createOperationsHandlers<ExampleCdnOperations>({
	keyMap: {
		width: "w",
		height: false, // Height parameter will be removed
		quality: "q",
	},
});

3. Implement Core Functions

// Extract operations from existing URL
export const extract: URLExtractor<"example-cdn"> = (url) => {
	const parsedUrl = toUrl(url);
	const operations = operationsParser(parsedUrl);
	parsedUrl.search = "";

	return {
		src: toCanonicalUrlString(parsedUrl),
		operations,
		options: {
			baseUrl: parsedUrl.origin,
		},
	};
};

// Generate new URL with operations
export const generate: URLGenerator<"example-cdn"> = (
	src,
	operations,
	options = {},
) => {
	const url = toUrl(src, options.baseUrl);
	url.search = operationsGenerator(operations);
	return toCanonicalUrlString(url);
};

// Transform existing URL with new operations
export const transform: URLTransformer<"example-cdn"> =
	createExtractAndGenerate(extract, generate);

4. Add Comprehensive Tests

// example-cdn.test.ts
import { assertEquals } from "jsr:@std/assert";
import { extract, generate, transform } from "./example-cdn.ts";
import { assertEqualIgnoringQueryOrder } from "../test-utils.ts";

const img = "https://example-cdn.com/image.jpg";

Deno.test("Example CDN", async (t) => {
	// Test extraction
	await t.step("should extract operations from URL", () => {
		const url = `${img}?w=300&h=200&q=80&fmt=webp&specialCrop=smart`;
		const result = extract(url);
		assertEquals(result, {
			src: img,
			operations: {
				width: 300,
				height: 200,
				quality: 80,
				format: "webp",
				specialCrop: "smart",
			},
			options: {
				baseUrl: "https://example-cdn.com",
			},
		});
	});

	// Test URL generation
	await t.step("should generate URL with operations", () => {
		const result = generate(img, {
			width: 400,
			height: 300,
			quality: 90,
			specialCrop: "center",
		});
		assertEqualIgnoringQueryOrder(
			result,
			`${img}?w=400&h=300&q=90&specialCrop=center`,
		);
	});

	// Test transformation
	await t.step("should transform existing URL", () => {
		const url = `${img}?w=300&h=200`;
		const result = transform(url, {
			width: 500,
			blur: 5,
		});
		assertEqualIgnoringQueryOrder(
			result,
			`${img}?w=500&h=200&blur=5`,
		);
	});

	// Test error cases
	await t.step("should handle invalid URLs", () => {
		const result = extract("invalid-url");
		assertEquals(result, null);
	});

	// Test relative URLs
	await t.step("should handle relative URLs", () => {
		const result = generate("/image.jpg", { width: 300 });
		assertEqualIgnoringQueryOrder(
			result,
			"/image.jpg?w=300",
		);
	});
});

5. Update Types and Add Examples

// types.ts
export interface ProviderOperations {
	"example-cdn": ExampleCdnOperations;
	// ...
}

export interface ProviderOptions {
	"example-cdn": ExampleCdnOptions;
	// ...
}
// examples.json
{
	"example-cdn": [
		"Example CDN",
		"https://example-cdn.com/demo-image.jpg"
	]
}

Parameter Handling Patterns

Query Parameters vs Path Segments

Providers use different URL patterns for operations:

// Standard query parameters
// https://example.com/image.jpg?width=100&height=200
{
  kvSeparator: "=",
  paramSeparator: "&"
}

// Path segments
// https://example.com/image/w_100/h_200/image.jpg
{
  kvSeparator: "_",
  paramSeparator: "/"
}

// Custom separators
// https://example.com/image:w=100,h=200/image.jpg
{
  kvSeparator: "=",
  paramSeparator: ","
}

Best Practices

When to Use Utilities

Use the provided utilities when:

  • You need standard parameter mapping
  • Your provider follows common URL patterns
  • You want automatic parameter normalization

When to Create Custom Solutions

Create custom handlers when:

  • Your provider has unique URL structures
  • Parameters have complex interdependencies
  • You need special encoding or encryption
  • The provider requires custom protocols

Error Handling

Always handle:

  • Invalid URLs
  • Missing parameters
  • Malformed parameters
  • Unsupported formats
  • Edge cases

Type Safety

  • Define clear interfaces extending Operations
  • Only add provider-specific operations
  • Use proper TypeScript generics
  • Document supported operations

Testing Requirements

  1. Basic Operations

    • Width/height resizing
    • Format conversion
    • Quality settings
    • Provider-specific features

  2. URL Handling

    • Absolute URLs
    • Relative URLs
    • URL with existing parameters
    • Invalid URLs

  3. Parameter Edge Cases

    • Missing parameters
    • Invalid values
    • Parameter combinations
    • Default values

  4. Common Scenarios

    • Standard transformations
    • Format conversion
    • Quality adjustment
    • Size constraints

Final Checklist

Before submitting:

  • Implementation complete with proper types
  • Comprehensive tests covering all features
  • Types updated in all files listed above
  • Example added to examples.json
  • Detection domains or paths added if needed
  • All tests passing, including unit tests and E2E tests

Development Environment

Deno Setup

This project uses Deno for development. If you haven't already, install Deno from https://deno.com.

Basic commands:

# Run tests
deno test

# Run tests with watch mode
deno test --watch

# Type checking
deno check

# Format code
deno fmt

Running Tests

Tests are written using Deno's built-in test framework. Run them from the project root:

# Run all tests
deno test

# Run tests for a specific provider
deno test src/providers/example-cdn.test.ts

# Run E2E tests. These need network access.
deno test --allow-net e2e.test.ts

Image Defaults

When implementing a provider, follow these default behaviors for consistency across CDNs. If the provider does not support a feature then it can be omitted, but these are the defaults to aim for so that users have a consistent experience.

Format Handling

  • Enable auto format detection/content negotiation when supported
  • When supported, priority order for formats should be. For services that generate images locally, it is ok to prefer WebP over AVIF for performance reasons.
    1. AVIF
    2. WebP
    3. Original format

Image Fitting

Default to fit=cover behavior (equivalent to CSS object-fit: cover). This means:

  • Image should fill requested dimensions
  • Maintain aspect ratio
  • Crop if necessary
  • Avoid distortion

Size Handling

  • Never upscale images beyond their original dimensions
  • Return largest available size when requested size is too large
  • Maintain requested aspect ratio even when size is constrained

Local Development Server

The project includes a playground application in the demo directory for testing providers visually:

  1. Start the development server:
cd demo
pnpm install
pnpm dev
  1. Open http://localhost:1234

The playground is crucial for testing as it:

  • Provides real-world testing with actual CDN endpoints
  • Allows visual verification of image operations
  • Tests responsive image behavior
  • Verifies URL generation patterns

When adding a new provider:

  1. Add an example URL to demo/src/examples.json
    • Ideally use a public sample image from the CDN's documentation
    • If unavailable, use any publicly-accessible image on that CDN
    • Do not skip this - no provider can be added without an example URL, because otherwise it cannot be tested
  2. Test comprehensively:
    • Verify resizing behavior
    • Check that defaults are properly applied
    • Test format conversion
    • Verify responsive behavior
    • Ensure upscaling limits work
    • Check aspect ratio handling

End-to-End Testing

The E2E tests in e2e.test.ts verify that providers work with real CDN endpoints. They use the images from examples.json to test real operations:

deno test --allow-net e2e.test.ts

Getting Help

If you need help:

  1. Review existing provider implementations
  2. Check test files for patterns
  3. Open an issue for discussion
  4. Ask questions in pull requests

Remember that clear, well-tested code is more important than clever solutions. Take time to write comprehensive tests and documentation.