Cap URN - JavaScript Implementation

JavaScript implementation of Cap URN (Capability Uniform Resource Names), built on Tagged URN.

Features

• Required Direction Specifiers - in/out tags for input/output media types
• Media URN Validation - Validates direction spec values are valid Media URNs
• Special Pattern Values - * (must-have-any), ? (unspecified), ! (must-not-have)
• Graded Specificity - Exact values score higher than wildcards
• Cross-Language Compatible - Identical behavior to Rust, Go, and Objective-C implementations
• Production Ready - No fallbacks, fails hard on invalid input

Installation

npm install capns

Quick Start

const { CapUrn, CapUrnBuilder, CapMatcher } = require('capns');

// Create from string (with required direction specifiers)
const cap = CapUrn.fromString('cap:in="media:binary";op=extract;out="media:object"');
console.log(cap.toString());

// Use builder pattern
const built = new CapUrnBuilder()
  .inSpec('media:void')
  .outSpec('media:object')
  .tag('op', 'generate')
  .tag('target', 'thumbnail')
  .build();

// Matching
const request = CapUrn.fromString('cap:in="media:binary";op=extract;out="media:object"');
console.log(cap.accepts(request)); // true

// Find best match by specificity
const caps = [
  CapUrn.fromString('cap:in=*;op=extract;out=*'),
  CapUrn.fromString('cap:in="media:binary";op=extract;out="media:object"'),
  CapUrn.fromString('cap:ext=pdf;in="media:binary";op=extract;out="media:object"')
];
const best = CapMatcher.findBestMatch(caps, request);
console.log(best.toString()); // Most specific match

API Reference

CapUrn Class

Static Methods

• CapUrn.fromString(s) - Parse Cap URN from string
  • Throws CapUrnError on invalid format or missing direction specifiers

Instance Methods

• toString() - Get canonical string representation
• getTag(key) - Get tag value (case-insensitive)
• getInSpec() - Get input media type
• getOutSpec() - Get output media type
• hasTag(key, value) - Check if tag exists with value
• withTag(key, value) - Add/update tag (returns new instance)
• withoutTag(key) - Remove tag (returns new instance)
• accepts(request) - Check if this cap (as pattern) accepts a request
• specificity() - Get specificity score for matching
• isMoreSpecificThan(other) - Compare specificity
• equals(other) - Check equality

CapUrnBuilder Class

Fluent builder for constructing Cap URNs:

const cap = new CapUrnBuilder()
  .inSpec('media:binary')
  .outSpec('media:object')
  .tag('op', 'extract')
  .tag('target', 'metadata')
  .build();

CapMatcher Class

Utility for matching sets of caps:

• CapMatcher.findBestMatch(caps, request) - Find most specific match
• CapMatcher.findAllMatches(caps, request) - Find all matches (sorted by specificity)

Error Handling

const { CapUrnError, ErrorCodes } = require('capns');

try {
  const cap = CapUrn.fromString('cap:op=extract'); // Missing in/out
} catch (error) {
  if (error instanceof CapUrnError) {
    console.log(`Error code: ${error.code}`); // MISSING_IN_SPEC
  }
}

Cap-specific error codes:

• ErrorCodes.MISSING_IN_SPEC - Missing required in tag
• ErrorCodes.MISSING_OUT_SPEC - Missing required out tag
• ErrorCodes.INVALID_MEDIA_URN - Invalid Media URN in direction spec

For base Tagged URN error codes, see Tagged URN documentation.

Documentation

• RULES.md - Cap-specific rules
• Tagged URN RULES.md - Base format rules (case, quoting, wildcards, etc.)

Testing

npm test

Cross-Language Compatibility

This JavaScript implementation produces identical results to:

• Rust reference implementation
• Go implementation
• Objective-C implementation

All implementations pass the same test cases and follow identical rules.