Intl LocaleMatcher

We've migrated the docs to https://formatjs.github.io/docs/polyfills/intl-localematcher.

Performance

This package implements a highly optimized three-tier locale matching algorithm that provides excellent performance even with large locale sets (700+ locales).

Benchmark Results

Benchmarked with 725 CLDR locales on Node.js:

| Scenario | Latency | Throughput | Relative Performance | | ------------------------------------------------------- | ------- | ---------- | -------------------- | | Tier 1: Exact Match (en) | 1.38ms | 730 ops/s | Baseline | | Tier 2: 1-level Fallback (en-US → en) | 1.39ms | 725 ops/s | 1.01x slower | | Tier 2: Maximized Match (zh-TW → zh-Hant) | 1.40ms | 720 ops/s | 1.02x slower | | Tier 3: CLDR Distance (sr-Latn-BA → sr-Latn-BA) | 1.38ms | 730 ops/s | 1.00x slower | | Tier 3: Fuzzy Match (en-XZ → en) | 1.50ms | 670 ops/s | 1.09x slower |

Real-world Impact

The optimization in this package resolved issue #4936, where DurationFormat instantiation was taking 610ms on React Native/Hermes due to slow locale matching against 700+ auto-loaded locales.

After optimization:

• Common case (en-US): 1.39ms per instantiation
• Chinese locales (zh-TW): 1.40ms per instantiation
• Serbo-Croatian locales: 1.38ms per instantiation

Performance improvement: 439x faster 🚀

Three-Tier Optimization

The algorithm uses three tiers for maximum performance:

1. Tier 1 (Exact Match): O(1) Set lookup for exact locale matches
2. Tier 2 (Maximization + Fallback): Progressive subtag removal with locale maximization
3. Tier 3 (CLDR Distance): Full UTS #35 Enhanced Language Matching with memoization

This design ensures that common cases (exact matches and simple fallbacks) are extremely fast, while complex scenarios (script/region matching, language distances) still perform well.