LEI Validation in JavaScript — GLEIF Lookup Without Rate Limits

If you work in finance, compliance, or regulatory reporting, you've seen LEIs everywhere. Every MiFID II transaction report, every EMIR derivative trade, every SEC filing — they all require a valid Legal Entity Identifier . Yet most developers treat LEI validation as a simple format check. It's not. A syntactically valid LEI can belong to an entity that merged, dissolved, or had its registration lapsed years ago. If you're not checking against the GLEIF database, you're not really validating. This guide covers the full stack: format validation, mod-97 checksum, and live entity lookup — with code you can use today. What Is an LEI? An LEI (ISO 17442) is a 20-character alphanumeric code that uniquely identifies legal entities participating in financial transactions. It was introduced after the 2008 crisis because regulators couldn't trace exposure across institutions — nobody had a universal ID for companies. 7 L T W F Z Y I C N S X 8 D 6 2 1 K 8 6 ├─────┤ ├─┤ ├─────────────────────┤ ├─┤