Idempotency Middleware for Express

express-idempotency is an Express.js middleware designed to add idempotency to API routes, inspired by Stripe's implementation. It ensures that repeated requests with the same idempotency key result in the same outcome, preventing duplicate processing of non-idempotent operations. The current stable version is 2.0.0. The package has seen consistent updates, with recent releases focusing on dependency upgrades and security fixes, indicating an active maintenance cadence rather than rapid feature development. Key differentiators include its high level of customization for data adapters, intent validators, and response validators, allowing developers to integrate it deeply with their existing infrastructure and persistence layers. It also provides helpers to detect idempotency hits and report errors, facilitating robust error handling within route handlers. It requires Node.js >=18.0.0 and npm >=9.0.0, aligning with modern JavaScript ecosystem standards. The library ships with TypeScript types, promoting strong typing in projects.