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koa-cors and @koa/cors: async middleware stacks, delegated methods, and origin checks with downstream routers
Koa’s onion model means CORS middleware wraps later layers—ensure ctx.set headers survive after await next().
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koacorsnodejs
Streaming bodies
When streaming uploads, respond to OPTIONS before consuming the stream or clients may see timeouts disguised as CORS failures.
Server-sent events need explicit flushing of CORS headers on initial 200 responses.
Testing
Use supertest with Origin headers in every integration test matrix entry.
Snapshot koa-cors defaults after upgrades—minor semver releases can widen allowed methods.