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webpack-dev-server: CORS, devServer.proxy, and keeping HMR WebSocket origins aligned with your API
Hot Module Replacement uses a second connection; if your API runs on another port, proxy requests so the browser sees one origin for fetch and WS.
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webpackdev-servercors
Proxy vs headers
Proxying `/api` to the backend removes cross-origin fetches in development, which eliminates most preflight noise in Network tabs.
Global devServer.headers apply to all responses; accidental duplication with backend middleware can emit conflicting Allow-Origin values.
WebSockets
sockjs-client and webpack HMR need allowed origins on the dev server—do not conflate them with REST CORS on Nest or Express.
When using HTTPS locally, ensure WSS matches the certificate SANs your browser trusts.