@microsoft/fetch-event-source

@microsoft/fetch-event-source replaces the browser’s built-in EventSource with an implementation built on fetch(), removing that API’s restriction to GET requests with no custom headers or body. It streams and parses Server-Sent Events (SSE) the same way EventSource does, but lets callers set arbitrary headers (for auth tokens), send a POST body, control retry/backoff behavior, and cleanly cancel an in-flight stream via AbortController.

What You Get

• An fetchEventSource() function with the same event callbacks as native EventSource (onopen, onmessage, onclose, onerror) plus full control over method, headers, and body
• Automatic reconnect-with-backoff on connection drops, matching the SSE spec’s retry semantics, with a hook to override or disable it
• First-class AbortController/signal support so a caller can cancel a stream cleanly instead of leaking an open connection
• A standards-compliant SSE parser (parse.ts) that handles multi-line data fields, comments, and the retry:/id: control fields per spec

Common Use Cases

• Streaming token-by-token completions from an LLM API (OpenAI, Azure OpenAI) that requires a POST body and bearer-token header
• Building a live-updating dashboard or notification feed that needs SSE but must attach an auth header the native EventSource can’t send
• Replacing polling with a long-lived streamed connection for progress updates on a long-running server-side job
• Any browser client that needs SSE semantics with the flexibility of fetch() — custom retry logic, request cancellation, or non-GET methods

Under The Hood

Architecture The library is deliberately small: fetch.ts (154 lines) wraps the browser’s native fetch() call, wires it to an AbortController, and manages the retry/backoff loop and event-callback dispatch, while parse.ts (183 lines) implements a dedicated line-based SSE parser that incrementally consumes the streamed response body and emits parsed {event, data, id, retry} messages. index.ts simply re-exports the public fetchEventSource function. Tech Stack Pure TypeScript with zero runtime dependencies, compiled to both CJS (lib/cjs) and ESM (lib/esm) output via two separate tsconfig targets, keeping the published bundle framework-agnostic and usable in any environment with a fetch and ReadableStream implementation. Code Quality The parser has a dedicated Jasmine spec file (parse.spec.ts, 353 lines) exercising SSE parsing edge cases (multi-line fields, comments, partial chunks) in detail, though the higher-level fetch.ts reconnect/backoff logic itself has no equivalent test coverage — test depth is concentrated on the parsing layer, the part most prone to subtle spec-compliance bugs. API Design The API is intentionally a drop-in shape match for the native EventSource callbacks (onopen/onmessage/onerror/onclose), so migrating existing SSE consumer code mostly means swapping the import and adding a fetch-style options object (method, headers, body, signal) — there’s no new mental model to learn, just an extended options surface.