npm / eventsource-parser

eventsource-parser

What changed in eventsource-parser 3 to 4, from its official release notes, upgrade guides and advisories. Sources checked 2026-10-17.

4 breaking2 new APIs and options2 performance1 behavior and defaults4 fixes and other

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eventsource-parser 49 changes

The CommonJS build and its main field and require export conditions are removed; Node 22.12 and above supports require(esm) transparently.
4.0.0release notes
The package now requires Node.js >=22.12; Node.js 18 and 20 are no longer supported or guaranteed.
4.0.0release notes
Invalid lines are discarded as soon as they cannot become a valid SSE field; onError is not called and no line, field, or value is retained.
4.0.0release notes
The root stream.js file is removed; import eventsource-parser/stream instead and ensure your bundler resolves the exports field.
4.0.0release notes

Adds a callback that reports ID fields when an event block ends, including blocks without data.
4.1.0release notes

Partial comment lines are buffered only when an onComment callback is configured; otherwise, they are discarded.
4.0.0release notes

Buffer limits are enforced while processing a chunk.
4.1.2release notes
Handles leading byte-order marks across empty input chunks and split raw UTF-8 BOM chunks so the first event is not dropped.
4.1.1release notes
A leading byte order mark is stripped whether it arrives as decoded U+FEFF or as the raw three-byte sequence.
4.0.0release notes

Avoids O(N²) behavior when a single SSE event spans many chunks.
3.0.8release notes
Parsing is ~3.75x faster via inline fast-paths and single-pass line processing.
3.0.7release notes

An id: line containing a NULL character no longer wipes the buffered event id.
3.1.1release notes