Patch Changes

• #363 c4d6b9a Thanks @brennanbutler01! - Abort the underlying request when a response fails EventSource validation.

• #362 627eb17 Thanks @brennanbutler01! - Stop dispatching buffered messages when an event listener closes the connection.

Patch Changes

• #358 de3c0ae Thanks @pacocartones! - Fixed MessageEvent.lastEventId being empty when a message omits the id field

  The lastEventId attribute is the last event ID string of the event source, so an explicit id field persists until another one replaces it. Message events were dispatched with the current event's own id instead of that buffer, which left lastEventId empty on every event that omitted id, even though the buffer still held the earlier value and would have been sent as Last-Event-ID on reconnect.

• 17a9772 Thanks @rexxars! - Fixed ID-only event blocks not updating MessageEvent.lastEventId or Last-Event-ID on reconnect.

Minor Changes

• #355 d8370e4 Thanks @rexxars! - Typed response body chunks as buffers instead of unknown

  ReaderLike typed chunks read off the response body as unknown, but they are handed to a TextDecoder, which only accepts buffers and throws on anything else. So a reader yielding anything else was never usable, and the type said otherwise. It also hid a type error in the client itself, which only surfaces when compiling against TypeScript's dom library: the node typings resolve the chunk to any, while the dom library resolves it to {}.

  Chunks are now typed as Uint8Array | DataView | ArrayBuffer, which is what both the node and DOM typings accept, and which every fetch() implementation yields. Nothing changes at runtime.

  If you pass a custom fetch() that returns a hand-rolled body, and your reader's chunk type is wider than the above, eg unknown or any, it will no longer be assignable. Returning a real Response, or a reader that yields Uint8Array chunks, is unaffected.

• #355 008f07e Thanks @rexxars! - Declared the this type for the onerror, onmessage and onopen properties

  addEventListener() already declared that listeners are called with the EventSource instance as this, but the on* properties did not, so this was an implicit any in handlers assigned to them (an error under noImplicitThis). They now match addEventListener() and the native EventSource:

  eventSource.onmessage = function (event) {
    console.log(this.url, event.data) // `this` is now typed
  }

  Handlers that declare an incompatible this, eg an unbound class method typed with this: MyClass, will now be rejected where they were previously accepted. Arrow functions and handlers that ignore this are unaffected.

• #355 3512add Thanks @rexxars! - Made the exported EventSource type structural, so other implementations can satisfy it

  EventSource was exported as a class holding hard-private (#) fields, which makes TypeScript emit a #private brand into the declaration file and turns the exported type nominal. A consumer writing function connect(es: EventSource) against this package could not pass the native EventSource, a mock, or any other implementation, even when the shape matched exactly.

  The implementation class is now internal, and EventSource is exported as an interface plus a const holding the constructor. As a result:

  • The native EventSource, along with mocks and stubs, is assignable to the exported EventSource type
  • The exported value and globalThis.EventSource are interchangeable in both directions, which helps libraries that accept an EventSource implementation to construct
  • A new EventSourceConstructor type is exported for that case

  Nothing changes at runtime: new EventSource(...), instanceof, subclassing, the readyState statics, EventSource.name, inspected output and the eventsource.supports-fetch-override symbol all behave as before, and the internal state is still held in real #private fields.

Patch Changes

• #357 8e5c691 Thanks @rexxars! - Fixed origin and lastEventId being empty on Cloudflare Workers

  workerd accepts data from the MessageEvent constructor's init dictionary but silently drops origin and lastEventId, so message events dispatched on Cloudflare Workers arrived with origin set to null and lastEventId to an empty string. Both are now assigned explicitly when the constructor did not take them, which leaves every other runtime untouched.

Major Changes

• #349 b195e70 Thanks @rexxars! - - BREAKING CHANGE: The client now fails the connection, emitting an error without reconnecting, if the parser buffers 100 MB without receiving a valid, complete EventSource line. Configure a different limit with maxBufferSize; ideally, servers should emit smaller chunks or newlines more frequently.
  • BREAKING CHANGE: Node.js 22.12 or later is now required. Older Node.js versions may still work, but are not supported or guaranteed going forward because Node.js 20 is out of LTS.
  • BREAKING CHANGE: The separate CommonJS variant is no longer published. Node.js 22.12 and later transparently supports require() of ESM, so most CommonJS consumers should continue to work. This removes the dual-package hazard.
  • BREAKING CHANGE: Chrome versions before 84, Safari before 15, Firefox before 105, Edge before 84, and JavaScript environments without private fields, methods, and accessors are no longer supported.
  • BREAKING CHANGE: When both an on* property handler and an addEventListener() listener are registered for the same event, they now run in registration order instead of always running the on* handler first. Code that relied on the old order can observe a different callback sequence.
  • Unref reconnection timers where the runtime supports it, so pending reconnects do not keep the process alive.

Patch Changes

• #351 6127f40 Thanks @rexxars! - Allow a value of any type in ReaderLike done-results. TypeScript 5.9's DOM lib types ReadableStreamDefaultReader.read()'s done-result as {done: true, value: T | undefined}, so a custom fetch returning a Response-shaped body no longer satisfied FetchLikeResponse on TypeScript 5.9 and newer, forcing consumers to hand-wrap the reader.

4.1.0 (2025-11-19)

Features

• declare fetch override support symbol on class (5575122)

This release is also available on:

• npm package (@latest dist-tag)

4.0.0 (2025-05-13)

⚠ BREAKING CHANGES

• FetchLikeInit is now removed. Use
  EventSourceFetchInit.
• Drop support for Node.js v18, as it is end-of-life.

Features

• require node.js v20 or higher (91a3a48)

Bug Fixes

• drop FetchLikeInit type. Use EventSourceFetchInit instead. (6786e46)

This release is also available on:

• npm package (@latest dist-tag)

3.0.7 (2025-05-09)

Bug Fixes

• mark fetch init properties required in typings (1282872)

This release is also available on:

• npm package (@latest dist-tag)

3.0.6 (2025-03-27)

Bug Fixes

• upgrade parser to latest version, improving performance (59a5ddd)

This release is also available on:

• npm package (@latest dist-tag)

3.0.5 (2025-01-28)

Bug Fixes

• include message and code on errors when logging in node.js and deno (f2596b3)

This release is also available on:

• npm package (@latest dist-tag)