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Is eventsource-polyfill ESM-only? Can you require() it?

A browser polyfill for W3C EventSource (http://www.w3.org/TR/eventsource/)

Last verified Sep 9, 2026 against eventsource-polyfill@0.9.6. JSON

CommonJS CommonJS — require() works on every Node.js version

eventsource-polyfill ships CommonJS only. require("eventsource-polyfill") works everywhere. import "eventsource-polyfill" also works because Node.js and Bun can import CommonJS from ES modules, but only the default export is guaranteed; named imports depend on Node's static analysis of the module.

Latest
0.9.6 (Feb 28, 2015)
Module format
CommonJS
Last CommonJS version
0.9.6
TypeScript types
None
Node engines
not declared
Weekly downloads
85K
License
MIT

Does eventsource-polyfill@0.9.6 actually load? Runtime test results

Installed and executed on Sep 9, 2026 with require("eventsource-polyfill") from a CommonJS file and import "eventsource-polyfill" from an ES module.

Runtimerequire("eventsource-polyfill")import "eventsource-polyfill"
Node.js 18✗ failsReferenceError✗ failsReferenceError
Node.js 20✗ failsReferenceError✗ failsReferenceError
Node.js 22✗ failsReferenceError✗ failsReferenceError
Node.js 24✗ failsReferenceError✗ failsReferenceError
Node.js 26✗ failsReferenceError✗ failsReferenceError
Bun 1.4✗ failsReferenceError✗ failsReferenceError

A failing require() on a CommonJS package usually means the package needs a browser/DOM global, a peer dependency, or a native binary that is not available in a bare Node.js process.

How to import eventsource-polyfill

ES modules (import)

import eventsourcePolyfill from "eventsource-polyfill";

Named imports from a CommonJS package only work when Node.js can statically detect them. If a named import fails, use the default import and destructure.

CommonJS (require)

const eventsourcePolyfill = require("eventsource-polyfill");

Module format by version

Derived from the type, main and exports fields of every stable release on npm (2 versions; the newest 2 shown).

VersionsFormatFirst published
0.9.5 – 0.9.6CommonJSFeb 28, 2015

Frequently asked

Is eventsource-polyfill ESM-only?

eventsource-polyfill@0.9.6 is a CommonJS package. eventsource-polyfill ships CommonJS only. require("eventsource-polyfill") works everywhere. import "eventsource-polyfill" also works because Node.js and Bun can import CommonJS from ES modules, but only the default export is guaranteed; named imports depend on Node's static analysis of the module.

Can I import eventsource-polyfill with ES module syntax?

Yes. import eventsourcePolyfill from "eventsource-polyfill" works in Node.js and Bun because they support importing CommonJS modules. Named imports work only when Node.js can statically detect the exports.

Does eventsource-polyfill work in Bun?

Importing eventsource-polyfill failed in Bun 1.4 on Sep 9, 2026: ReferenceError: window is not defined

Does eventsource-polyfill ship TypeScript types?

No. eventsource-polyfill ships no type declarations and there is no @types package for it.

Links

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