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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. Markdown · JSON

CommonJS CommonJS on paper — require() failed in our runtime test

eventsource-polyfill declares a CommonJS entry only, but require("eventsource-polyfill") failed on every Node.js version in our probe: ReferenceError: window is not defined. import "eventsource-polyfill" resolves to the same file, so it fails the same way. That usually means a missing peer dependency, a browser-only global, or a package that is not meant to be loaded directly; see the runtime table below.

What to do

  1. require("eventsource-polyfill") failed in our probe on every Node.js version (ReferenceError: window is not defined); check the package README for peer dependencies or setup before relying on it.
  2. From ES modules use the default import: import eventsourcePolyfill from "eventsource-polyfill"; then destructure. Named imports only work when Node.js can statically detect the exports.
  3. TypeScript: no types are available for eventsource-polyfill; declare the module yourself if you need them.
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 1.4.0✗ 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)

// require("eventsource-polyfill") failed in our probe on every Node.js version we test (ReferenceError: window is not defined); see the runtime table.
const eventsourcePolyfill = require("eventsource-polyfill");

Module format by version

Derived from the type, main and exports fields of every stable release on npm, corrected where a runtime test of that version contradicted them (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 declares a CommonJS entry, but require() fails at runtime. eventsource-polyfill declares a CommonJS entry only, but require("eventsource-polyfill") failed on every Node.js version in our probe: ReferenceError: window is not defined. import "eventsource-polyfill" resolves to the same file, so it fails the same way. That usually means a missing peer dependency, a browser-only global, or a package that is not meant to be loaded directly; see the runtime table below.

Can I require eventsource-polyfill?

Not in practice. eventsource-polyfill declares a CommonJS entry, but require("eventsource-polyfill") failed on every Node.js version in our probe: ReferenceError: window is not defined. Check the package README for peer dependencies or setup before relying on it.

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 1.4.0 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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