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

> eventsource-polyfill@0.9.6 declares a CommonJS entry, but require() fails at runtime. CommonJS on paper — require() failed in our runtime test.

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

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

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.

## Facts

- 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

## Runtime test results for eventsource-polyfill@0.9.6

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

| Runtime | require("eventsource-polyfill") | import "eventsource-polyfill" |
|---|---|---|
| Node.js 18 | fails (ReferenceError) | fails (ReferenceError) |
| Node.js 20 | fails (ReferenceError) | fails (ReferenceError) |
| Node.js 22 | fails (ReferenceError) | fails (ReferenceError) |
| Node.js 24 | fails (ReferenceError) | fails (ReferenceError) |
| Node.js 26 | fails (ReferenceError) | fails (ReferenceError) |
| Bun 1.4.0 | fails (ReferenceError) | fails (ReferenceError) |

## How to import eventsource-polyfill

ES modules:

```js
import eventsourcePolyfill from "eventsource-polyfill";
```

CommonJS:

```js
// 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 (2 versions; the newest 2 shown).

| Versions | Format | First published |
|---|---|---|
| 0.9.5 – 0.9.6 | CommonJS | Feb 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

- [eventsource-polyfill on npm](https://www.npmjs.com/package/eventsource-polyfill)
- [Source repository](https://github.com/amvtek/EventSource)
- [HTML version](https://canirequire.com/package/eventsource-polyfill/)
- [JSON version](https://canirequire.com/package/eventsource-polyfill.json)

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