# Is assertion-error ESM-only? Can you require() it?

> assertion-error@2.0.1 is ESM-only. ESM-only since 2.0.0 — last CommonJS version is 1.1.0.

Error constructor for test and validation frameworks that implements standardized AssertionError specification.

Last verified Sep 9, 2026 against assertion-error@2.0.1.

assertion-error dropped its CommonJS build in 2.0.0 (October 2021). The last version with a plain require()-able CommonJS entry is 1.1.0, published Jan 7, 2018; it declares support for Node.js *.

On Node.js 20.19+, 22.12+ and newer, require("assertion-error") works with the latest version: Node.js loads synchronous ES modules through require(). Verified on Node.js 20, 22, 24 and 26. On Node.js 18, which has no require(esm), require("assertion-error") fails with ERR_REQUIRE_ESM.

## What to do

1. On Node.js 20.19+, 22.12+ and newer, require("assertion-error") works with the latest version: Node.js loads synchronous ES modules through require(). Verified on Node.js 20, 22, 24 and 26.
2. From a CommonJS file, use a dynamic import inside an async function: const assertionError = await import("assertion-error"). That works on every Node.js version the package supports (>=12).
3. If you need a synchronous require() or run Node.js 18: npm install assertion-error@1.1.0, the last CommonJS release (published Jan 7, 2018); it declares Node.js *.
4. TypeScript: types are bundled; install nothing extra.

## Facts

- Latest: 2.0.1 (Oct 18, 2023)
- Module format: ESM-only
- Last CommonJS version: 1.1.0
- ESM-only since: 2.0.0 (Oct 5, 2021)
- TypeScript types: bundled
- Node engines: >=12
- Weekly downloads: 89.6M
- License: MIT

## Runtime test results for assertion-error@2.0.1

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

| Runtime | require("assertion-error") | import "assertion-error" |
|---|---|---|
| Node.js 18 | fails (ERR_REQUIRE_ESM) | works |
| Node.js 20 | works | works |
| Node.js 22 | works | works |
| Node.js 24 | works | works |
| Node.js 26 | works | works |
| Bun 1.4.0 | works | works |

## How to import assertion-error

ES modules:

```js
import * as assertionError from "assertion-error";
```

Namespace import shown because the ESM entry may have no default export; use named imports for what it exports.

CommonJS:

```js
// Node.js 20.19+, 22.12+ and newer (verified):
const assertionError = require("assertion-error");

// Any Node.js version the package supports (>=12), from an async function:
async function main() {
  const assertionError = await import("assertion-error");
}

// Or pin the last CommonJS release (declares Node.js *):
// npm install assertion-error@1.1.0
```

## Module format by version

Derived from the type, main and exports fields of every stable release on npm (7 versions; the newest 7 shown).

| Versions | Format | First published |
|---|---|---|
| 2.0.0 – 2.0.1 | ESM-only | Oct 5, 2021 |
| 0.1.0 – 1.1.0 | CommonJS | Apr 7, 2013 |

## Frequently asked

**Is assertion-error ESM-only?**

assertion-error@2.0.1 is ESM-only. assertion-error dropped its CommonJS build in 2.0.0 (October 2021). The last version with a plain require()-able CommonJS entry is 1.1.0, published Jan 7, 2018; it declares support for Node.js *.

**Which version of assertion-error still supports CommonJS / require()?**

1.1.0 is the last version of assertion-error with a CommonJS build; it declares Node.js *. Install it with "npm install assertion-error@1.1.0". Every version from 2.0.0 on is ESM-only.

**How do I use assertion-error from a CommonJS file?**

From a CommonJS file, use a dynamic import inside an async function: const assertionError = await import("assertion-error"). That works on every Node.js version the package supports (>=12). On Node.js 20.19+, 22.12+ and newer, require("assertion-error") works with the latest version: Node.js loads synchronous ES modules through require(). Verified on Node.js 20, 22, 24 and 26. Alternatively pin assertion-error@1.1.0 (declares Node.js *).

**Does assertion-error work in Bun?**

Yes. import "assertion-error" loaded successfully in Bun 1.4.0 on Sep 9, 2026.

**Does assertion-error ship TypeScript types?**

Yes. assertion-error bundles its own type declarations; no @types package is needed.

## Links

- [assertion-error on npm](https://www.npmjs.com/package/assertion-error)
- [Source repository](https://github.com/chaijs/assertion-error)
- [HTML version](https://canirequire.com/package/assertion-error/)
- [JSON version](https://canirequire.com/package/assertion-error.json)

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