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

> chai@6.2.2 is ESM-only. ESM-only since 5.0.0 — last CommonJS version is 4.5.0.

BDD/TDD assertion library for node.js and the browser. Test framework agnostic.

Last verified Sep 9, 2026 against chai@6.2.2.

chai dropped its CommonJS build in 5.0.0 (December 2023). The last version with a plain require()-able CommonJS entry is 4.5.0, published Jul 25, 2024; it declares support for Node.js >=4.

On Node.js 20.19+, 22.12+ and newer, require("chai") 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("chai") fails with ERR_REQUIRE_ESM.

## What to do

1. On Node.js 20.19+, 22.12+ and newer, require("chai") 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 chai = await import("chai"). That works on every Node.js version the package supports (>=18).
3. If you need a synchronous require() or run Node.js 18: npm install chai@4.5.0, the last CommonJS release (published Jul 25, 2024); it declares Node.js >=4.
4. TypeScript: npm install -D @types/chai

## Facts

- Latest: 6.2.2 (Dec 22, 2025)
- Module format: ESM-only
- Last CommonJS version: 4.5.0
- ESM-only since: 5.0.0 (Dec 28, 2023)
- TypeScript types: @types/chai
- Node engines: >=18
- Weekly downloads: 105.1M
- License: MIT

## Runtime test results for chai@6.2.2

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

| Runtime | require("chai") | import "chai" |
|---|---|---|
| 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 chai

ES modules:

```js
import * as chai from "chai";
```

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 chai = require("chai");

// Any Node.js version the package supports (>=18), from an async function:
async function main() {
  const chai = await import("chai");
}

// Or pin the last CommonJS release (declares Node.js >=4):
// npm install chai@4.5.0
```

## Module format by version

Derived from the type, main and exports fields of every stable release on npm (104 versions; the newest 60 shown).

| Versions | Format | First published |
|---|---|---|
| 5.0.0 – 6.2.2 | ESM-only | Dec 28, 2023 |
| 4.3.0 – 4.5.0 | Dual (CommonJS + ESM) | Feb 4, 2021 |
| 1.7.2 – 4.2.0 | CommonJS | Jun 27, 2013 |

## Frequently asked

**Is chai ESM-only?**

chai@6.2.2 is ESM-only. chai dropped its CommonJS build in 5.0.0 (December 2023). The last version with a plain require()-able CommonJS entry is 4.5.0, published Jul 25, 2024; it declares support for Node.js >=4.

**Which version of chai still supports CommonJS / require()?**

4.5.0 is the last version of chai with a CommonJS build; it declares Node.js >=4. Install it with "npm install chai@4.5.0". Every version from 5.0.0 on is ESM-only.

**How do I use chai from a CommonJS file?**

From a CommonJS file, use a dynamic import inside an async function: const chai = await import("chai"). That works on every Node.js version the package supports (>=18). On Node.js 20.19+, 22.12+ and newer, require("chai") works with the latest version: Node.js loads synchronous ES modules through require(). Verified on Node.js 20, 22, 24 and 26. Alternatively pin chai@4.5.0 (declares Node.js >=4).

**Does chai work in Bun?**

Yes. import "chai" loaded successfully in Bun 1.4.0 on Sep 9, 2026.

**Does chai ship TypeScript types?**

Not bundled. Install the community types with "npm install -D @types/chai".

## Links

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

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