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

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

Last verified Sep 9, 2026 against chai@6.2.2. Markdown · JSON

ESM-only ESM-only since 5.0.0 — last CommonJS version is 4.5.0

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
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

Does chai@6.2.2 actually load? Runtime test results

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

Runtimerequire("chai")import "chai"
Node.js 18✗ failsERR_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 (import)

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 (require)

// 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, corrected where a runtime test of that version contradicted them (104 versions; the newest 60 shown).

VersionsFormatFirst published
5.0.0 – 6.2.2ESM-onlyDec 28, 2023
4.3.0 – 4.5.0Dual (CommonJS + ESM)Feb 4, 2021
1.7.2 – 4.2.0CommonJSJun 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

Other esm-only packages

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