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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. 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 you can require() on every Node.js version, including 18 and older, is 4.5.0, published Jul 25, 2024.

On Node.js 20.19+, 22.12+ and every newer release, require("chai") works anyway: Node can load synchronous ES modules through require(). Verified on Node 20, 22, 24, 26. Node.js 18 and older still fail with ERR_REQUIRE_ESM.

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✓ works✓ works

How to import chai

ES modules (import)

import chai from "chai";

CommonJS (require)

// Node.js 20.19+ / 22.12+ / newer (package has no top-level await):
const chai = require("chai");

// Any Node.js version:
const chai = await import("chai");

// Or pin the last CommonJS release:
// 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).

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 you can require() on every Node.js version, including 18 and older, is 4.5.0, published Jul 25, 2024.

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

4.5.0 is the last version of chai with a CommonJS build. 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?

Use a dynamic import: const mod = await import("chai"). On Node.js 20.19+, 22.12+ and newer, a plain require("chai") also works for ESM packages that do not use top-level await. Alternatively pin chai@4.5.0.

Does chai work in Bun?

Yes. import "chai" loaded successfully in Bun 1.4 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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