chaijs/chai

BDD / TDD assertion framework for node.js and the browser that can be paired with any testing framework.

Source code at GitHub https://chaijs.github.io

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  "description": "BDD / TDD assertion framework for node.js and the browser that can be paired with any testing framework.",
  "fullName": "chaijs/chai",
  "homepage": "https://chaijs.github.io",
  "language": "JavaScript",
  "name": "chai",
  "pushedAt": "2025-11-24T08:59:57Z",
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  "topics": [
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  "updatedAt": "2025-11-27T03:33:02Z",
  "url": "https://github.com/chaijs/chai"
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chai

Chai is a BDD / TDD assertion library for node and the browser that can be delightfully paired with any javascript testing framework.

For more information or to download plugins, view the documentation.

What is Chai?

Chai is an assertion library, similar to Node’s built-in assert. It makes testing much easier by giving you lots of assertions you can run against your code.

Installation

Node.js

chai is available on npm. To install it, type:

$ npm install --save-dev chai

Browsers

You can also use it within the browser; install via npm and use the index.js file found within the download. For example:

<script unsrc="./node_modules/chai/index.js" type="module"></script>

Usage

Import the library in your code, and then pick one of the styles you’d like to use - either assert, expect or should:

import { assert } from 'chai';  // Using Assert style
import { expect } from 'chai';  // Using Expect style
import { should } from 'chai';  // Using Should style

Register the chai testing style globally

import 'chai/register-assert';  // Using Assert style
import 'chai/register-expect';  // Using Expect style
import 'chai/register-should';  // Using Should style

Import assertion styles as local variables

import { assert } from 'chai';  // Using Assert style
import { expect } from 'chai';  // Using Expect style
import { should } from 'chai';  // Using Should style
should();  // Modifies `Object.prototype`

import { expect, use } from 'chai';  // Creates local variables `expect` and `use`; useful for plugin use

Usage with Mocha

mocha spec.js --require chai/register-assert.js  # Using Assert style
mocha spec.js --require chai/register-expect.js  # Using Expect style
mocha spec.js --require chai/register-should.js  # Using Should style

Plugins

Chai offers a robust Plugin architecture for extending Chai’s assertions and interfaces.

Need a plugin? View the official plugin list.
Want to build a plugin? Read the plugin api documentation.
Have a plugin and want it listed? Simply add the following keywords to your package.json:
- chai-plugin
- browser if your plugin works in the browser as well as Node.js
- browser-only if your plugin does not work with Node.js

chaijs / chai-docs: The chaijs.com website source code.
chaijs / assertion-error: Custom Error constructor thrown upon an assertion failing.
chaijs / deep-eql: Improved deep equality testing for Node.js and the browser.
chaijs / check-error: Error comparison and information related utility for Node.js and the browser.
chaijs / loupe: Inspect utility for Node.js and browsers.
chaijs / pathval: Object value retrieval given a string path.