BDD / TDD assertion framework for node.js and the browser that can be paired with any testing framework.
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Latest commit a7e1200 Feb 14, 2017 @meeber meeber committed with lucasfcosta fix: wrong assertion types (#925)
BREAKING CHANGE: `.change`, `.increase`, and `.decrease` changed from
Chainable Method Assertions to Method Assertions. They don't have any
chaining behavior, and there's no generic semantic benefit to chaining
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lib fix: wrong assertion types (#925) Feb 14, 2017
support/sauce tests: [sauce] error in setup Sep 18, 2013
test test: add `.own.include` and `.nested.include` Feb 2, 2017
.gitignore Automate release process as much as possible Jun 3, 2015
.mailmap Add a .mailmap file to fix my name in shortlogs. May 20, 2012
.npmignore Add coverage to npmignore Jan 26, 2014
.travis.yml chore: update supported Node versions Jan 11, 2017 Add email to Mar 12, 2016 Add lucasfcosta to contributors on and Mar 11, 2016 Automate release process as much as possible Jun 3, 2015
LICENSE Create LICENSE Oct 25, 2016
Makefile Run clean-browser before testing with karma Dec 19, 2015 Center repo name on README (#889) Dec 18, 2016 Automate release process as much as possible Jun 3, 2015
bower.json Remove moot `version` property from bower.json Jun 12, 2015
chai.js improved documentation for .throws Apr 22, 2016
index.js clean up index.js to not check for cov, revert package.json to use in… Nov 29, 2012
karma.conf.js Refactor global.err to test error properties Apr 12, 2016
karma.sauce.js chore: tweak saucelab build settings Feb 8, 2016
package.json fix(package): update get-func-name to version 2.0.0 Jan 24, 2017
register-assert.js Auto register side effects and Update documentation (#872) Nov 18, 2016
register-expect.js Auto register side effects and Update documentation (#872) Nov 18, 2016
register-should.js Auto register side effects and Update documentation (#872) Nov 18, 2016
sauce.browsers.js chore: tweak saucelab build settings Feb 8, 2016


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

license:mit tag:? node:?
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For more information or to download plugins, view the documentation.

What is Chai?

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



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

$ npm install chai


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

<script src="./node_modules/chai/chai.js"></script>


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

var chai = require('chai');  
var assert = chai.assert;    // Using Assert style
var expect = chai.expect;    // Using Expect style
var should = chai.should();  // Using Should style

Pre-Native Modules Usage (registers the chai testing style globally)

require('chai/register-assert');  // Using Assert style
require('chai/register-expect');  // Using Expect style
require('chai/register-should');  // Using Should style

Pre-Native Modules Usage (as local variables)

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

const { expect, use } = require('chai');  // Creates local variables `expect` and `use`; useful for plugin use

Native Modules Usage (registers 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

Native Modules Usage (local import only)

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`

Usage with Mocha

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

Read more about these styles in our docs.


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

Related Projects


Thank you very much for considering to contribute!

Please make sure you follow our Code Of Conduct and we also strongly recommend reading our Contributing Guide.

Here are a few issues other contributors frequently ran into when opening pull requests:

  • Please do not commit changes to the chai.js build. We do it once per release.
  • Before pushing your commits, please make sure you rebase them.


Please see the full Contributors Graph for our list of contributors.

Core Contributors

Feel free to reach out to any of the core contributors with your questions or concerns. We will do our best to respond in a timely manner.

Jake Luer Veselin Todorov Keith Cirkel Lucas Fernandes da Costa Grant Snodgrass