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

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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 built-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

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