chai

What is chai?

Chai is a BDD / TDD assertion library for node and the browser that can be delightfully paired with any javascript testing framework. It provides functions to express assertions in a readable language, supporting both 'should' and 'expect' style syntaxes, as well as a traditional 'assert' style interface.

What are chai's main functionalities?

Should Style Assertions

Allows you to write assertions in a more human-readable form, which reads like English. The 'should' style extends each object with a 'should' property to start your chain.

chai.should();
var should = chai.should();
should.exist(value, 'value should not be null or undefined');

Expect Style Assertions

Provides a function called 'expect' to make assertions about values. This style is similar to 'should' but does not extend any objects, which is useful for working with libraries that don't allow object extensions.

var expect = chai.expect;
expect(value).to.exist.and.be.an('object');

Assert Style Assertions

Offers a more traditional testing interface, which is similar to Node.js' core 'assert' module but with more capabilities and better error messages.

var assert = chai.assert;
assert.typeOf(value, 'string', 'value is a string');

Chainable Language Chains

Provides chainable getters to improve the readability of your assertions, allowing you to write more expressive tests.

expect(value).to.be.an('array').that.includes(3);

Plugins

Supports plugins to extend its core functionalities, which can be used to add new assertions, matchers, or methods to the library.

var chaiHttp = require('chai-http');
chai.use(chaiHttp);

Other packages similar to chai

jest

Jest is a delightful JavaScript Testing Framework with a focus on simplicity. It works out of the box for any React project and includes its own assertion library, which provides a similar but more integrated testing experience compared to Chai.

jasmine

Jasmine is a behavior-driven development framework for testing JavaScript code. It does not rely on browsers, DOM, or any JavaScript framework, making it suitable for websites, Node.js projects, or anywhere JavaScript can run. It has its own assertion syntax that is somewhat similar to Chai's 'expect' syntax.

expect.js

Expect.js is a minimalistic BDD-style assertions library that can be considered an alternative to Chai's 'expect' interface. It provides a similar API but is no longer actively maintained.

README

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 chai.js file found within the download. For example:

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

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.

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

Related Projects

• 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.

Contributing

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.

Contributors

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.