chai

name	chai
description	Chai assertion library for JavaScript. Use for JS assertions.

Chai

Chai is an assertion library. It pairs naturally with Mocha. It supports TDD (assert) and BDD (expect, should) styles.

When to Use

• With Mocha: The default pair.
• Expressive Tests: You want tests to read like English ("expect foo to be a string").

Quick Start

import { expect } from "chai";

const foo = "bar";
const beverages = { tea: ["chai", "matcha", "oolong"] };

expect(foo).to.be.a("string");
expect(foo).to.equal("bar");
expect(foo).to.have.lengthOf(3);
expect(beverages).to.have.property("tea").with.lengthOf(3);

Core Concepts

Styles

• Assert: Classic. assert.equal(foo, 'bar').
• Expect: BDD. Chainable. expect(foo).to.be.equal('bar').
• Should: BDD. Extends Object prototype. foo.should.be.equal('bar'). (Less common now due to side effects).

Plugins

Chai has a rich ecosystem.

• chai-http: For API testing.
• chai-as-promised: For asserting promises (return expect(promise).to.eventually.equal(2)).

Best Practices (2025)

Do:

• Stick to one style: Usually Expect. It's clean and doesn't modify prototypes.
• Use Descriptive Chains: to.be.true reads better than to.equal(true).

Don't:

• Don't mix Assert and Expect: Confuses the reader.

References

• Chai Documentation