chai

Original：🇺🇸 English
Translated

Chai assertion library for JavaScript. Use for JS assertions.

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npx skill4agent add g1joshi/agent-skills chai

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chai-assertionmocha-integrationjs-testingchai-pluginstesting-best-practices

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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

javascript
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