Chai

assert

expect

should

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-http

  : For API testing.

• chai-as-promised

  : For asserting promises (

  return expect(promise).to.eventually.equal(2)

  ).

Best Practices (2025)

• 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 mix Assert and Expect: Confuses the reader.

References

• Chai Documentation