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March 03, 2025 00:33
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Chai Assertion Library Cheatsheet

A comprehensive cheat sheet for the Chai assertion library, covering various assertion styles and methods for effective testing in JavaScript.

Introduction to Chai

Chai Overview

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

It provides a clean and readable syntax for writing test assertions.

Chai supports several interfaces: should, expect, and assert. This cheat sheet will primarily focus on expect and assert styles, which are commonly used.

Installation: npm install chai --save-dev

Importing Chai:

const chai = require('chai');
const expect = chai.expect; // for 'expect' style
const assert = chai.assert; // for 'assert' style

Expect Style Assertions

The expect style provides a chainable way to express assertions.

Basic Syntax: expect(value).to.assertion

Example:

expect(foo).to.equal('bar');
expect(list).to.have.lengthOf(3);

Assert Style Assertions

The assert style provides a more traditional way to express assertions, similar to standard assert.

Basic Syntax: assert.assertion(value, message)

Example:

assert.equal(foo, 'bar', 'foo should equal bar');
assert.lengthOf(list, 3, 'list should have a length of 3');

Common Assertions with Expect

Equality Assertions

expect(value).to.equal(value2)

Checks for deep equality (using ===)

expect(value).to.eql(value2)

Checks for deep equality of objects and arrays

expect(value).to.deep.equal(value2)

Alias for eql

expect(value).to.be.a(type)

Checks the type of a value

expect(value).to.be.an(type)

Alias for a

Existence and Truthiness

expect(value).to.exist

Checks if a value is not null or undefined

expect(value).to.not.exist

Checks if a value is null or undefined

expect(value).to.be.true

Checks if a value is strictly true

expect(value).to.be.false

Checks if a value is strictly false

expect(value).to.be.ok

Checks if a value is truthy

Number Assertions

expect(number).to.be.above(number2)

Checks if number is greater than number2

expect(number).to.be.below(number2)

Checks if number is less than number2

expect(number).to.be.at.least(number2)

Checks if number is greater than or equal to number2

expect(number).to.be.at.most(number2)

Checks if number is less than or equal to number2

expect(number).to.be.within(min, max)

Checks if number is within the range [min, max]

Common Assertions with Assert

Equality Assertions

assert.equal(actual, expected, message)

Tests shallow, coercive equality with the equal comparison operator ( == )

assert.strictEqual(actual, expected, message)

Tests strict equality ( === )

assert.deepEqual(actual, expected, message)

Tests for deep equality

assert.notEqual(actual, expected, message)

Tests shallow, coercive inequality with the not equal comparison operator ( != )

assert.notStrictEqual(actual, expected, message)

Tests strict inequality ( !== )

assert.notDeepEqual(actual, expected, message)

Tests for deep inequality

Type Assertions

assert.isOk(value, message)

Tests if a value is truthy

assert.isNotOk(value, message)

Tests if a value is falsy

assert.isTrue(value, message)

Tests if a value is strictly true

assert.isFalse(value, message)

Tests if a value is strictly false

assert.isDefined(value, message)

Tests if a value is not undefined

assert.isUndefined(value, message)

Tests if a value is undefined

assert.isNull(value, message)

Tests if a value is null

assert.isNotNull(value, message)

Tests if a value is not null

assert.typeOf(value, type, message)

Tests if the type of value is as expected

Number Assertions

assert.isAbove(value, above, message)

Tests if a value is greater than another value

assert.isBelow(value, below, message)

Tests if a value is less than another value

assert.isAtLeast(value, atLeast, message)

Tests if a value is greater than or equal to another value

assert.isAtMost(value, atMost, message)

Tests if a value is less than or equal to another value

Additional Chai Features

String Assertions

expect(string).to.include(substring)

Checks if string contains substring

expect(string).to.contain(substring)

Alias for include

expect(string).to.match(regex)

Checks if string matches the regular expression regex

assert.include(haystack, needle, message)

Asserts that haystack contains needle.

assert.match(string, regexp, message)

Asserts that string matches the regular expression regexp.

Array Assertions

expect(array).to.include(value)

Checks if array contains value

expect(array).to.contain(value)

Alias for include

expect(array).to.have.lengthOf(number)

Checks if array has a length of number

assert.lengthOf(object, length, message)

Asserts that object has expected length.

assert.isArray(value, message)

Asserts that value is an array.

Object Assertions

expect(object).to.have.property(key)

Checks if object has a property named key

expect(object).to.have.property(key, value)

Checks if object has a property named key with value value

expect(object).to.have.nested.property(path)

Checks if object has a nested property specified by path

assert.property(object, property, message)

Asserts that object has property.

assert.deepProperty(object, property, message)

Asserts that object has a deep property.

assert.propertyVal(object, property, value, message)

Asserts that object has property with expected value.