jest

name	jest
description	Write tests, mock functions, assert values, and configure test suites for Roblox/Luau code using Jest Roblox — a Luau port whose API diverges from JS Jest in subtle, non-obvious ways that LLM training data does not cover. Use whenever encountering Jest in Roblox/Luau code: editing .spec test files, calling jest.fn/jest.mock/spyOn, using describe/it/expect, or debugging Jest tests.
metadata	{"author":"Christopher Buss","version":"2026.5.4","source":"Generated from https://github.com/Roblox/jest-roblox, scripts located at https://github.com/christopher-buss/skills"}

Based on Jest Roblox v3.x, generated 2026-02-07.

Jest Roblox is a Luau port of Jest for the Roblox platform. It closely follows the upstream Jest API but has critical deviations due to Luau language constraints.

Critical deviations from JS Jest:

• .never instead of .not (reserved keyword)
• jest.fn() returns two values: mock object + forwarding function
• 0, "", {} are truthy in Luau (only false and nil are falsy)
• All globals (describe, expect, jest, etc.) must be explicitly imported
• .each uses table syntax, not tagged template literals
• Custom matchers take self as first parameter

Read core-deviations first when working with this codebase.

Core References

Topic	Description	Reference
Deviations	All Luau/Roblox differences from JS Jest	core-deviations
Test Structure	describe, test/it, hooks, .each, .only/.skip	core-test-structure
Matchers	toBe, toEqual, toContain, toThrow, mock matchers	core-matchers
Asymmetric Matchers	expect.anything/any/nothing/callable, .resolves/.rejects	core-asymmetric-matchers
Mocking	jest.fn(), spyOn, mock.calls, return values	core-mocking
Configuration	jest.config.lua, runCLI, reporters, options	core-configuration

Features

Testing Patterns

Topic	Description	Reference
Async Testing	Promises, done callbacks, .resolves/.rejects	feature-async-testing
Custom Matchers	expect.extend(), self parameter, isNever	feature-custom-matchers
Snapshot Testing	toMatchSnapshot, property matchers, serializers	feature-snapshot-testing
Test Filtering	testMatch, testPathPattern, testNamePattern	feature-test-filtering

Mocking

Topic	Description	Reference
Timer Mocks	useFakeTimers, Roblox timers, engineFrameTime	feature-timer-mocks
Global Mocks	jest.globalEnv, spyOn globals, library mocks	feature-global-mocks
Module Mocking	jest.mock(), isolateModules, resetModules	feature-module-mocking

Extended Matchers

Topic	Description	Reference
Jest Extended	When core matchers aren't expressive enough: exact booleans, ranges, membership, entries, call order, side effects	feature-jest-extended

Advanced

Topic	Description	Reference
Benchmarking	benchmark(), Reporter, Profiler, CustomReporters	advanced-benchmarking