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