Introduction

deride is a TypeScript-first mocking library that works with frozen objects, sealed classes, and any coding style — because it composes rather than monkey-patches.

What it is

A test-doubles library. You use it to replace collaborators during testing: databases, HTTP clients, loggers, event emitters, clocks, anything. Typical testing-library shape:

import { stub } from 'deride'

const mockDb = stub<Database>(['query'])
mockDb.setup.query.toResolveWith(rows)

const service = new UserService(mockDb)  // inject the mock
await service.getAll()

mockDb.expect.query.called.once()

What makes it different

Most JS mocking libraries mutate objects in place — they swap a method on the real object with a spy wrapper and rely on restoring it afterwards. That works until it doesn't: frozen objects reject the mutation, ES2015 classes with private fields break, and any test that forgets to call restore() leaks state into the next test.

deride doesn't touch your real objects. stub(), wrap(), and func() all produce a fresh object with the same shape but independent behaviour. Your production code is never modified during tests.

What you get

Capability	What it looks like
Create test doubles from interfaces, instances, classes, or standalone functions	stub / wrap / func
Configure behaviour (return, resolve, throw, yield, sequence…)	setup.*
Assert how the mock was called	expect.*.called
Inspect call history without throwing	spy.*
Match arguments expressively	match.*
Verify cross-mock call order	inOrder(...)
Group mocks, snapshot, restore	sandbox / snapshot
Framework integrations	deride/vitest, deride/jest, deride/clock

When to reach for deride

Use it when you want clean, composable, type-safe mocking that doesn't require a beforeEach/afterEach dance to clean up after itself — especially in codebases with Object.freeze, immutable-by-convention objects, or dependency-injected services.

Reach for something else if you primarily need module-level mocking (vi.mock / jest.mock) without injection — deride works alongside those but isn't a replacement for them. See Module mocking for the preferred pattern.

Stability & support

• Node.js 20+ (required for vitest 4; CI tests against Node 20 and 22 on Ubuntu, macOS, and Windows).
• ESM-first with CJS fallback.
• Published under MIT, maintained at guzzlerio/deride.

Next up: installation, then a 5-minute quick start.