mentoss

library0.13.0jsnpmunverified

Mentoss is a lightweight, TypeScript-first utility for mocking fetch() requests and responses in Node.js (>=18.18) and browser environments. Version 0.13.0 (July 2024) provides three core classes — MockServer, FetchMocker, and MockAgent — to define server routes, replace global fetch, and intercept undici requests. Unlike similar libraries (e.g., nock, msw), Mentoss focuses exclusively on fetch() and undici, supporting both ESM and CommonJS through conditional exports, shipping type definitions, and offering a minimal API with built-in route tracking and assertion helpers. The package is authored by Nicholas C. Zakas (ESLint creator) and has a release cadence of approximately monthly updates.

testing

npm install mentoss

github ↗package ↗homepage ↗

INSTALL

IMPORT

SIG · MENTOSS

mentoss

testingjavascriptv0.13.0

harness data pending

Install & Compatibility

Where this runs

No compatibility data collected yet for this library.

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

MockServer

✓ import { MockServer } from 'mentoss'

✗ const { MockServer } = require('mentoss')

Mentoss is ESM-only since v0.9.0; require() fails with ERR_REQUIRE_ESM.

FetchMocker

✓ import { FetchMocker } from 'mentoss'

✗ import { FetchMocker } from 'mentoss/src/index.js'

Do not import from internal paths; they are not part of public API.

MockAgent

✓ import { MockAgent } from 'mentoss'

✗ import { MockAgent } from 'mentoss/types'

Types are re-exported; no separate type-only import path.

Sets up a MockServer with a GET route, creates a FetchMocker, makes a fetch call, and verifies the route was called.

import { MockServer, FetchMocker } from 'mentoss'; const server = new MockServer('https://api.example.com'); server.get('/foo/bar', { status: 200, body: { message: 'OK' } }); const mocker = new FetchMocker({ servers: [server] }); const { fetch } = mocker; const response = await fetch('https://api.example.com/foo/bar'); const data = await response.json(); console.log(data); console.assert(mocker.called('https://api.example.com/foo/bar')); console.assert(mocker.allRoutesCalled()); mocker.clearAll();

Debug

Known issues

breakingESM-only since v0.9.0: require('mentoss') throws ERR_REQUIRE_ESM. Use import or dynamic import().

fix

Switch to ESM imports (import { MockServer } from 'mentoss') or use dynamic import() in CJS context.

affects: >=0.9.0

gotchaUsing Mentoss inside Jest requires --experimental-vm-modules flag and Jest 28+ with transformIgnorePatterns adjusted.

fix

Run Jest with node --experimental-vm-modules ./node_modules/.bin/jest and add 'mentoss' to transformIgnorePatterns exception.

affects: >=0.0.0

gotchaMockAgent does not support baseUrl or credentials options; they are silently ignored (only relevant for browser fetch()).

fix

Do not pass baseUrl or credentials to MockAgent; configure those via MockServer routes if needed.

affects: >=0.0.0

deprecatedMethod mocker.routesCalled() was deprecated in v0.10.0 and removed in v0.11.0. Use mocker.allRoutesCalled() instead.

fix

Replace mocker.routesCalled() with mocker.allRoutesCalled().

affects: >=0.10.0 <0.11.0

Errors

Common errors & fixes

ERR_REQUIRE_ESM: require() of ES Module /path/to/node_modules/mentoss/src/index.js from /path/to/project/test.js not supported.

Mentoss is ESM-only; using require() fails since v0.9.0.

fix

Use import { MockServer } from 'mentoss' instead of require().

TypeError: MockServer is not a constructor

Importing MockServer incorrectly (e.g., as default import or from wrong path).

fix

Use named import: import { MockServer } from 'mentoss'.

Jest encountered an unexpected token - SyntaxError: Unexpected token 'export'

Jest's default transform doesn't transpile ESM from node_modules.

fix

Enable ESM support in Jest: use node --experimental-vm-modules jest and add 'mentoss' to transformIgnorePatterns in jest.config.

Error: Cannot find module 'undici'

MockAgent requires undici to be installed; Mentoss does not include it as a dependency.

fix

Install undici: npm install undici.

Upgrade

Version history

0.13.0latest on npm

Audit

Dependencies

No dependency data recorded yet.

Agent activity

4 hits · last 30 days

node

Resources

githubhttps://github.com/humanwhocodes/mentoss ↗

packagementoss ↗

homepagehttps://github.com/humanwhocodes/mentoss#readme ↗