Mocha To Jest Migrator Agent Skill

Specialized guidance for migrating Node.js packages from the legacy Mocha, Sinon, and Istanbul/c8 ecosystem to Jest. It focuses on surgical test transformation, type safety, and resolving ESM/CommonJS interoperability issues.

742 tokens

context cost

the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

3194

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/googleapis/google-cloud-node --skill mocha-to-jest-migrator

The instruction itself

as written by the author

Infrastructure Migration
Dependency Swap: Remove mocha, @types/mocha, sinon, @types/sinon, c8, codecov, proxyquire, and @types/proxyquire. Install

jest, ts-jest, and @types/jest.

Script Alignment: Update package.json to use jest for the test and system-test commands. Avoid NODE_OPTIONS or

experimental flags if possible by using mocking.

Config Generation: Create a jest.config.js using the modern transform block for ts-jest:

   transform: { '^.+\\.tsx?$': ['ts-jest', { tsconfig: 'tsconfig.json' }] },
   clearMocks: true

In the config file, include this license header:

    // Copyright 2026 Google LLC
    //
    // Licensed under the Apache License, Version 2.0 (the "License");
    // you may not use this file except in compliance with the License.
    // You may obtain a copy of the License at
    //
    //      https://www.apache.org/licenses/LICENSE-2.0
    //
    // Unless required by applicable law or agreed to in writing, software
    // distributed under the License is distributed on an "AS IS" BASIS,
    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    // See the License for the specific language governing permissions and
    // limitations under the License.

Syntax Transformation (Cheat Sheet)
Structure: Jest supports describe, it, beforeEach, etc., natively. No changes are needed to the block structure.
Assertions:
assert.strictEqual(a, b) → expect(a).toBe(b)
assert.deepStrictEqual(a, b) → expect(a).toEqual(b)
assert.ifError(err) → expect(err).toBeNull()
somePromise.then(result => assert.deepStrictEqual(result, expected)) → expect(somePromise).resolves.toEqual(expected)
Mocking:
sinon.stub(obj, 'meth') → jest.spyOn(obj, 'meth').mockImplementation(...)
sinon.fn() → jest.fn()
proxyquire(path, { 'dep': mock }) → jest.mock('dep', () => mock)
Advanced Patterns
Strong Typing: If possible without adding to much complexity, remove the use of any from the tests as you go.
Top-Level Mocking: If a dependency uses ESM dynamic imports (like import('node-fetch')) that trigger TypeError in Jest,

mock that dependency at the top level of the test file using jest.mock(). This bypasses the need for the

--experimental-vm-modules flag.

Async Robustness: For tests using done(), always wrap expectations in try-catch blocks and pass the error to done(e) to

prevent tests from hanging on failure.

Cleanup Workflow
Config Deletion: Remove .mocharc.js, .nycrc, and old .coverage or .nyc_output directories.
Git: Update .gitignore to ignore coverage/.

Other skills for the same job

different authors, same section of the catalogue

Webapp Testing

by anthropics

vendor ×12

Toolkit for interacting with and testing local web applications using Playwright. Supports verifying frontend functionality, debugging UI behavior, capturing browser screenshots, and viewing browser logs.

6k tokens scripts

Finishing A Development Branch

by ZhanlinCui

×7

Use when implementation is complete, all tests pass, and you need to decide how to integrate the work - guides completion of development work by presenting structured options for merge, PR, or cleanup

1k tokens

Test Driven Development

by w95

×7

Use when implementing any feature or bugfix, before writing implementation code

Use when encountering any bug, test failure, or unexpected behavior, before proposing fixes

10k tokens scripts

Verification Before Completion

by ZhanlinCui

×6

Use when about to claim work is complete, fixed, or passing, before committing or creating PRs - requires running verification commands and confirming output before making any success claims; evidence before assertions always

Expert guidance for systematic backtesting of trading strategies. Use when developing, testing, stress-testing, or validating quantitative trading strategies. Covers "beating ideas to death" methodology, parameter robustness testing, slippage modeling, bias prevention, and interpreting backtest results. Applicable when user asks about backtesting, strategy validation, robustness testing, avoiding overfitting, or systematic trading development.

Cloud laboratory platform for automated protein testing and validation. Use when designing proteins and needing experimental validation including binding assays, expression testing, thermostability measurements, enzyme activity assays, or protein sequence optimization. Also use for submitting experiments via API, tracking experiment status, downloading results, optimizing protein sequences for better expression using computational tools (NetSolP, SoluProt, SolubleMPNN, ESM), or managing protein design workflows with wet-lab validation.

This skill should be used for time series machine learning tasks including classification, regression, clustering, forecasting, anomaly detection, segmentation, and similarity search. Use when working with temporal data, sequential patterns, or time-indexed observations requiring specialized algorithms beyond standard ML approaches. Particularly suited for univariate and multivariate time series analysis with scikit-learn compatible APIs.

19k tokens

How to use it

Copy the folder

Take googleapis/mocha-to-jest-migrator from the repository into ~/.claude/skills for personal use, or into .claude/skills inside a project.

Check the name does not clash

The agent identifies a skill by the name field in its header. Two skills with the same name cannot sit side by side — one of them will be ignored.