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Mocha To Jest Migrator by googleapis

Name: Mocha To Jest Migrator
Author: googleapis

Mocha To Jest Migrator

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.

googleapis Updated Sep 17, 2026

File contents

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/.

---name: mocha-to-jest-migratordescription: 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.---1. 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: ```javascript transform: { '^.+\\.tsx?$': ['ts-jest', { tsconfig: 'tsconfig.json' }] }, clearMocks: true ``` * In the config file, include this license header: ```javascript // 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. ```2. 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)3. 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.4. Cleanup Workflow * Config Deletion: Remove .mocharc.js, .nycrc, and old .coverage or .nyc_output directories. * Git: Update .gitignore to ignore `coverage/`.

Frequently asked questions