Mocha To Jest Migrator

• 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/.