docs: Document getMockImplementation()

[ide]

Summary

getMockImplementation() was made a public method in commit 89119e4 in 2016 and in public TypeScript types since commit 95b94e8 in 2020 but it was never documented. Closes #9735.

Motivation

This method has become more necessary with Jest's support for ESM. Previously, you could extend manual mocks using jest.spyOn():

// __mocks__/module.js
export function fn() { /* default mock implementation */ }

// __tests__/module.test.js
const originalFn = module.fn;
jest.spyOn(module, 'fn').mockImplementation((...args) => {
  // Add custom logic for the test here
  return originalFn(...args);
});

However, spyOn doesn't work with ESM because module object properties are read-only. The workaround is to use getMockImplementation():

// __mocks__/module.js
export const fn = jest.fn(() => { /* default mock implementation */ });

// __tests__/module.test.js
const originalImpl = jest.mocked(module.fn).getMockImplementation();
jest.mocked(module.fn).mockImplementation((...args) => {
  // Add custom logic for the test here
  return originalImpl?.(...args);
});

Test plan

1. Build and serve the docs locally: cd website && yarn install && yarn fetchSupporters && yarn start
2. Browse to http://localhost:3000/docs/next/mock-function-api#mockfngetmockimplementation

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commit: 37267a5

[cpojer]

Thank you!

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