jestでwindowオブジェクトの要素をmockする

例えば window.navigator.languageを変更したい場合などに用いる

テスト対象コード例 isSupportedLang.ts

export function isSupportedLang(): boolean {
  return ['ja','en', 'fr'].indexOf(window.navigator.language) >= 0;
}

テストコード例 isSupportedLang.test.ts

import { isSupportedLang } from '../../src/utils/isSupportedLang';

describe("when browser language is not in [ja,en.fr]", () => {
  let windowSpy;
  beforeEach(() => {
    const originalWindow = { ...window };
    windowSpy = jest.spyOn(global, "window", "get");
    windowSpy.mockImplementation(() => ({
      ...originalWindow,
      navigator: {
        ...originalWindow.navigator,
        language: 'Mocked_language'
      }
    }));
  });
  afterEach(() => {
    windowSpy.mockRestore();
  });
  it('will be falsy', () => {
    expect(isSupportedLang()).toBeFalsy();
  });
});

describe("when browser language is fr", () => {
  let windowSpy;
  beforeEach(() => {
    const originalWindow = { ...window };
    windowSpy = jest.spyOn(global, "window", "get");
    windowSpy.mockImplementation(() => ({
      ...originalWindow,
      navigator: {
        ...originalWindow.navigator,
        language: 'fr'
      }
    }));
  });
  afterEach(() => {
    windowSpy.mockRestore();
  });
  it('will be truehy', () => {
    expect(isSupportedLang()).toBeTruthy();
  });
});