Add fake_date test helper and use it in spec

https://gitlab.com/gitlab-org/gitlab/-/issues/236004

doc/development/testing_guide/frontend_testing.md

@@ -691,6 +691,38 @@ unit tests.
 Instead of `setImmediate`, use `jest.runAllTimers` or `jest.runOnlyPendingTimers` to run pending timers.
 The latter is useful when you have `setInterval` in the code. **Remember:** our Jest configuration uses fake timers.
 
+## Avoid non-deterministic specs
+
+Non-determinism is the breeding ground for flaky and brittle specs. Such specs end up breaking the CI pipeline, interrupting the work flow of other contributors.
+
+1. Make sure your test subject's collaborators (e.g., axios, apollo, lodash helpers) and test environment (e.g., Date) behave consistently across systems and over time.
+1. Make sure tests are focused and not doing "extra work" (e.g., needlessly creating the test subject more than once in an individual test)
+
+### Faking `Date` for determinism
+
+Consider using `useFakeDate` to ensure a consistent value is returned with every `new Date()` or `Date.now()`.
+
+```javascript
+import { useFakeDate } from 'helpers/fake_date';
+
+describe('cool/component', () => {
+  useFakeDate();
+
+  // ...
+});
+```
+
+### Faking `Math.random` for determinism
+
+Consider replacing `Math.random` with a fake when the test subject depends on it.
+
+```javascript
+beforeEach(() => {
+  // https://xkcd.com/221/
+  jest.spyOn(Math, 'random').mockReturnValue(0.4);
+});
+```
+
 ## Factories
 
 TBU

ee/spec/frontend/threat_monitoring/components/__snapshots__/network_policy_list_spec.js.snap

@@ -90,7 +90,7 @@ exports[`NetworkPolicyList component renders policies table 1`] = `
       >
         <div>
           
-      2 weeks ago
+      2 months ago
     
         </div>
       </td>

ee/spec/frontend/threat_monitoring/components/network_policy_list_spec.js

@@ -3,17 +3,15 @@ import { GlTable } from '@gitlab/ui';
 import createStore from 'ee/threat_monitoring/store';
 import NetworkPolicyList from 'ee/threat_monitoring/components/network_policy_list.vue';
 import { PREDEFINED_NETWORK_POLICIES } from 'ee/threat_monitoring/constants';
+import { useFakeDate } from 'helpers/fake_date';
 import { convertObjectPropsToCamelCase } from '~/lib/utils/common_utils';
 import { mockPoliciesResponse } from '../mock_data';
 
-jest.mock('timeago.js', () => ({
-  format: jest.fn().mockReturnValue('2 weeks ago'),
-  register: jest.fn(),
-}));
-
 const mockData = mockPoliciesResponse.map(policy => convertObjectPropsToCamelCase(policy));
 
 describe('NetworkPolicyList component', () => {
+  useFakeDate();
+
   let store;
   let wrapper;
 

spec/frontend/helpers/fake_date.js

+// Frida Kahlo's birthday (6 = July)
+export const DEFAULT_ARGS = [2020, 6, 6];
+
+const RealDate = Date;
+
+const isMocked = val => Boolean(val.mock);
+
+export const createFakeDateClass = ctorDefault => {
+  const FakeDate = new Proxy(RealDate, {
+    construct: (target, argArray) => {
+      const ctorArgs = argArray.length ? argArray : ctorDefault;
+
+      return new RealDate(...ctorArgs);
+    },
+    apply: (target, thisArg, argArray) => {
+      const ctorArgs = argArray.length ? argArray : ctorDefault;
+
+      return RealDate(...ctorArgs);
+    },
+    // We want to overwrite the default 'now', but only if it's not already mocked
+    get: (target, prop) => {
+      if (prop === 'now' && !isMocked(target[prop])) {
+        return () => new RealDate(...ctorDefault).getTime();
+      }
+
+      return target[prop];
+    },
+    getPrototypeOf: target => {
+      return target.prototype;
+    },
+    // We need to be able to set props so that `jest.spyOn` will work.
+    set: (target, prop, value) => {
+      // eslint-disable-next-line no-param-reassign
+      target[prop] = value;
+      return true;
+    },
+  });
+
+  return FakeDate;
+};
+
+export const useFakeDate = (...args) => {
+  const FakeDate = createFakeDateClass(args.length ? args : DEFAULT_ARGS);
+  global.Date = FakeDate;
+};
+
+export const useRealDate = () => {
+  global.Date = RealDate;
+};

spec/frontend/helpers/fake_date_spec.js

+import { createFakeDateClass, DEFAULT_ARGS, useRealDate } from './fake_date';
+
+describe('spec/helpers/fake_date', () => {
+  describe('createFakeDateClass', () => {
+    let FakeDate;
+
+    beforeAll(() => {
+      useRealDate();
+    });
+
+    beforeEach(() => {
+      FakeDate = createFakeDateClass(DEFAULT_ARGS);
+    });
+
+    it('should use default args', () => {
+      expect(new FakeDate()).toEqual(new Date(...DEFAULT_ARGS));
+      expect(FakeDate()).toEqual(Date(...DEFAULT_ARGS));
+    });
+
+    it('should have deterministic now()', () => {
+      expect(FakeDate.now()).not.toBe(Date.now());
+      expect(FakeDate.now()).toBe(new Date(...DEFAULT_ARGS).getTime());
+    });
+
+    it('should be instanceof Date', () => {
+      expect(new FakeDate()).toBeInstanceOf(Date);
+    });
+
+    it('should be instanceof self', () => {
+      expect(new FakeDate()).toBeInstanceOf(FakeDate);
+    });
+  });
+});