fix: remove side-effect from runWithRealTimers (#887)

Author: ph-fritsche

src/__tests__/helpers.js

@@ -5,10 +5,6 @@ import {
   runWithRealTimers,
 } from '../helpers'
 
-const globalObj = typeof window === 'undefined' ? global : window
-
-afterEach(() => jest.useRealTimers())
-
 test('returns global document if exists', () => {
   expect(getDocument()).toBe(document)
 })
@@ -53,42 +49,47 @@ describe('query container validation throws when validation fails', () => {
   })
 })
 
-test('should always use realTimers before using callback when timers are faked with useFakeTimers', () => {
-  const originalSetTimeout = globalObj.setTimeout
+describe('run with real timers', () => {
+  const realSetTimeout = global.setTimeout
 
-  // legacy timers use mocks and do not rely on a clock instance
-  jest.useFakeTimers('legacy')
-  runWithRealTimers(() => {
-    expect(originalSetTimeout).toEqual(globalObj.setTimeout)
+  afterEach(() => {
+    // restore timers replaced by jest.useFakeTimers()
+    jest.useRealTimers()
+    // restore setTimeout replaced by assignment
+    global.setTimeout = realSetTimeout
   })
-  expect(globalObj.setTimeout._isMockFunction).toBe(true)
-  expect(globalObj.setTimeout.clock).toBeUndefined()
 
-  jest.useRealTimers()
-
-  // modern timers use a clock instance instead of a mock
-  jest.useFakeTimers('modern')
-  runWithRealTimers(() => {
-    expect(originalSetTimeout).toEqual(globalObj.setTimeout)
+  test('use real timers when timers are faked with jest.useFakeTimers(legacy)', () => {
+    // legacy timers use mocks and do not rely on a clock instance
+    jest.useFakeTimers('legacy')
+    runWithRealTimers(() => {
+      expect(global.setTimeout).toBe(realSetTimeout)
+    })
+    expect(global.setTimeout._isMockFunction).toBe(true)
+    expect(global.setTimeout.clock).toBeUndefined()
   })
-  expect(globalObj.setTimeout._isMockFunction).toBeUndefined()
-  expect(globalObj.setTimeout.clock).toBeDefined()
-})
 
-test('should not use realTimers when timers are not faked with useFakeTimers', () => {
-  const originalSetTimeout = globalObj.setTimeout
-
-  // useFakeTimers is not used, timers are faked in some other way
-  const fakedSetTimeout = callback => {
-    callback()
-  }
-  fakedSetTimeout.clock = jest.fn()
+  test('use real timers when timers are faked with jest.useFakeTimers(modern)', () => {
+    // modern timers use a clock instance instead of a mock
+    jest.useFakeTimers('modern')
+    runWithRealTimers(() => {
+      expect(global.setTimeout).toBe(realSetTimeout)
+    })
+    expect(global.setTimeout._isMockFunction).toBeUndefined()
+    expect(global.setTimeout.clock).toBeDefined()
+  })
 
-  globalObj.setTimeout = fakedSetTimeout
+  test('do not use real timers when timers are not faked with jest.useFakeTimers', () => {
+    // useFakeTimers is not used, timers are faked in some other way
+    const fakedSetTimeout = callback => {
+      callback()
+    }
+    fakedSetTimeout.clock = jest.fn()
+    global.setTimeout = fakedSetTimeout
 
-  runWithRealTimers(() => {
-    expect(fakedSetTimeout).toEqual(globalObj.setTimeout)
+    runWithRealTimers(() => {
+      expect(global.setTimeout).toBe(fakedSetTimeout)
+    })
+    expect(global.setTimeout).toBe(fakedSetTimeout)
   })
-
-  globalObj.setTimeout = originalSetTimeout
 })

src/helpers.js

@@ -5,52 +5,42 @@ const TEXT_NODE = 3
 
 // Currently this fn only supports jest timers, but it could support other test runners in the future.
 function runWithRealTimers(callback) {
-  const fakeTimersType = getJestFakeTimersType()
-  if (fakeTimersType) {
+  return _runWithRealTimers(callback).callbackReturnValue
+}
+
+function _runWithRealTimers(callback) {
+  const timerAPI = {
+    clearImmediate,
+    clearInterval,
+    clearTimeout,
+    setImmediate,
+    setInterval,
+    setTimeout,
+  }
+
+  // istanbul ignore else
+  if (typeof jest !== 'undefined') {
     jest.useRealTimers()
   }
 
   const callbackReturnValue = callback()
 
-  if (fakeTimersType) {
-    jest.useFakeTimers(fakeTimersType)
-  }
+  const usedJestFakeTimers = Object.entries(timerAPI).some(
+    ([name, func]) => func !== globalObj[name],
+  )
 
-  return callbackReturnValue
-}
-
-function getJestFakeTimersType() {
-  // istanbul ignore if
-  if (
-    typeof jest === 'undefined' ||
-    typeof globalObj.setTimeout === 'undefined'
-  ) {
-    return null
+  if (usedJestFakeTimers) {
+    jest.useFakeTimers(timerAPI.setTimeout?.clock ? 'modern' : 'legacy')
   }
 
-  if (
-    typeof globalObj.setTimeout._isMockFunction !== 'undefined' &&
-    globalObj.setTimeout._isMockFunction
-  ) {
-    return 'legacy'
-  }
-
-  if (
-    typeof globalObj.setTimeout.clock !== 'undefined' &&
-    typeof jest.getRealSystemTime !== 'undefined'
-  ) {
-    try {
-      // jest.getRealSystemTime is only supported for Jest's `modern` fake timers and otherwise throws
-      jest.getRealSystemTime()
-      return 'modern'
-    } catch {
-      // not using Jest's modern fake timers
-    }
+  return {
+    callbackReturnValue,
+    usedJestFakeTimers,
   }
-  return null
 }
 
-const jestFakeTimersAreEnabled = () => Boolean(getJestFakeTimersType())
+const jestFakeTimersAreEnabled = () =>
+  Boolean(_runWithRealTimers(() => {}).usedJestFakeTimers)
 
 // we only run our tests in node, and setImmediate is supported in node.
 // istanbul ignore next