fix: Don't assume mocked timers imply jest fake timers (#900)

* fix: Don't assume mocked timers imply jest fake timers

* Create dedicated jest timer functions

The naming should indicate that they should only be called in a jest-like environment

* No implicit return

* I don't know how istanbul works

and I don't care

src/helpers.js

@@ -5,10 +5,16 @@ const TEXT_NODE = 3
 
 // Currently this fn only supports jest timers, but it could support other test runners in the future.
 function runWithRealTimers(callback) {
-  return _runWithRealTimers(callback).callbackReturnValue
+  // istanbul ignore else
+  if (typeof jest !== 'undefined') {
+    return runWithJestRealTimers(callback).callbackReturnValue
+  }
+
+  // istanbul ignore next
+  return callback()
 }
 
-function _runWithRealTimers(callback) {
+function runWithJestRealTimers(callback) {
   const timerAPI = {
     clearImmediate,
     clearInterval,
@@ -18,29 +24,33 @@ function _runWithRealTimers(callback) {
     setTimeout,
   }
 
-  // istanbul ignore else
-  if (typeof jest !== 'undefined') {
-    jest.useRealTimers()
-  }
+  jest.useRealTimers()
 
   const callbackReturnValue = callback()
 
-  const usedJestFakeTimers = Object.entries(timerAPI).some(
+  const usedFakeTimers = Object.entries(timerAPI).some(
     ([name, func]) => func !== globalObj[name],
   )
 
-  if (usedJestFakeTimers) {
+  if (usedFakeTimers) {
     jest.useFakeTimers(timerAPI.setTimeout?.clock ? 'modern' : 'legacy')
   }
 
   return {
     callbackReturnValue,
-    usedJestFakeTimers,
+    usedFakeTimers,
   }
 }
 
-const jestFakeTimersAreEnabled = () =>
-  Boolean(_runWithRealTimers(() => {}).usedJestFakeTimers)
+function jestFakeTimersAreEnabled() {
+  // istanbul ignore else
+  if (typeof jest !== 'undefined') {
+    return runWithJestRealTimers(() => {}).usedFakeTimers
+  }
+
+  // istanbul ignore next
+  return false
+}
 
 // we only run our tests in node, and setImmediate is supported in node.
 // istanbul ignore next

src/wait-for.js

@@ -52,8 +52,8 @@ function waitFor(
 
     const overallTimeoutTimer = setTimeout(handleTimeout, timeout)
 
-    const usingFakeTimers = jestFakeTimersAreEnabled()
-    if (usingFakeTimers) {
+    const usingJestFakeTimers = jestFakeTimersAreEnabled()
+    if (usingJestFakeTimers) {
       checkCallback()
       // this is a dangerous rule to disable because it could lead to an
       // infinite loop. However, eslint isn't smart enough to know that we're
@@ -107,7 +107,7 @@ function waitFor(
       finished = true
       clearTimeout(overallTimeoutTimer)
 
-      if (!usingFakeTimers) {
+      if (!usingJestFakeTimers) {
         clearInterval(intervalId)
         observer.disconnect()
       }