[Scheduler] Get current time from performance.now in non-DOM environments (#19532)

* Get current time from performance.now in non-DOM environments

* Use local references to native APIs for Date and Performance

* Refactored to read globals directly

Author: emilisb

packages/scheduler/src/__tests__/SchedulerBrowser-test.js

@@ -42,14 +42,16 @@ describe('SchedulerBrowser', () => {
     );
 
     runtime = installMockBrowserRuntime();
-    performance = window.performance;
+    performance = global.performance;
     Scheduler = require('scheduler');
     cancelCallback = Scheduler.unstable_cancelCallback;
     scheduleCallback = Scheduler.unstable_scheduleCallback;
     NormalPriority = Scheduler.unstable_NormalPriority;
   });
 
   afterEach(() => {
+    delete global.performance;
+
     if (!runtime.isLogEmpty()) {
       throw Error('Test exited without clearing log.');
     }
@@ -63,17 +65,17 @@ describe('SchedulerBrowser', () => {
 
     let eventLog = [];
 
-    const window = {};
-    global.window = window;
-
     let currentTime = 0;
 
-    window.performance = {
+    global.performance = {
       now() {
         return currentTime;
       },
     };
 
+    const window = {};
+    global.window = window;
+
     // TODO: Scheduler no longer requires these methods to be polyfilled. But
     // maybe we want to continue warning if they don't exist, to preserve the
     // option to rely on it in the future?

packages/scheduler/src/forks/SchedulerHostConfig.default.js

@@ -16,6 +16,18 @@ export let requestPaint;
 export let getCurrentTime;
 export let forceFrameRate;
 
+const hasPerformanceNow =
+  typeof performance === 'object' && typeof performance.now === 'function';
+
+if (hasPerformanceNow) {
+  const localPerformance = performance;
+  getCurrentTime = () => localPerformance.now();
+} else {
+  const localDate = Date;
+  const initialTime = localDate.now();
+  getCurrentTime = () => localDate.now() - initialTime;
+}
+
 if (
   // If Scheduler runs in a non-DOM environment, it falls back to a naive
   // implementation using setTimeout.
@@ -40,10 +52,6 @@ if (
       }
     }
   };
-  const initialTime = Date.now();
-  getCurrentTime = function() {
-    return Date.now() - initialTime;
-  };
   requestHostCallback = function(cb) {
     if (_callback !== null) {
       // Protect against re-entrancy.
@@ -68,8 +76,6 @@ if (
   requestPaint = forceFrameRate = function() {};
 } else {
   // Capture local references to native APIs, in case a polyfill overrides them.
-  const performance = window.performance;
-  const Date = window.Date;
   const setTimeout = window.setTimeout;
   const clearTimeout = window.clearTimeout;
 
@@ -98,16 +104,6 @@ if (
     }
   }
 
-  if (
-    typeof performance === 'object' &&
-    typeof performance.now === 'function'
-  ) {
-    getCurrentTime = () => performance.now();
-  } else {
-    const initialTime = Date.now();
-    getCurrentTime = () => Date.now() - initialTime;
-  }
-
   let isMessageLoopRunning = false;
   let scheduledHostCallback = null;
   let taskTimeoutID = -1;