diff --git a/packages/@react-stately/virtualizer/src/tween.ts b/packages/@react-stately/virtualizer/src/tween.ts
index 0a57aa8bc48..0f6e44b43cc 100644
--- a/packages/@react-stately/virtualizer/src/tween.ts
+++ b/packages/@react-stately/virtualizer/src/tween.ts
@@ -16,15 +16,11 @@ import {Point} from './Point';
 let perf = typeof window !== 'undefined' ? window.performance : null;
 // @ts-ignore
 let perfNow = perf && (perf.now || perf.webkitNow || perf.msNow || perf.mozNow);
-let getTime = perfNow ? perfNow.bind(perf) : function () {
-  return Date.now ? Date.now() : new Date().getTime();
-};
-
-// check if we need to get the time each frame (see below)
-let fixTs = false;
-requestAnimationFrame(function (t) {
-  fixTs = (t > 1e12) !== (getTime() > 1e12);
-});
+let getTime = perfNow
+  ? perfNow.bind(perf)
+  : function () {
+    return Date.now ? Date.now() : new Date().getTime();
+  };
 
 export interface CancelablePromise<T> extends Promise<T> {
   cancel(): void
@@ -32,14 +28,15 @@ export interface CancelablePromise<T> extends Promise<T> {
 
 export function tween(begin, end, duration, ease, fn): CancelablePromise<void> {
   let canceled = false;
-  let raf_id;
+  let raf_id: number = 0;
 
-  let promise = new Promise(resolve => {
-    let start = getTime();
-    let diffX = end.x - begin.x;
-    let diffY = end.y - begin.y;
+  const promise = new Promise((resolve) => {
+    const start = getTime();
+    const diffX = end.x - begin.x;
+    const diffY = end.y - begin.y;
 
     raf_id = requestAnimationFrame(function run(t) {
+      const fixTs = t > 1e12 !== getTime() > 1e12;
       // if we're using a high res timer, make sure timestamp is not the old epoch-based value.
       // http://updates.html5rocks.com/2012/05/requestAnimationFrame-API-now-with-sub-millisecond-precision
       if (fixTs) {
@@ -47,16 +44,18 @@ export function tween(begin, end, duration, ease, fn): CancelablePromise<void> {
       }
 
       // check if we're done
-      let delta = t - start;
+      const delta = t - start;
       if (delta > duration) {
         fn(end);
         resolve();
       } else {
         // call frame callback after computing eased time and get the next frame
-        let proceed = fn(new Point(
-          begin.x + diffX * ease(delta / duration),
-          begin.y + diffY * ease(delta / duration)
-        ));
+        const proceed = fn(
+          new Point(
+            begin.x + diffX * ease(delta / duration),
+            begin.y + diffY * ease(delta / duration)
+          )
+        );
         if (proceed !== false && !canceled) {
           raf_id = requestAnimationFrame(run);
         }
@@ -78,5 +77,5 @@ export function linearEasing(t) {
 }
 
 export function easeOut(t) {
-  return Math.sin(t * Math.PI / 2);
+  return Math.sin((t * Math.PI) / 2);
 }