After updating react-dom to version 17 the build now includes Array.fill() which isn't supported by ie11 without polyfilling. Coming from this line. Tested with just the default cra production build to make sure it wasn't a problem with my webpack config and the same issue.

https://github.com/facebook/react/blob/96ac799eace5d989de3b4f80e6414e94a08ff77a/packages/react-reconciler/src/ReactFiberLane.js#L671

Tested using CRA on version 16.9.8 of react and react-dom which works with the production build for ie11. 


return set & ~subset;
}
 Seems redundant, but it changes the type from a single lane (used for
 updates) to a group of lanes (used for flushing work).
export function laneToLanes(lane: Lane): Lanes {
  return lane;
}
export function higherPriorityLane(a: Lane, b: Lane) {
   
  return a !== NoLane && a < b ? a : b;
}
export function higherLanePriority(
  a: LanePriority,
  b: LanePriority,
): LanePriority {
  return a !== NoLanePriority && a > b ? a : b;
}
export function createLaneMap<T>(initial: T): LaneMap<T> {
  return new Array(TotalLanes).fill(initial);
}
export function markRootUpdated(
  root: FiberRoot,
  updateLane: Lane,
  eventTime: number,
) {
  root.pendingLanes |= updateLane;