in packages/fbjs/src/core/TouchEventUtils.js [21:30]
extractSingleTouch: function(nativeEvent) {
const touches = nativeEvent.touches;
const changedTouches = nativeEvent.changedTouches;
const hasTouches = touches && touches.length > 0;
const hasChangedTouches = changedTouches && changedTouches.length > 0;
return !hasTouches && hasChangedTouches ? changedTouches[0] :
hasTouches ? touches[0] :
nativeEvent;
}