Carousel in React Native

carousel.js

const CardList = (props) => {
  let snapScroll, lastx, scrollingRight
  const scrollBack = () => {
    const len = props.cards.length
    if (len == 0 || lastx==undefined) {
      return
    }

    let index = scrollingRight? Math.ceil(lastx / cardWith) : Math.floor(lastx / cardWith)
    
    if (index < 0) {
      index = 0
    }
    if (index > len-1) {
      index = len-1
    }
    
    props.onChangeIndex(index)

    const scrollTo = index * cardWith
    snapScroll.scrollTo({x:scrollTo, y:0 , animated:true})
  }

  return (
    <ScrollView 
      ref={(scroll) => { snapScroll = scroll }}
      style={{
        width:cardWith*3,
        left:0.5*(deviceWidth - cardWith*3),
      }}
      decelerationRate={0}
      onMomentumScrollEnd={Platform.select({
        ios: () => {},
        android: scrollBack,
      })}
      onResponderRelease={Platform.select({
        ios: scrollBack,
        android: () => {},
      })}
      scrollEventThrottle={32}
      showsHorizontalScrollIndicator={false} 
      horizontal={true} 
      onScroll={(event)=>{
        const nextx = event.nativeEvent.contentOffset.x;
        scrollingRight = (nextx > lastx);
        lastx = nextx;
      }} >
      <View style={{width:cardWith}}/>
      {
        props.cards.map( (obj) => 
          <View 
            key={obj.cardType} 
            style={{
            width:cardWith,
          }} >
            <Card
              renderCode={props.renderCode}
              info={obj} />
          </View>
          
        )
      }
      <View style={{width:cardWith}}/>
    </ScrollView>
  )
}