react-native array.map() performance

Question

I have an app that's fetching a JSON array and array mapping them. There are two nested array maps in the original map.

There are 75 separate results in the array - which contain 6 objects and 1 array. So on average there are around 1800 elements to map.

The fetch takes around 2 seconds to complete and the array mapping can take anywhere between 8-10 secs.

Is there any way I can improve on this speed?

trains.results.services.map((traininfo,index) => (

<ScrollView refreshControl={
<RefreshControl refreshing={this.state.refreshing} onRefresh= {this._onRefresh.bind(this)} /> }  key={traininfo.std+index.toString()} style={styles.slide1}>

<View key={'row'+index.toString()}  style={{flex: 1, flexDirection: 'row'}}>

{traininfo.platform===null ? (
<Text key={traininfo.platform+index.toString()} style={styles.platform}>Platform _</Text>
          ) : (
 <Text key={traininfo.platform+index.toString()} style={styles.platform}>Platform {traininfo.platform}</Text>
          )}  

{traininfo.etd==='On time' || traininfo.etd==='Cancelled' || traininfo.etd==='Delayed' ? (
<Text key={traininfo.etd+index.toString()} style={styles.time}>{traininfo.etd}</Text>
          ) : (
<Text key={traininfo.etd+index.toString()} style={styles.time}>Exp: {traininfo.etd}</Text>
          )}

         </View>

         {Object.keys(traininfo.destination).length>1 ? (

           traininfo.destination.map((dest,index) => (

          <View key={'cont'+index.toString()}>
             <View key={'row1'+index.toString()} style={{flex: 1, flexDirection: 'row'}}>
             {index===0 ? (
               <Text key={traininfo.std+index.toString()} style={styles.std}>{traininfo.std}</Text>
               ) : (<Text key={traininfo.std+index.toString()} style={styles.std1}>{traininfo.std}</Text>)}

             <Text style={styles.destination}>{dest.locationName}</Text>
             </View>
             <View key={'row11'+index.toString()} style={{flex: 1, flexDirection: 'row'}}>    
             {dest.via !== null ? (
             <Text key={dest.via+index.toString()} style={styles.via}>{dest.via}</Text>
            ) : (null)}
            </View>
             </View>

            ))

          ) : (

          <View>
           <View key={'row1'+index.toString()} style={{flex: 1, flexDirection: 'row'}}>
           <Text key={traininfo.std+index.toString()} style={styles.std}>{traininfo.std}</Text>
          <Text key={traininfo.destination[0].locationName+index.toString()}  style={styles.destination}>{traininfo.destination[0].locationName}</Text>  
          </View>

           <View key={'row11'+index.toString()} style={{flex: 1, flexDirection: 'row'}}>    
             {traininfo.destination[0].via !== null ? (
             <Text key={traininfo.destination[0].via+index.toString()} style={styles.via}>{traininfo.destination[0].via}</Text>
            ) : (null)}
            </View>

            </View>

          )}

           {traininfo.delayReason || traininfo.cancelReason  ? (

          <View key={'delay'+index.toString()} style={{flex: 1, flexDirection: 'row'}}>

          {traininfo.delayReason !== "undefined" ? (
            <Text key={'delayReason1'+index.toString()} style={{color:'#fff',fontFamily:'Lato-Medium', fontSize:16,marginBottom:0,marginTop:10}}>{traininfo.delayReason}</Text>
            ) : (null)}

           {traininfo.cancelReason !== "undefined" ? (
            <Text key={'delayReason2'+index.toString()} style={{color:'#fff',fontFamily:'Lato-Medium', fontSize:16,marginBottom:0,marginTop:10}}>{traininfo.cancelReason}</Text>
            ) : (null)}

         </View>             

          ) : (
          null
          )}

           <View key={'length'+index.toString()} style={{flex: 1, flexDirection: 'row'}}>    
             {traininfo.length !== null ? (
             <Text key={traininfo.length+index.toString()} style={styles.length}>{traininfo.length} Car Train</Text>
            ) : (null)}
          </View>   

        <View key={'row2'+index.toString()} style={{flex: 1, flexDirection: 'row'}}>
         <Text key={'calling'+index.toString()} style={{color:'#fff',fontFamily:'Lato-Bold', fontSize:18,marginBottom:0,marginTop:10}}>Callling at:</Text>
         </View>

         <View key={'dest'+index.toString()} style={{marginBottom:40,marginTop:10}}>

         <View key={'destview'+index.toString()} style={{backgroundColor:'black'}}>


               {traininfo.dest[0].map((stops,index) => (


                 <View key={stops.crs}>

              {index===Object.keys(traininfo.dest[0]).length-1 ? (
              <Image key={stops.crs+'1'}  source={require('../Images/dest.png')} style={{width:22,height:22,position:'absolute',top:8,zIndex:9999}}/>
            ) : (

               <Image key={stops.crs+'2'} source={require('../Images/dest1.png')} style={{marginLeft:7,width:30,height:52,position:'absolute',top:15,zIndex:9999}}/>
            ) }

              {stops.et==='On time' ? (

             <View key={stops.crs+'3'}>
             <Text key={stops.crs+'4'} style={{color:'#fff',fontSize:20,fontFamily:'Lato-Medium',marginTop:4,paddingLeft:37}}>{stops.loc}</Text>
             <Text key={stops.crs+'5'} style={{fontSize:14,color:'#fff001',marginLeft:37}}>{stops.st}</Text>
             </View>

            ) : (

            <View key={stops.crs+'6'}>
            <Text key={stops.crs+'7'} style={{color:'#fff',fontSize:20,fontFamily:'Lato-Medium',marginTop:4,paddingLeft:37}}>{stops.loc}</Text>
             {stops.et==='Cancelled' || stops.et==='Delayed'  ? (
               <Text key={stops.crs+'8'} style={{fontSize:14,color:'#fff001',marginLeft:37}}>{stops.st}<Text style={{fontSize:14,color:'#33b2f4',marginLeft:37}}>  {stops.et}</Text></Text>
              ) : (
               <Text key={stops.crs+'9'} style={{fontSize:14,color:'#fff001',marginLeft:37}}>{stops.st}<Text style={{fontSize:14,color:'#33b2f4',marginLeft:37}}>  Exp: {stops.et}</Text></Text>
              ) }

             </View>
            )}

              </View>

              ))}

               {traininfo.dest.length===2 ? (

                    traininfo.dest[1].map((stops,index) => (


                                 <View key={stops.crs}>

              {index===Object.keys(traininfo.dest[1]).length-1 ? (
              <Image key={stops.crs+'1'}  source={require('../Images/dest.png')} style={{width:22,height:22,position:'absolute',top:8}}/>
            ) : (

               <Image key={stops.crs+'2'} source={require('../Images/dest1.png')} style={{marginLeft:7,width:30,height:52,position:'absolute',top:15}}/>
            ) }



              {stops.et==='On time' ? (

             <View key={stops.crs+'3'}>
             <Text key={stops.crs+'4'} style={{color:'#fff',fontSize:20,fontFamily:'Lato-Medium',marginTop:4,paddingLeft:37}}>{stops.loc}</Text>
             <Text key={stops.crs+'5'} style={{fontSize:14,color:'#fff001',marginLeft:37}}>{stops.st}</Text>
             </View>

            ) : (

            <View key={stops.crs+'6'}>
            <Text key={stops.crs+'7'} style={{color:'#fff',fontSize:20,fontFamily:'Lato-Medium',marginTop:4,paddingLeft:37}}>{stops.loc}</Text>
             {stops.et==='Cancelled' || stops.et==='Delayed'  ? (
               <Text key={stops.crs+'8'} style={{fontSize:14,color:'#fff001',marginLeft:37}}>{stops.st}<Text style={{fontSize:14,color:'#33b2f4',marginLeft:37}}>  {stops.et}</Text></Text>
              ) : (
               <Text key={stops.crs+'9'} style={{fontSize:14,color:'#fff001',marginLeft:37}}>{stops.st}<Text style={{fontSize:14,color:'#33b2f4',marginLeft:37}}>  Exp: {stops.et}</Text></Text>
              ) }

             </View>
            )}

              </View>

                ))

                ) : (null)}

         </View>
         <Text style={{color:'#fff',fontSize:22,marginTop:20, fontFamily:'Lato-Bold'}}>{traininfo.operator}</Text>
         </View>

      </ScrollView>

        ))

Those numbers suggest that it is the processing of each element that takes significant time. The cost of the map call is inconsequential. — Aluan Haddad
– Aluan Haddad, Commented Apr 11, 2018 at 9:16
That depends. Are you going to show what you've done or are we just supposed to play "guess the problem with your code"? — Patrick Roberts
– Patrick Roberts, Commented Apr 11, 2018 at 9:17
Interesting question but we definitely need to see your code. — sjahan
– sjahan, Commented Apr 11, 2018 at 9:20
One simple solution would be to not get that amount of data at once... But then again, I can't give you a proper answer if I don't see some code. — Ray
– Ray, Commented Apr 11, 2018 at 9:22

HungrySoul · Accepted Answer · 2018-04-11 09:50:10Z

1

I recommend you to use Flatlist. This can be more flexible and fully cross platform. It also has pagination which can be used to increase speed and also performance. Please go through this link

answered Apr 11, 2018 at 9:50

HungrySoul

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3 Comments

Pete Nice Over a year ago

Instead off using ScrollView? These results are being rendered in a swiper.

HungrySoul Over a year ago

in place of .map() .

Pete Nice Over a year ago

How do I deal with the conditionals that I have in my .map() ?

Andy Gaskell · Accepted Answer · 2018-04-11 09:33:30Z

0

While you might see a slight benefit to switching to a for loop here are a few other things to try first:

Do this transformation on the server and use caching if possible.
Require the user to interact with the application before calculating more detail; the user doesn't need ~1800 pieces of information on the screen before rendering.
Improve perceived performance by only translating what the user will immediately see before rendering. For example translate the first three records, render them, and translate the rest in the background.

answered Apr 11, 2018 at 9:33

Andy Gaskell

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3 Comments

Pete Nice Over a year ago

Thanks Andy. Can you explain points 1 & 3 in more detail. I can't cache the results because the information is constantly updated but what transformation could I do server side? I would also be interested in how I can translate in the background.

Andy Gaskell Over a year ago

I wrote my answer before seeing that you posted code. I wasn't sure what kind of mapping you were doing; number 1 doesn't really apply. For number 3, the idea would be trains.results.services.slice(0, 3).map() before doing trains.results.services.map(). You'd create a function that returns the results array that you want to render.

Pete Nice Over a year ago

Interesting. Does .map() have a callback?

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