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If componentWillMount() function is running first time, items array is empty. Why?

componentWillMount() 
 items.splice(0, items.length);
 RNFS.readDir(RNFS.DocumentDirectoryPath)
 .then((result) => 
 console.warn('result: ', result); // debug

 for(var i = 0; i < result.length; i++) 
 if(result[i].name.endsWith('.txt')) 
 var tmp = result[i].name;
 console.warn(tmp); // debug
 items.push(tmp);
 
 
 )
 .catch((err) => 
 console.log(err.message, err.code);
 );

 console.warn(items); // empty

And using readFile() for get .txt's content, con is empty

renderItem(item) 
 return (
 <View style = styles.renderItemContainer
 key=items.indexOf(item) >
 <TouchableOpacity style=styles.renderItemTouchableOpacity
 onPress = () => 
 fileName = item;

 var path = RNFS.DocumentDirectoryPath + '/' + fileName;

 RNFS.readFile(path, 'utf8')
 .then((contents) => 
 console.warn(contents); 
 con = contents; // not working
 )
 .catch((err) => 
 console.log(err.message, err.code);
 );

 console.warn(con); // empty

 this.props.navigation.navigate('editorScreen');
 >
 <Text style = styles.renderItemText>
 item
 </Text>
 </TouchableOpacity>
 </View>
 );

in render() function, using items array for list .txt files in ScrollView

render() 
 return(
 ....
 <ScrollView>
 
 items.map((items) => this.renderItem(items))
 
 </ScrollView>
 ....
 );

Try :

componentWillMount() 
items.splice(0, items.length);
RNFS.readDir(RNFS.DocumentDirectoryPath)
.then((result) => 
 console.warn('result: ', result); // debug

 for(var i = 0; i < result.length; i++) 
 if(result[i].name.endsWith('.txt')) 
 var tmp = result[i].name;
 console.warn(tmp); // debug
 items.push(tmp);
 

 
 console.warn(items); // empty
)
.catch((err) => 
 console.log(err.message, err.code);
);

Your promise runs async => the console.warn is called before the promise resolves (.then(...))