未定义不是this.state对象上的对象

Question

I am setting up a React Native App. In this App i am trying to include a reusable Top Navigation Component, which accepts an array of strings and then renders them appropriately. When i am trying to compare the array string to a given string, it throws the following error:

Unhandeld JS Exception: TypeError: undefined is not an object (evaluating 'this.state.choseTopNavigation'

Inserting a string to compare works just fine

Setting up state object in constructor

import React, {Component} from 'react';
import {Platform, StyleSheet, Text, View, ScrollView, FlatList} from 'react-native';
import {Header, TopNavigationButton}  from './components';

type Props = {};
export default class App extends Component<Props> {


constructor(props) {
  super(props);
  this.state = { 
    topNavigationOptions : [
      {key: 'Quick Search'},
      {key: 'Person Search'}],
      chosenTopNavigation: 
      "Quick Search",
  };

}

}



renderItem({item}){
  if (item.key == this.state.chosenTopNavigation) {
    return (
        <TopNavigationButton>
          {item.key}
        </TopNavigationButton>
      )
  } else {
    return (
      <Text style = {styles.unchosenTextStyle}>
        {item.key}
      </Text>
    )
  }
}


  render() {
    return (
      <View>

        <Header>Header</Header>
        <ScrollView horizontal = {true}>
          <View style = {styles.navigationContainer}>
            <TopNavigationButton>Quick Search</TopNavigationButton>
            <Text style = {styles.unchosenTextStyle}>Person Search</Text>
          </View>
        </ScrollView>

      <FlatList 
        horizontal
        data={this.state.topNavigationOptions}
        renderItem = {this.renderItem}
      />

      </View>  
    );
  }
}

Note This Works

renderItem({item} ){
  if (item.key == "Quick Search") { //inserting a string directly works
    return (
        <TopNavigationButton>
          {item.key}
        </TopNavigationButton>
      )
  } else {
    return (
      <Text style = {styles.unchosenTextStyle}>
        {item.key}
      </Text>
    )
  }
}

Thanks a lot for your help!

Answer 1

Either you can make your function as arrow function like this:

renderItem = ({item}) => {
// your existing code
}

or you need to bind your function in constructor like this:

this.renderItem = this.renderItem.bind(this);