开发者问题收集

TypeError：无法读取未定义反应的属性“长度”

2020-04-14
261
javascript reactjs undefined create-react-app

我正在 React 中做一个简单的练习。练习的目标很简单：

  • 从数组中随机抽取水果
  • 记录消息“我想要一个 RANDOMFRUIT，谢谢。”
  • 记录消息“给你：RANDOMFRUIT”
  • 记录消息“太好吃了！我可以再来一个吗？”
  • 从水果数组中移除水果
  • 记录消息“很抱歉，我们全部用完了。我们只剩下 FRUITSLEFT。

在运行此代码时，我遇到了以下错误。出于某种原因，“长度”似乎是问题所在。 

TypeError: Cannot read property 'length' of undefined
Module../src/index.js
C:/Users/jaina/Desktop/ReactAPPS/exercise1/exercise-one/src/index.js:15
  12 | // Remove the fruit from the array of fruits
  13 | let remaining = remove(foods, fruit);
  14 | // Log the message “I’m sorry, we’re all out. We have FRUITSLEFT left.”
> 15 | console.log(`I’m sorry, we’re all out. We have ${remaining.length} other foods left.`);
  16 |

它还显示 __webpack_require__ 以及其他 webpack 警告。但我假设无法运行的主要原因是“长度”未定义。

index.js 

import foods from './foods';
import { choice, remove } from './helpers';

// Randomly draw a fruit from the array
let fruit = choice(foods);
// Log the message “I’d like one RANDOMFRUIT, please.”
console.log(`I’d like one ${fruit}, please.`);
// Log the message “Here you go: RANDOMFRUIT”
console.log(`Here you go: ${fruit}`);
// Log the message “Delicious! May I have another?”
console.log('Delicious! May I have another?');
// Remove the fruit from the array of fruits
let remaining = remove(foods, fruit);
// Log the message “I’m sorry, we’re all out. We have FRUITSLEFT left.”
console.log(`I’m sorry, we’re all out. We have ${remaining.length} other foods left.`);

Foods.js 

const foods = [
    "🍇", "🍈", "🍉", "🍊", "🍋", "🍌", "🍍", "🍎",
    "🍏", "🍐", "🍒", "🍓", "🥝", "🍅", "🥑",
];

export default foods;

helper.js 

function choice(items){
    let idx = Math.floor(Math.random()* items.length);
}

function remove(items, item){
    for (let i = 0; i < items.length; i++){
        if(items[i] === item){
            return [ ...items.slice(0,i), ...items.slice(i+1)];
        }
    }
}

export {choice, remove};

如能得到任何帮助，我们将不胜感激。

3个回答

在 helper.js 中，如果您的函数 remove 未找到任何内容，它将返回未定义。然后，当您说... 

console.log(`I’m sorry, we’re all out. We have ${remaining.length} other foods left.`);

...您假设 remaining 是一个数组，但它实际上是未定义的。

尝试将 return items; 放在 remove 函数的末尾，在 for 循环之后。

Jack Robinson
2020-04-14

在上面的代码中，如果在 remove 函数中未找到任何内容，则返回 items 。 

function remove(items, item){
    for (let i = 0; i < items.length; i++){
        if(items[i] === item){
            return [ ...items.slice(0,i), ...items.slice(i+1)];
        }
    }
    // change this to whatever you want in case it was able to find item to remove
    return items;
}
Mukund Goel
2020-04-14

修改您的选择并删除功能 

function choice(items) {
    return Math.floor(Math.random()*items.length);
}

function remove(items, item) {
    return items.filter(e => e !== item);
};
Shantanu
2020-04-14