Fetch 不存储数组 for 循环后的值 - Javascript

Question

我想为从维基媒体 API 获取的位置数组中的每个对象添加描述属性但是当我在循环中记录它的值时，它就在那里，但在循环之外, 它被删除了。

我用 async/await 函数或 Promise.all() 寻找解决方案，但没有成功。

有没有办法正确存储该值以便以后访问它？？

let locations = [
    {
        latLng: [33.975561111111,28.555830555556],
        name: 'Saint Catherine\'s Monastery',
        searchTerm: 'Saint Catherine\'s Monastery',
        urlSerchTerm: 'Saint%20Catherine\'s%20Monastery'
    },
    {
        latLng: [29.91667,31.2],
        name: 'Bibliotheca Alexandrina',
        searchTerm: 'Bibliotheca Alexandrina',
        urlSerchTerm: 'Bibliotheca%20Alexandrina'
    }
];

async function fetchAsync (site, location) {
    // await response of fetch call
    let response = await fetch(site);
    // only proceed once promise is resolved
    let data = await response.json();
    // only proceed once second promise is resolved
    location.description = data[2][0];

    return location.description;
  }

// let fetches = [];
for (let i = 0; i < locations.length; i++) {
    let site = `https://en.wikipedia.org/w/api.php?action=opensearch&search=${locations[i].urlSerchTerm}&limit=1&namespace=0&format=json&origin=*`;

    fetchAsync(site, locations[i])

}
console.log(locations[1].description)

Answer 1

这是我的 Promise.all 解决方案：

我正在通过 .mapping locations 数组来创建 Promise 数组。

let locations = [
 {
  latLng: [33.975561111111, 28.555830555556],
  name: "Saint Catherine's Monastery",
  searchTerm: "Saint Catherine's Monastery",
  urlSerchTerm: "Saint%20Catherine's%20Monastery"
 },
 {
  latLng: [29.91667, 31.2],
  name: "Bibliotheca Alexandrina",
  searchTerm: "Bibliotheca Alexandrina",
  urlSerchTerm: "Bibliotheca%20Alexandrina"
 }
];

Promise.all(
    locations.map( location => new Promise(async (resolve, reject) => {
        let site = `https://en.wikipedia.org/w/api.php?action=opensearch&search=${location.urlSerchTerm}&limit=1&namespace=0&format=json&origin=*`,
            response = await fetch(site),
            data = await response.json();
        location.description = data[2][0];
        // console.log("Got description = ", location.description)
        resolve();
    })))
.then(() => {
 console.log("locations[1].description = ", locations[1].description);
});

Answer 2

这只是一个时间问题。您的 fetch 调用正在 异步执行 而代码片段最后一行中的 console.log(...) 语句正在 同步执行 .换句话说，对 fetch 发出的请求的响应将在 console.log(...) 之后 return，并且 description 属性仍未定义。

您可以通过查看下面的代码让自己相信这一点，其中 console.log(...) 语句包含在超时中。现在将记录获取的描述而不是 undefined.

let locations = [
    {
        latLng: [33.975561111111,28.555830555556],
        name: 'Saint Catherine\'s Monastery',
        searchTerm: 'Saint Catherine\'s Monastery',
        urlSerchTerm: 'Saint%20Catherine\'s%20Monastery'
    },
    {
        latLng: [29.91667,31.2],
        name: 'Bibliotheca Alexandrina',
        searchTerm: 'Bibliotheca Alexandrina',
        urlSerchTerm: 'Bibliotheca%20Alexandrina'
    }
];

async function fetchAsync (site, location) {
    // await response of fetch call
    let response = await fetch(site);
    // only proceed once promise is resolved
    let data = await response.json();
    // only proceed once second promise is resolved
    location.description = data[2][0];

    return location.description;
  }

// let fetches = [];
for (let i = 0; i < locations.length; i++) {
    let site = `https://en.wikipedia.org/w/api.php?action=opensearch&search=${locations[i].urlSerchTerm}&limit=1&namespace=0&format=json&origin=*`;

    fetchAsync(site, locations[i])

}

window.setTimeout(() => {console.log(locations);}, 5000);

您可以按照@JeremyThille 的建议使用Promise.all 解决此问题。解释了 Promise.all 的第二次使用，以防混淆。

let locations = [
        {
            latLng: [33.975561111111,28.555830555556],
            name: 'Saint Catherine\'s Monastery',
            searchTerm: 'Saint Catherine\'s Monastery',
            urlSerchTerm: 'Saint%20Catherine\'s%20Monastery'
        },
        {
            latLng: [29.91667,31.2],
            name: 'Bibliotheca Alexandrina',
            searchTerm: 'Bibliotheca Alexandrina',
            urlSerchTerm: 'Bibliotheca%20Alexandrina'
        }
    ];


    const fetchDescription = (location) => fetch(`https://en.wikipedia.org/w/api.php?action=opensearch&search=${location.urlSerchTerm}&limit=1&namespace=0&format=json&origin=*`);

    const descriptionRequests = locations.map(fetchDescription);
    Promise.all(descriptionRequests)
    .then(responses => Promise.all(responses.map(r => r.json())))
    .then(descriptions => {
      descriptions.forEach((description, index) => { locations[index].description = description[2][0]; });
    })
    .then(() => {
      console.log(locations);
    });

Fetch 不存储数组 for 循环后的值 - Javascript

Fetch doesn't store the values after the array for loop - Javascript

javascript

arrays

promise

async-await

fetch-api