我们如何在 JavaScript 中等待结果？

Question

我一直在到处寻找有关 Promise 或 async/await 的信息，但它们似乎不起作用。我需要从查询函数中获取当前日期，然后才能在后续函数中使用该日期。这是迄今为止我最好的猜测：

    // Retrieve current session date
    var currentDate;
    async function getMaxDate() {

        fetch(url).then((data) => {
            return data.json();
        }).then((json) => {

            // This is the date used in queries
            currentDate = json[obj].max.substr(0, 10);
            console.log(currentDate);

            return new Promise(resolve => {
                setTimeout(() => {
                    resolve(currentDate);
                }, 5000);
            });

        }).catch((e) => {
            console.error('There was an error:');
            console.log(e);
        });

    }


  async function getAllData() {
        try {
            currentDate = await getMaxDate();
            await getMarket(currentDate);
            console.log(currentDate);
        } catch (error) {
            console.log('An error occurred.');
        }
    }

    // This function is called when the document is first loaded
    $(function () {
        let promise = getAllData();
    });

但是无论有没有 promise/async/await，行为都是相同的 - currentDate 直到几秒钟后仍未定义。我也试过这样调用：

    getAllData()
        .then(function (currentDate) {
            console.log(currentDate);
        });

这里的 currentDate 直到稍后才定义。如需进一步说明 - getMaxDate 会进行一次提取，它可能会慢到足以导致此问题。因为稍后在该函数中，console.log(currentDate) 确实输出了正确的值。 （currentDate 被定义为一个全局变量。）但这就像我的代码正在以随机顺序执行......

Answer 1

您的函数不返回任何内容。如果你希望它返回currentDate，你必须明确地这样做。

async function getAllData() {
    try {
        let currentDate = await getMaxDate();
        await getMarket(currentDate);
        console.log(currentDate);

        return currentDate;
    } catch (error) {
        console.log('An error occurred.');
    }
}

返回值将是在 then 或 await 情况下分配的值：

let currentDate = await getAllData();

getAllData.then(currentDate => {
  // ...
});

Answer 2

看看下面： 工作示例jsFiddle

let currentDate;
const url = 'https://jsonplaceholder.typicode.com/posts/1';

function getMaxDate() {
  return new Promise(resolve => {
    fetch(url).then(data => {
      return data.json();
    }).then(json => {
      console.log('data from api', json);
      currentDate = new Date(); // here I simulate to get Date
      console.log(currentDate);
      resolve(currentDate);
    }).catch(error => {
      console.error('There was an error:');
      console.log(e);
      reject(error);
    });
  })

}


function getMarket(date) {
  return new Promise(resolve => {
    resolve({
      market: 'US',
      date
    });
  });
}

async function getAllData() {
  try {
    let currentDate = await getMaxDate();
    console.log('getAllData', currentDate);
    return await getMarket(currentDate);
  } catch (error) {
    console.log('An error occurred.');
  }
}

// This function is called when the document is first loaded
$(function () {
  getAllData().
  then(result => {
    console.log('do sth...', result);
  });
});