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Question

我正在尝试使用async / await从postresql数据库获取数据。我面临的问题是fetchMembers会加载数据，但不会返回该数据，而只会返回一个空白对象。

//server.js
const members = require('./memberDB')

app.get('/' , (req, res) => {
    res.json(members)
    console.log('Members: ',JSON.stringify(members))
})

//MemberDB.js

const fetchMembers =  async function() {

    const { Client, Pool } = require('pg')

    const pool = new Pool({
      user: 'user',
      host: 'localhost',
      database: 'whatevs',
      password: '',
      port: 5432,
    })

    const loadData = await pool.query('SELECT * FROM members;')
    const data = await loadData.rows
    console.log('data -> ', data)//I get this data correctly

    return data // but I don't get the return value even if I remove 'await' from the 'loadData.rows'
}

module.exports.members = fetchMembers()

Answer 1

make fetchMembers返回一个promise，并在请求处理程序中等待它。

 const fetchMembers =  async function() {

    const { Client, Pool } = require('pg')

    const pool = new Pool({
      user: 'user',
      host: 'localhost',
      database: 'whatevs',
      password: '',
      port: 5432,
    })

    return pool.query('SELECT * FROM members;')        
}
module.exports.members = fetchMembers;

在路线中：

    app.get('/' , async (req, res) => {
       const loadData = await fetchMembers();
       const data = loadData.rows;
       res.json(data);           
   })

问题在于您调用fetchMembers并等待其中的诺言，但这不会阻塞代码。继续执行req处理程序，并且尚未解析成员。因此，您有两种解决方案要么在fetchMembers和请求处理程序中等待，要么返回承诺并仅在请求处理程序中等待。就像保罗所说的那样，最好是导出函数本身。

Answer 2

# file = "C:/Users/PycharmProjects/MindNotez/" file_path = "C:/Users/PycharmProjects/MindNotez/" df_content = {"title": [], "author": [], "year": [], "others": [], "note": []} # for file in os.listdir(file): for file in os.listdir(file_path): if file.endswith("Note.csv"): # file_content = file + "Note.csv" file_content = file_path + "Note.csv" return file_content else: df = pd.DataFrame(data=df_content) # df.to_csv(file + "Note.csv", sep=",", index=False) df.to_csv(file_path + "Note.csv", sep=",", index=False) file_content = file + "Note.csv" return file_content导出调用counting: for i = 1:5 println(i) end counting的结果，该结果是MemberDB.js函数，因此将始终返回fetchMembers()。

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async

Answer 3

您在需要文件时调用该函数，但不等待它。这有两个效果。首先，您的members变量将没有数据，其次，由于您需要调用它，因此由于缓存的原因，您在第一次需要该文件时只会获得数据库中的任何成员。正确的方法是像这样导出函数：

    module.exports = fetchMembers; // note no parentheses

然后正确呼叫您的路线：

    const fetchMembers = require(“./dbMembers”);

     app.get(‘/‘, async (req, res) => {
            const members = await fetchMembers();
            res.send(members); // obviously add erro catching and stuff 
    });

Answer 4

将函数包装在try catch语句中，这样就不会出现静默错误，例如：

const fetchMembers =  async () => {
    try
    {
       const { Pool } = require('pg')

       const pool = new Pool({
         user: 'user',
         host: 'localhost',
         database: 'whatevs',
         password: '',
         port: 5432,
       })

       return await pool.query('SELECT * FROM members;')
    }
    catch(reason){
        console.log(reason)
    }
}

console.log(fetchMembers())

异步函数不需要返回值

4 个答案:

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