無法從函數獲得輸出

Question

我想弄清楚如何從xml文件獲取值並將該值輸出到控制檯。

const path = require('path'); 
const fs = require('fs'); 
const parseString = require('xml2js').parseString; 

const sourceFile = path.join(__dirname, 'books.xml'); 

const document = (sourceFile, callback) => { 
    fs.readFile(sourceFile, { encoding: 'utf-8' }, function (err, data) { 
    if (err) { 
     return err; 
    } 

    parseString(data, (err, result) => { 
     result.catalog.book.forEach(users => { 
     callback(users.email); 
     }); 
    }); 
    }); 
}; 

console.log(document(sourceFile, data => data)); 

XML本身是很簡單的：

<catalog> 
    <book id="bk101"> 
     <author>Gambardella, Matthew</author> 
     <title>XML Developer's Guide</title> 
     <email>test@test.com</email> 
    </book> 
</catalog> 

但是我從功能上得到的是undefined。爲什麼會發生？

更新： 我想送返回值nodemailer

transporter.sendMail({ from: 'mail@mail.com', to: document(sourceFile, (data) => data), subject: 'Message', text: 'I hope this message gets sent!', }, (err, info) => { if (err) { console.log('error!'); console.log(err.message); } else { console.log('sucess!'); console.log(info.messageId); console.log(info.response); } 

，但我得到一個錯誤： 「缺少必需的頭‘要’。」

Answer 1

您不打印數據值，而是打印返回值document函數（它不返回任何內容，因此undefined）。

我已經修改了代碼，所以現在按你們的要求，將工作，

const path = require('path'); 
const fs = require('fs'); 
const parseString = require('xml2js').parseString; 

const sourceFile = path.join(__dirname, 'books.xml'); 

const document = (sourceFile, callback) => { 
    fs.readFile(sourceFile, { encoding: 'utf-8' }, function (err, data) { 
    if (err) { 
     return err; 
    } 

    parseString(data, (err, result) => { 
     result.catalog.book.forEach(users => { 
     callback(users.email); 
     }); 
    }); 
    }); 
}; 

document(sourceFile, data => console.log(data)); 

更新：由於每個操作的評論，

const path = require('path'); 
const fs = require('fs'); 
const parseString = require('xml2js').parseString; 

const sourceFile = path.join(__dirname, 'books.xml'); 

const document = (sourceFile, callback) => { 
    fs.readFile(sourceFile, { encoding: 'utf-8' }, function (err, data) { 
    if (err) { 
     return err; 
    } 

    parseString(data, (err, result) => { 
     result.catalog.book.forEach(users => { 
     callback(users.email); 
     }); 
    }); 
    }); 
}; 

document(sourceFile, data => { 
    transporter.sendMail({ from: 'mail@mail.com', to: data, subject: 'Message', text: 'I hope this message gets sent!', }, (err, info) => { 
    if (err) { 
     console.log('error!'); 
     console.log(err.message); 
    } else { 
     console.log('sucess!'); 
     console.log(info.messageId); 
     console.log(info.response); 
    } 
    }); 
});