无法从函数获得输出

Question

我想弄清楚如何从xml文件获取值并将该值输出到控制台。

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本身是很简单的：

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

但是我从功能上得到的是undefined。为什么会发生？

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); } 

，但我得到一个错误： “缺少必需的头‘要’。”

Answer 1

您不打印数据值，而是打印返回值document函数（它不返回任何内容，因此undefined）。

我已经修改了代码，所以现在按你们的要求，将工作，

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：由于每个操作的评论，

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); 
    } 
    }); 
});