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如何在节点js中访问查询回调之外的变量

node.js

Asad ullah · 技术社区 · 6 年前

var emailsToReturn = [];

    emails.forEach(( email, index ) => {
        var msgId = email.message_id;

        //fetch attachments against this attachment

        var fetchAttachments = `SELECT * FROM email_attachments WHERE message_id='${msgId}'`;
        connection.query(fetchAttachments, (err, attachmentsResult)=>{

            email['attachments'] = attachmentsResult;                
            emailsToReturn.push(email); // this show desired results here.
        });


    });
    console.log(JSON.stringify(emailsToReturn, null, 4)); // this shows empty array here outside of query callback.

我知道这是范围问题,我试图使用 global 我也是。但那也没用。任何帮助将不胜感激。谢谢!

2 回复 | 直到 6 年前

Rajan Lagah 6 年前

这里的基本情况是emailsToReturn是全局变量,可以在任何地方访问。您正在更新emailsToReturn的值,然后尝试打印它。

如何解决?
您可以使用async/await( https://www.geeksforgeeks.org/using-async-await-in-node-js/

我有用处等等,现在Javascript将等待查询结果,然后更新数组,然后转到下一条指令。

var emailsToReturn = [];
async function functionName(emails,index){
 emails.forEach(( email, index ) => {
    var msgId = email.message_id;

    //fetch attachments against this attachment

    var fetchAttachments = `SELECT * FROM email_attachments WHERE message_id='${msgId}'`;
    let attachmentsResult = await connection.query(fetchAttachments)
    if(attachmentsResult.err){
      console.log(err);
   }else{
        email['attachments'] = attachmentsResult;                
        emailsToReturn.push(email); // this show desired results here.
    }
});
console.log(JSON.stringify(emailsToReturn, null, 4));
//req.send(JSON.stringify(emailsToReturn, null, 4));//use to send response
}
 functionName(emails,index)

Ankit 6 年前

const async = require("async");
var emailsToReturn = [];
async.waterfall([function(cb){
 emails.forEach(( email, index ) => {
        var msgId = email.message_id;

        //fetch attachments against this attachment

        var fetchAttachments = `SELECT * FROM email_attachments WHERE message_id='${msgId}'`;
        connection.query(fetchAttachments, (err, attachmentsResult)=>{

            email['attachments'] = attachmentsResult;                
            emailsToReturn.push(email); // this show desired results here.
            if(index == emails.length - 1){
             cb();  
            }
        });
     });
}],function(){
  console.log(JSON.stringify(emailsToReturn, null, 4));
})