Javascript 将变量传递到回调函数

我有一段代码如下：

var guid = 'unique_guid';
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, function(err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);
        // do more things which require guid
    }
}

var checkExistence = function(guid, err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    }
}
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence.bind(null, guid));

为了避免回调地狱，我给回调函数起了一个名字，并按照如下方式重构它：

var checkExistence = function(err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    }
}
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence);

con是从中创建的连接

现在我的问题是我不能在checkExistence中引用guid，我不想将guid作为全局变量

---

是否可以在checkExistence中获取guid？

您可以使用Function.bind函数，如下所示：

var guid = 'unique_guid';
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, function(err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);
        // do more things which require guid
    }
}

var checkExistence = function(guid, err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    }
}
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence.bind(null, guid));

---

您可以将Function.bind函数用于thet，如下所示：

var guid = 'unique_guid';
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, function(err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);
        // do more things which require guid
    }
}

var checkExistence = function(guid, err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    }
}
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence.bind(null, guid));

---

您可以添加guid作为参数并返回函数：

var checkExistence = function(guid) {
    return function(err, rows) {
        if(err) throw err;
        if(rows.length == 0) {
            console.log('new guid: ' + guid);       // guid can't be referenced here
            // do more things which require guid
        } else {
            console.log('old guid: ' + guid);       // guid can't be referenced here
            // do more things which require guid
        }
    };
};
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence(guid));

---

您可以添加guid作为参数并返回函数：

var checkExistence = function(guid) {
    return function(err, rows) {
        if(err) throw err;
        if(rows.length == 0) {
            console.log('new guid: ' + guid);       // guid can't be referenced here
            // do more things which require guid
        } else {
            console.log('old guid: ' + guid);       // guid can't be referenced here
            // do more things which require guid
        }
    };
};
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence(guid));

---

也许你可以使用bind函数

var checkExistence = function(guid, err, rows) { ...

然后像这样调用方法查询

con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence.bind(null, guid);

---

也许你可以使用bind函数

var checkExistence = function(guid, err, rows) { ...

然后像这样调用方法查询

con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence.bind(null, guid);

---

回调函数将是{checkExistenceguid；}并接受guid作为参数..为什么不只添加一个参数？回调函数将是{checkExistenceguid；}并接受guid作为参数..为什么不只添加一个参数？谢谢：-。我是javascript新手。谢谢：-。我是javascript新手。