将variables传入callback函数

我有这样一段代码：

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 } } 

为了避免callback地狱，我给callback函数一个名字和重构它如下：

 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node-mysql创build的连接

现在我的问题是，我不能在checkExistence()引用guid，我不想让guid成为一个全局variables。

是否有可能在checkExistence()得到guid ？

Answer 1

你可以添加guid作为参数并返回一个函数：

 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)); 

Answer 2

你可以使用Function.bind函数，如下所示：

 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)); 

Answer 3

也许你可以使用绑定function，

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

并像这样调用方法查询

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

Answer 4

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