JavaScript函数回调变量的作用域

Question

我想一个对象变量传递给回调

var sql = require('mssql'); 
var asset_update = function (connection, addr) { 
    this.connection = connection; 
    this.addr = addr; 
    this.addr_long = parseInt(addr, 16); 
} 

asset_update.prototype.getFromMac = function() { 
    var ps = new sql.PreparedStatement(this.connection); 
    ps.input('addr', sql.Binary); 
    ps.prepare('SELECT asset_id FROM asset_addr WHERE addr = @addr', function (err) { 
     ps.execute({ addr: this.addr_long }, function (err, recordset) { 
      ps.unprepare(); 
      console.log(recordset.length); 
      console.log(this.addr_long); 
     }) 
    }); 
} 

如何传递this.addr_long到ps.execute（）回调？

Answer 1

您可以使用箭头功能保留此：

ps.prepare('SELECT asset_id FROM asset_addr WHERE addr = @addr', (err) => { 
    ps.execute({ mac: this.addr_long }, function (err, recordset) { //this has been preserved 
     ps.unprepare(); 
     console.log(recordset.length); 
     console.log(this.addr_long); 
    }) 
}); 

Check out this doc for the ES5 alternatives，你不应该再使用。

一个受欢迎的方法是将this重命名为that或self。但现在已经过时了。它看起来像：

var self = this; 
ps.prepare('SELECT asset_id FROM asset_addr WHERE addr = @addr', function (err) { 
     ps.execute({ mac: self.addr_long }, function (err, recordset) { 
      ps.unprepare(); 
      console.log(recordset.length); 
      console.log(that.addr_long); 
     }) 
    }); 

当你是新来的节点/ JS，我建议您阅读以下资源：

What does "this" mean?

Callbacks overview and tricks

Answer 2

asset_update.prototype.getFromMac = function() { 
    var that = this; 
    var ps = new sql.PreparedStatement(this.connection); 
    ps.input('addr', sql.Binary); 
    ps.prepare('SELECT asset_id FROM asset_addr WHERE addr = @addr', function (err) { 
     ps.execute({ mac: that.addr_long }, function (err, recordset) { 
      ps.unprepare(); 
      console.log(recordset.length); 
      console.log(that.addr_long); 
     }) 
    }); 
}