Question

我正在尝试将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);
        })
    });
}

如何将this.addr_long传递给ps.execute（）回调？

Answer 1

您可以使用箭头功能来保留它：

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，你不应该再使用了。

一种流行的方法是将this重命名为that或self。但现在已经过时了。它看起来像是：

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);
        })
    });

当您是node / JS的新手时，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);
        })
    });
}

Javascript函数回调变量范围

2 个答案: