Javascript function callback variable scope

Question

I am trying to pass an object variable to a callback

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);
        })
    });
}

How can I pass this.addr_long to the ps.execute() callback?

score 0 · Accepted Answer · edited May 23 '17 at 12:24

You can use an arrow function to preserve this :

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, which you shouldn't use anymore.

A popular one would be to rename this to that or self. But this is now obsolete. It would look like :

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);
        })
    });

As you're new to node/JS, I suggest you read the following resources :

What does "this" mean?

Callbacks overview and tricks

Thank you... I am going to use lambda as it is more familiar to me, too — Magician, Dec 01 '16 at 07:57

user1554295 · Answer 2 · 2016-12-01T08:30:49.800

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 function callback variable scope

2 Answers2