Proper usage of this inside callback function in node.js

Question

Proper usage of this inside callback function in node.js

I'm getting an error accessing the variables in this class when running a 'constructor' script. Any thoughts?

/**
 * Info
 */
exports.sqlWhiteList = function(){

    var sqlImport = require('../DBConnectors/MYSQLConn.js');
    var sql = new sqlImport.sqlConn();

    //fill up static variables
    this.tables = {};
    this.attributes = {};

    //populate the table names
    sql.performQuery(('xxxxxx xxxxx = \'BASE TABLE\' ' +
        'AND TABLE_SCHEMA=\'' + sql.credentials.database + '\''),function(rows){

        for (var index in rows){
            this.tables[rows[index]['TABLE_NAME']] = true; // this fails
        }
    });
};

Error = "TypeError: Cannot read property 'tables' of undefined"

javascript
node.js
callback

Answer 1

You just need to stash this in the outer function so that the callback can get to the right value:

var moduleObj = this;

// ...

//populate the table names
sql.performQuery(('xxxxxx xxxxx = \'BASE TABLE\' ' +
    'AND TABLE_SCHEMA=\'' + sql.credentials.database + '\''),function(rows){

    for (var index in rows){
        moduleObj.tables[rows[index]['TABLE_NAME']] = true; // this fails
    }
});

Also I would very strongly suggest not using a for ... in loop to iterate over the rows returned from the query. I'm not familiar with the API but I'm willing to bet that it's a real JavaScript array. You should use either a plain for loop with an index variable, or else a .forEach() call. Using for ... in for arrays is sort-of a bad practice for a few reasons, not the least of which is that it causes V8 (and maybe other runtimes) to abandon any attempt at serious optimization of a function's code in some cases. One of those cases is when you're using it on an array, apparently.