callback is not a function node js
<p>I am new to javascript and i am having trouble solving this error. I get the message: "callback is not a function" at:"return callback(rolesArray)". 
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<pre class="snippet-code-js lang-js prettyprint-override"><code>Rol.getAllRoles = function(callback){
    sql = "select role from Role;";
    var rolesArray = [];
    var role;
    mysql.connection(function(err,conn){
        if (err){
            return callback(err);
        }
        conn.query(sql,function(err,rows){
            if (err){
                return callback(err);
            }
            for(var i=0; i &lt; rows.length; i++){
                role = rows[i].role;
                rolesArray.push(rol);
            }
            console.log("roles: " + rolesArray);
            return callback(rolesArray);
        });
    });   
} </code></pre>
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<p>The console.log outputs:"roles: admin,customer" so the connection with the database works. </p>