callback is not a function node js

Question

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)".

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 < rows.length; i++){
                role = rows[i].role;
                rolesArray.push(rol);
            }
            console.log("roles: " + rolesArray);
            return callback(rolesArray);
        });
    });   
} 

The console.log outputs:"roles: admin,customer" so the connection with the database works.

Answer 1

That error means that you are not passing a function to Rol.getAllRoles(fn) when you call it.

In addition, so that you can have proper error handling in your callback and so you can more easily distinguish between an error and the actual data, you should always pass a first argument to the callback that indicates whether there was an error or not and then the second argument (if not an error) can be your results array like this:

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 < rows.length; i++){
                role = rows[i].role;
                rolesArray.push(rol);
            }
            console.log("roles: " + rolesArray);
            // make sure the first argument to the callback
            // is an error value, null if no error
            return callback(null, rolesArray);
        });
    });   
} 

And, then you should be calling it like this:

Rol.getAllRoles(function(err, rolesArray) {
    if (err) {
        // handle error here
    } else {
        // process rolesArray here
    }
});

This style of calling an async callback as in callback(err, data) is a very common async callback design pattern. It allows all callers to see if there was an error or not and if there was no error to get access to the final result.