Question

What is the correct way to add context to a callback used in a Model query? For example…

function doSomething(param) {
    var magic = function(context, err, results) {
        if(err) { console.log('fail'); }
        else {
            // do the magic with context and results
        }
    }

    for( var i=0; i < 5; i++ ) {
        var myObject = {'secret' : i};
        MyModel.find({number:param[i]}, magic(myObject, err, results));
    }
}

I’d like to loop through results for each query and have the context of myObject. The above solution does not work. ‘err’ and ‘results’ are not defined.

I normally do this with an anonymous function, but I can’t rely on the context inside the for loop.

Answer 1

err and results are undefined because you arer passing a variables called err and results to magic which you never defined.

MyModel.find can’t do anything with the result from magic because you don’t return anything from it, you should be returning a function that takes err and result.

function doSomething(param) {
    var magic = function(context) {
        // return a function here
        return function(err, results) {
            if(err) { console.log('fail'); }
            else {
                // do the magic with context and results
            }
        };
    }

    for( var i=0; i < 5; i++ ) {
        var myObject = {'secret' : i};
        // do not pass err or results to magic
        // they are not defined anywhere in this scope
        MyModel.find({number:param[i]}, magic(myObject));
    }
}