Question

I’m trying to use async and request module together but i don’t understand how the callbacks get passed. My code is

var fetch = function(file, cb) {
    return request(file, cb);
};

async.map(['file1', 'file2', 'file3'], fetch, function(err, resp, body) {
    // is this function passed as an argument to _fetch_ 
    // or is it excecuted as a callback at the end of all the request?
    // if so how do i pass a callback to the _fetch_ function
    if(!err) console.log(body);
});

I’m trying to fetch 3 files in order and concatenate the results. My head is stuck in callbacks I tryed and the different combinations I could think of. Google wasn’t much help.

Answer 1

Request is asynchronous function, it does not return something, when its job is done, it calls back. From request examples, you should do something like:

var fetch = function(file,cb){
     request.get(file, function(err,response,body){
           if ( err){
                 cb(err);
           } else {
                 cb(null, body); // First param indicates error, null=> no error
           }
     });
}
async.map(["file1", "file2", "file3"], fetch, function(err, results){
    if ( err){
       // either file1, file2 or file3 has raised an error, so you should not use results and handle the error
    } else {
       // results[0] -> "file1" body
       // results[1] -> "file2" body
       // results[2] -> "file3" body
    }
});