为什么array.push不起作用？

Question

i have this code

function imagenesIn(errU,errores)
    {
        if(errU) throw errU;
        var directorios=new Array();
        var origenDir='';
        var destinoDir='';
        if(errores=='')
        {
            if(campos.img instanceof Array)
            {
                for(file in campos.img)
                {
                    origenDir='';
                    destinoDir='';
                    origenDir=campos.img[file].path;
                    destinoDir='/uploads/publish/alquiler/'+req.session.passport.user+campos.img[file].name;
                    fs.rename(origenDir,process.cwd()+'/public'+destinoDir,function(err)
                    {
                        if (err) throw err;
                        directorios.push(destinoDir);
                        console.dir(directorios)
                    })
                }
            }
        }else{
            res.send(errores)
        }
        return directorios;
    },

i want to get in directorios an array of the destiny of all files content in req.files.img that are in campos.img

but when i print in console this happend

"img": [
  "/uploads/publish/alquiler/andres@hotmail.comTulips.jpg",
  "/uploads/publish/alquiler/andres@hotmail.comTulips.jpg"
],

im trying to get this result

"img": [
  "/uploads/publish/alquiler/andres@hotmail.comTulips.jpg", //first img
  "/uploads/publish/alquiler/andres@hotmail.flowers.jpg"//second img
],

why .push() method put only the first image directory and not the second??? i miss something???

tnx

Answer 1

Your problem is that in

               fs.rename(origenDir,process.cwd()+'/public'+destinoDir,function(err)
                {
                    if (err) throw err;
                    directorios.push(destinoDir);
                    console.dir(directorios)
                })

your push() won't have actually run by the time you do

    return directorios;

You need to make sure that the call to fs.rename(...) that finishes last (which is not , I repeat not , necessarily going to be the same call that starts last) handles the case where all the calls have finished. Using asynchronous calls, you cannot just fall through after firing up a bunch of them and do a return ; you will have to put the code that you want to run after all the work is done in a callback that addresses what I called "handles" earlier.

Control-flow libraries like async.js could simplify your code, but you'll need to get your head around the notion that once your function goes async everything that follows it has to be async as well.

Answer 2

ebohlman pretty much called it. Right now your for loop is setting up the anonymous functions that the rename function will call once it finishes.

As soon as these are setup, imagenesIn will return directories. It may contain some or none of the directories, depending on whether or not rename finished before your return.

The power of node is that it is asynchronous. You could use fs.renameSync yes, and it would follow what you expect. Node is not like an apache php server. The php server gets a request, and reserves a small slice of memory for the request. That's why other requests can still be processed, because they all get their own memory. Node doesn't do this. It runs on a single thread and if you do anything that is blocking (like synchronous IO), other requests have to wait until it's finished before they can be processed.

Ideally, your imagenesIn should also be asynchronous as well, taking a function as the final parameter. The standard for the function usually follows function(error, data). Error should be null if there was none. fs.rename follows this pattern.

Also the function that calls imagenesIn should ideally handle the server response. This allows the function to be used in other types of cases. What if you don't want to send that specific response on error? What if you don't want to send a response at all? Right now this is a good recipe for accidentally sending headers twice (and getting an error).

If it were me, this is how I would write your function (I didn't test but should give you some direction).

function imagenesIn(callback) {

    var directorios=new Array();
    var origenDir='';
    var destinoDir='';
    if(campos.img instanceof Array) {

        function recursion(index){

            //the case that ends the recursion and returns directories
            if (index >= campos.img.length) {
                callback(null, directorios);
                return;
            }

            origenDir=campos.img[index].path;
            destinoDir='/uploads/publish/alquiler/'+req.session.passport.user+campos.img[index].name;

            fs.rename(origenDir, process.cwd() + '/public' + destinoDir, function(err) {

                //the case that ends recursion and sends an error
                if (err) {
                    callback(err);
                    return; 
                }
                directorios.push(destinoDir);
                console.dir(directorios);
                recursion(index++);
            })  
        }

        recursion(0);
    }
    else {
        callback('Campos.img was not an array.');
    }
}

and your code that calls it might look something like this

imagenesIn(function(err, directories) {
    if (err) {
        res.send(err);
    }
    else {
        //do cool stuff with directories.
    }
});

Also, I wanted to make sure you understood the unique difference between for( ; ; ) and for(key in object). "For in" iterates through the keys of an object. This works on an array because it is essentially an object with numeric keys. I could do this however

var array = ['data', 'data'];
array.nonNumericKey = 'otherdata';

If you did for (var i = 0; i < array.length; i++), you would only iterate through the array data. If you used for (key in array), you would also iterate through the nonNumericKey. This is why personally, I only use "for in" on objects that are not arrays.