Question

I have an array which keeps URL of several files. For example:

var files = ['1.html', '2.html', '3.html'];

I need to read them asynchronously and save them in an object named cache (cache = {}).
To do this I used the code:

for(var i = 0; i < files.length; i++){
    require('fs').readFile(files[i], 'utf8', function (error,data) {
        cache[files[i]]=data;
    });
}

In the end I have the result:

cache = { undefined : 'File 3 content' }

I do understand that the “readFile” acts after the loop is ended and it looses it’s scope. Is there a way to fix this or another method to read files from an array and cache them?

Answer 1

When your callback to readFile executes, the for loop will already have finished. So i will be files.length and files[i] will be undefined. To mitigate this, you need to wrap the variables in a closure. The simplest way to do this is to create a function which does your readFile call, and call that in the loop:

function read(file) {
    require('fs').readFile(file, 'utf8', function (error,data) {
        cache[file]=data;
    });
}

for(var i = 0; i < files.length; i++){
    read(files[i]);
}

For even better execution control, you might want to look into async:

function readAsync(file, callback) {
    fs.readFile(file, 'utf8', callback);
}

async.map(files, readAsync, function(err, results) {
    // results = ['file 1 content', 'file 2 content', ...]
});

Edit: Made use of helper function for async example.