Node async waterfall callback was already called - node.js

Question

I'm trying to read some files from dir with async.waterfall, it seems to me that I'm doing stuff right, but I get the specified error and the readData function is never called. What's wrong?
var fs = require("fs");
var async = require("async");
var folder = "./files/";
try {
async.waterfall([
function readDir(cb) {
fs.readdir(folder, function(err, files) {
cb(err, files);
});
},
function loopFiles(files, cb) {
files.forEach(function(fn) {
console.log("loop " + fn);
cb(null, fn);
});
},
function check(fn, cb) {
console.log("check "+fn);
fs.stat(folder + fn, function(err, stats) {
console.log(stats.isFile());
cb(err, stats, fn);
});
},
function readData(stats, fn, cb) {
console.log("read "+fn);
if (stats.isFile()) {
fs.readFile(folder + fn, "utf-8", function(err, data) {
cb(err, data);
});
}
}
], function(err, result) {
if (err) {
throw err;
}
console.log(result);
});
} catch (err) {
console.log(err);
}

Answer 1

The problem is that you're calling cb(null, fn) in loopFiles() multiple times if files.length > 1. You will probably need to perform a separate async.waterfall() or use some other async.* method for each file.
One other problem is in readData() where you aren't calling cb() in the case that stats.isFile() evaluates to false.