node.js callback after all callbacks excuted

callbacks.js

#!/usr/bin/env node

var FS = require('fs');

var pending = callback => {
  var count = 0;
  var returns = {};
  console.log("Start cound: %d", count);
  return key => { // a function done
    count++;
    console.log("return1: %d", count);
    return (error, data) => { // called by readFile
      count--;
      returns[key] = data; // or add content length: data.length
      console.log("return2: [%s] = %d", key, count);
      if (count === 0) {
        console.log("Callback Excute.");
        callback(returns);
      }
    };
  };
};

var done = pending( fileData => {
  console.log("done");
  console.log(fileData);
});
// current directory files for test
var fileName = ['get.js', 'pha.js', 'promise.js'];

for (var i = 0; i < fileName.length; i++) {
  FS.readFile(fileName[i], 'UTF-8', done(fileName[i]));
}
// from http://www.cnblogs.com/onlyfu/p/5062068.html