Async/delayed/cached

bad.js

var files = ['foo.js', 'bar.js', 'foo.js']

files.forEach(function (file) {
  // this will end up reading foo.js twice
  fs.readFile(file, function (err, data) {
    // ...
  })
})

good.js

var files = ['foo.js', 'bar.js', 'foo.js']

var readFile = magic(function (file, cb) {
  fs.readFile(file, cb)
})

files.forEach(function (file) {
  // this will start reading foo.js and bar.js right away,
  // but wait reading the 2nd foo.js and just use the cached
  // result of the first foo.js
  readfile(file, function (err, data) {
    // ...
  })
})