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easy, one-off immutable directories!

Pure functions are a powerful concept. They allow you to, given an input, produce the same deterministic output, without side effects.

On the filesystem, this is hard to achieve. Filesystems are all about side effects! Consider creating a directory as a function, and then creating a file within it:

$ mkdir foobar
$ touch foobar/quux

If this was part of a script you used in, say, a build process, you might run into some problems:

  1. your current directory is an implicit input; being in the wrong directory will have unintended side effects.
  2. mkdir foobar is not idempotent: multiple applications of it in the same directory yield different results (generally, an error).
  3. it creates global mutable state. what if another not-quite-pure function decided it wanted to use foobar/quux for a different purpose? Each script can clobber and conflict with the other!
  4. the resultant folder can be modified between functions. If I ran mkdir foo; sleep 10; touch foo/quux and, during those 10 seconds, another process did rm -rf foo, the result would be different than if they hadn't.

From this, we can say that a better solution would have the three inverse properties:

  1. the current directory should be irrelevant (that is, all paths should be absolute).
  2. each application of a function should produce a brand new folder.
  3. each brand new folder should be unique named, to prevent conflicts.
  4. each brand new folder should have write permissions removed, so that its contents are frozen.

Enter ice-box: a module that manages a store of uniquely-named, immutable directories, and makes it easy to create new ones.


Let's say we have a build system that takes a directory and puts its contents into a tarball. What might a script look like to do that, so we could invoke it using node make-tar.js some-directory/?

var icebox = require('ice-box')()
var fs = require('fs')
var path = require('path')
var tar = require('tar-fs')
var src = process.argv[2]
icebox(function (dst, done) {
    .pipe(fs.createWriteStream(path.join(dst, 'result.tar')))
    .on('finish', done)
}, function (err, finalDir) {

Running node make-tar.js some-directory/ will output


which contains the output file, result.tar.


Much like UNIX pipes, this enables the creation of UNIX-like pipes: programs that consume a directory can produce a new immutable directory and output that.

Imagine we had a program that took a directory of JS files and packaged them for Electron before the tarball step:

var icebox = require('ice-box')('./ice-box')
var packager = require('electron-packager')
var src = process.argv[2]
icebox(function (dst, done) {
    dir: src,  // use the input dir, 'src'
    arch: 'x64',
    platform: 'linux',
    out: dst,  // use the output dir, 'dst'
    tmpdir: false,
    prune: true,
    overwrite: true,
  }, done)
}, function (err, finalDir) {

Now we could run this as just

$ node build-electron.js .

to get the electron-ready output, or it can be piped into make-tar.js from the above section to produce the final .tar file!

$ node build-electron.js . | node make-tar.js

Now we have a directory with a tarball of the electron package!

Bonus: all intermediate steps are permanently cacheable, since they're immutable and permanent!


var iceBox = require('ice-box')

var icebox = iceBox([outDir], [tmpDir])

Creates a new function for adding new directories to an icebox. Both parameters are optional, and default to sane values.

  • outDir (string) - The location to place the immutable output directories. Defaults to ./ice-box.
  • tmpDir (string) - The temporary location to create in-progress directories that haven't yet finished being produced. These are cleaned up once they are frozen and placed in outDir.

icebox(work, done)

Creates a new directory for writing to.

work is a function of the form function (dir, done) { ... }. dir is the absolute path to the in-progress temporary directory. It has full write permissions. done is a function to call once you are done writing, to signify that the directory can be "frozen" and placed in the icebox. If you pass in an error (done(err)) then the entire operation will abort cleanly.

done is a function of the form function (dir) { ... }. It is called once the newly-frozen output directory is placed in the ice-box (outDir from the above section). path is a string containing the absolute path to the frozen, immutable, unique directory.


With npm installed, run

$ npm install ice-box


I was inspired by looking at how many codebases will use a many-step build process that involves transforming directories (source dir -> build dir -> packaged dir -> windows installer program), but suffer from side effects and shared global state. If build steps were interrupted the series of output directories would be inconsistent, hard to track down, etc. I really wanted to be able to make build and release pipelines that were as easy to reason about as UNIX pipes.

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