Simple promise example with bluebird and coffeescript works half the time - coffeescript

Question

So I'm basically attempting to write some simple code using promises and I'm having a bit of trouble understanding why this particular code works every other time.
Promise = require('bluebird')
mkdirp = Promise.promisify(require('mkdirp'))
rm = Promise.promisify(require('rimraf'))
console.log "Preparing build directory"
rm('build')
.then(mkdirp('build'))
This will complete successfully the first run, but the second will fail, and so on.
Here is the steps:
┌[adam#bigboi] [/dev/pts/5] [master ⚡]
└[~/Projects/bummyjab]> time coffee index.coffee ~/Dropbox/Articles/*.md
Preparing build directory
coffee index.coffee ~/Dropbox/Articles/*.md 0.25s user 0.02s system 100% cpu 0.267 total
┌[adam#bigboi] [/dev/pts/5] [master ⚡]
└[~/Projects/bummyjab]> stat build
File: ‘build’
Size: 4096 Blocks: 8 IO Block: 4096 directory
Device: 804h/2052d Inode: 17172395 Links: 2
Access: (0775/drwxrwxr-x) Uid: ( 1000/ adam) Gid: ( 1000/ adam)
Access: 2015-06-25 22:07:49.061331341 -0400
Modify: 2015-06-25 22:07:49.061331341 -0400
Change: 2015-06-25 22:07:49.061331341 -0400
Birth: -
┌[adam#bigboi] [/dev/pts/5] [master ⚡]
└[~/Projects/bummyjab]> time coffee index.coffee ~/Dropbox/Articles/*.md
Preparing build directory
Unhandled rejection Error: EEXIST: file already exists, mkdir '/home/adam/Projects/bummyjab/build'
at Error (native)
coffee index.coffee ~/Dropbox/Articles/*.md 0.20s user 0.03s system 100% cpu 0.235 total
Unfortunately, my google skills for this haven't come up with a reason why this is happening.
Thanks

Answer 1

If you're trying to control the order so that mkdirp('build') happens only after rm('build') has completed, then you need to pass a function reference to .then() like this:
rm('build').then(function () {
return mkdirp('build');
});
Or, you could use .bind():
rm('build').then(mkdirp.bind(null, 'build'));
What you were doing is executing mkdirp() immediately and passing that return value to .then() which doesn't wait to execute it until the promise is resolved.