npm: Why is a version "0.1" invalid?

I had to change the version of my npm app from 0.1 to 0.0.1 in order for npm not to do this.

$ npm install
npm ERR! install Couldn't read dependencies
npm ERR! Error: invalid version: 0.1
npm ERR!     at validVersion (/usr/local/Cellar/node/0.10.5/lib/node_modules/npm/node_modul
es/read-package-json/read-json.js:571:40)
npm ERR!     at final (/usr/local/Cellar/node/0.10.5/lib/node_modules/npm/node_modules/read
-package-json/read-json.js:323:23)
npm ERR!     at /usr/local/Cellar/node/0.10.5/lib/node_modules/npm/node_modules/read-packag
e-json/read-json.js:139:33
npm ERR!     at cb (/usr/local/Cellar/node/0.10.5/lib/node_modules/npm/node_modules/slide/l
ib/async-map.js:48:11)
npm ERR!     at /usr/local/Cellar/node/0.10.5/lib/node_modules/npm/node_modules/read-packag
e-json/read-json.js:301:48
npm ERR!     at fs.js:207:20
npm ERR!     at Object.oncomplete (fs.js:107:15)
npm ERR! If you need help, you may report this log at:
npm ERR!     <http://github.com/isaacs/npm/issues>
npm ERR! or email it to:
npm ERR!     <[email protected]>

npm ERR! System Darwin 12.3.0
npm ERR! command "/usr/local/Cellar/node/0.10.5/bin/node" "/usr/local/bin/npm" "install"
npm ERR! cwd /Users/lust/Documents/ply/dev-server
npm ERR! node -v v0.10.5
npm ERR! npm -v 1.2.18
npm ERR!
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/lust/Documents/ply/dev-server/npm-debug.log
npm ERR! not ok code 0

For completeness here's the working json

$ cat package.json
{
    "name": "ply",
    "description": "ply server for local dev testing deployments",
    "version": "0.0.1",
    "private": true,
    "dependencies": {
        "express": "3.x"
    }
} 

version used to be "0.1" when it made the error.

Is this some sort of API/ABI compatibility versioning concept requiring 3 sets of version numbers? Why is the error message not more friendly w.r.t. this?

Yes, this is required for semantic versioning, which is the versioning scheme npm packages use. Here's the snippet from npm help json:

> Version must be parseable by node-semver, which is bundled with npm as a dependency. (npm install semver to use it yourself.) > > Here's how npm's semver implementation deviates from what's on semver.org: > > * Versions can start with "v" > * A numeric item separated from the main three-number version by a hyphen will be interpreted as a "build" number, and will increase the version. But, if the tag is not a number separated by a hyphen, then it's treated as a pre-release tag, and is less than the version without a tag. So, 0.1.2-7 > 0.1.2-7-beta > 0.1.2-6 > 0.1.2 > 0.1.2beta

Simple answer - use 0.1.0

0.1 will not work

Happy coding!

So yea, the short answer is "You need to use semantic versioning"

But the reasoning behind that is to provide a sensible, uniform package version to all users of npm. When getting a version number of an package, you have some level of confidence that the author understands semver and is employing it properly.