NOTE: Work is ongoing to get an io.js formula into the official Homebrew tap. Follow the progress here!
A Homebrew formula for https://iojs.org.
brew install smockle/iojs/iojs
Or brew tap smockle/homebrew-iojs and then brew install iojs.
Or install via URL (which will not receive updates):
brew install https://raw.githubusercontent.com/smockle/homebrew-iojs/master/Formula/iojs.rb
brew help, man brew or check Homebrew's documentation.