index.js

#!/usr/bin/env node
const program = require('commander')
const shell = require('shelljs')
const shellescape = require('shell-escape')

// Show these many paths and then append " and x more"
const DEFAULT_SHOW_PATHS = 2

program
  .arguments('[paths...]')
  .description('Generates a conventional commit message based on staged changes')
  .parse(process.argv)

const PATHSPEC_DOCS = ['README.md', 'CHANGELOG.md', 'docs', 'examples', 'example']

commitmsg(program.args, function (msg) {
  if (msg !== '') {
    commitmsg(PATHSPEC_DOCS, function (docs) {
      if (docs === msg) {
        console.log('docs: ' + msg)
      } else {
        console.log(msg)
      }
    })
  }
})

function commitmsg(pathspecs, cb) {
  shell.exec(
    'git status -s -- ' + shellescape(pathspecs),
    {silent: true},
    function (code, stdout, stderr) {
      if (code != 0) {
        throw new Error(stderr)
      }
      cb(formatStatus(stdout))
    }
  )
}

function formatStatus(status) {
  var added = []
  var updated = []
  var deleted = []
  var lines = status.split(/\r?\n/)

  for (var n in lines) {
    var line = lines[n]
    var file = line.slice(3)
    var x = line[0] // tree -> index

    if (x === 'A') {
      added.push(file)
    } else if (x === 'M') {
      updated.push(file)
    } else if (x === 'D') {
      deleted.push(file)
    }
  }

  var changes = added.concat(updated).concat(deleted)
  if (changes.length === 0) {
    // No changes, no commit message
    return
  }

  var actions = []
  if (added.length > 0) {
    actions.push('add')
  }
  if (updated.length > 0) {
    actions.push('update')
  }
  if (deleted.length > 0) {
    actions.push('remove')
  }

  return actionlist(actions) + ' ' + filelist(changes)
}

function actionlist(actions) {
  return actions.sort().join('/')
}

function filelist(paths, options) {
  options = options || {}
  var show = options.show || DEFAULT_SHOW_PATHS

  if (paths.length === 1) {
    return paths[0]
  } else if (paths.length > show) {
    var rest = paths.slice(show)
    return paths.slice(0, show).join(', ') + ' and ' + rest.length + ' more'
  } else {
    var first = paths.slice(0, paths.length - 1)
    var last = paths[paths.length - 1]
    return first.join(', ') + ' and ' + last
  }
}