prepare-commit-msg.js

var
  fs = require('fs'),
  messageFile = process.argv[2],
  commitType = process.argv[3],
  branchNameIssuePattern = /On branch .+#(\d{1,5})/,
  issueNumber,
  contents;

function messageTemplate (issueNumber) {
  var
    byline = '\n\n# [General description above]\n\n' +
    '# [Details below]\n\n\n';

  if (issueNumber) {
    return byline + 'Issue: #' + issueNumber + '\n';
  }
  else {
    return byline + '# WARNING: Issue number not found in branch name.\n';
  }
}

if (['merge', 'commit', 'message'].indexOf(commitType) !== -1) {
  // Skip commits with existing messages.
  process.exit();
}

contents = fs.readFileSync(messageFile).toString();

if (contents.match(/^Merge branch/)) {
  // Skip commits resolving merge conflicts.
  process.exit();
}

issueNumber = (contents.match(branchNameIssuePattern) || [])[1];

fs.writeFileSync(messageFile, messageTemplate(issueNumber) + contents);