wiki-js/server/modules/rendering/markdown-core/underline.js

const renderEm = (tokens, idx, opts, env, slf) => {
  const token = tokens[idx]
  if (token.markup === '_') {
    token.tag = 'u'
  }
  return slf.renderToken(tokens, idx, opts)
}

module.exports = (md) => {
  md.renderer.rules.em_open = renderEm
  md.renderer.rules.em_close = renderEm
}