How to use the mjml-core.documentParser function in mjml-core

.then(content => {
      const MJMLDocument = documentParser(content.toString())
      const body = find(MJMLDocument.children, ['tagName', 'mj-body'])

      if (!body || body.children.length == 0) {
        return;
      }

      const report = MJMLValidator(body.children[0])
      const outputFormat = availableErrorOutputFormat[format] || availableErrorOutputFormat['text']

      error(outputFormat(report))
    })
    .catch(e => {