code: function one(h, node, parent) {
    var type = node && node.type
    var fn = own.call(h.handlers, type) ? h.handlers[type] : null
  
    // Fail on non-nodes.
    if (!type) {
      throw new Error('Expected node, got `' + node + '`')
    }
  
    return (typeof fn === 'function' ? fn : unknown)(h, node, parent)
  }
docs: Visit a node.