code: function getVisitorKeys(visitorKeys, node) {
      let keys = visitorKeys[node.type];
  
      if (!keys) {
          keys = vk.getKeys(node);
          debug("Unknown node type \"%s\": Estimated visitor keys %j", node.type, keys);
      }
  
      return keys;
  }
docs: Get the visitor keys of a given node.
  @param {Object} visitorKeys The map of visitor keys.
  @param {ASTNode} node The node to get their visitor keys.
  @returns {string[]} The visitor keys of the node.