@redocly/openapi-core/src/rules/oas3/no-servers-empty-enum.ts

See https://github.com/Redocly/openapi-cli

import { Oas3Server } from 'core/src/typings/openapi';
import { Oas3Rule } from '../../visitors';

enum enumError {
  empty = 'empty',
  invalidDefaultValue = 'invalidDefaultValue',
}

export const NoEmptyEnumServers: Oas3Rule = () => {
  return {
    DefinitionRoot(root, { report, location }) {
      if (!root.servers || root.servers.length === 0) return;

      const invalidVariables: enumError[] = [];

      if (Array.isArray(root.servers)) {
        for (const server of root.servers) {
          const enumErrors = checkEnumVariables(server);
          if (!enumErrors) continue;
          invalidVariables.push(...enumErrors);
        }
      } else {
        const enumErrors = checkEnumVariables(root.servers);
        if (!enumErrors) return;
        invalidVariables.push(...enumErrors);
      }

      for(const invalidVariable of invalidVariables) {
        if(invalidVariable === enumError.empty) {
          report({
            message: 'Server variable with `enum` must be a non-empty array.',
            location: location.child(['servers']).key(),
          });
        }
        if(invalidVariable === enumError.invalidDefaultValue) {
          report({
            message: 'Server variable define `enum` and `default`. `enum` must include default value',
            location: location.child(['servers']).key(),
          });
        }
      }
    },
  };
};

function checkEnumVariables(server: Oas3Server): enumError[] | undefined{
  if (server.variables && Object.keys(server.variables).length === 0)
    return;

  const errors: enumError[] = [];
  for(var variable in server.variables) {
    const serverVariable = server.variables[variable];
    if (!serverVariable.enum) continue;

    if (Array.isArray(serverVariable.enum) && serverVariable.enum?.length === 0)
      errors.push(enumError.empty);

    if (!serverVariable.default)
      continue;

    const defaultValue = server.variables[variable].default;
    if (serverVariable.enum && !serverVariable.enum.includes(defaultValue))
      errors.push(enumError.invalidDefaultValue);
  }
  if (errors.length) return errors;
  return;
}