dualCase = (items) => {
        /** @type string[] */
        const result = [];
        items.forEach(item => {
          result.push(item);
          if (item.toLowerCase() === item) {
            result.push(item.toUpperCase());
          } else {
            result.push(item.toLowerCase());
          }
        });
        return result;
      }
Dual-case keywords

["then","FILE"] =>
    ["then", "THEN", "FILE", "file"]

@param {string[]} items