code: function $var(params, fn, envs) {
      if (params == null) {
          return params;
      }
      if (typeof params === 'string') {
          if (envs && envs[params] !== undefined) {
              return envs[params];
          }
  
          return;
      }
      if (Array.isArray(params) && params.length === 2) {
          const key = params[0];
          if (typeof key === 'string') {
              const value = fn.$var(key, fn, envs);
  
              return value === undefined ? params[1] : value;
          }
      }
      throw new Error(
          '$var expects a string or an array with two elements of which the first one must be a string'
      );
  }
docs: Get value from envs with an optional fallback value
  
  @param {string|[string, any]} params
  @returns {any}