Default Value Handling in Query Parameters with Zod

[alberto-htl]

I have a scenario where I need to handle a date range in the search parameters. The date format used is MM-dd-yyyy, and I want to provide a default value server-side if the date range is not provided. Here's what I’ve implemented so far:

export const searchParamsParsers = {
  filters: parseAsJson(generalReportFilterServerSchema.parse).withDefault({
    to: DateTime.utc().endOf("day").toJSDate(),
    from: DateTime.utc().minus({ days: 7 }).startOf("day").toJSDate(),
  }),
};

export const generalReportSearchParamsCache =
  createSearchParamsCache(searchParamsParsers);

And the schema that I’m using:

export const generalReportFilterServerSchema = z.object({
  from: z
    .string()
    .optional()
    .transform((val) => {
      return val
        ? DateTime.fromFormat(val, "yyyy-MM-dd").toJSDate()
        : DateTime.utc().endOf("day").toJSDate();
    }),
  to: z
    .string()
    .optional()
    .transform((val) =>
      val
        ? DateTime.fromFormat(val, "yyyy-MM-dd").toJSDate()
        : DateTime.utc().minus({ days: 7 }).startOf("day").toJSDate(),
    ),
});

Issue:
The code above should handle both cases: when the query is missing from the URL and when a parameter is missing from the filters.

I was expecting the .transform method in the schema to be enough to handle the default values, but it seems like when the query is missing, the validation schema is not evaluated, and the default values aren't applied as expected.

Question:
Is there a better approach to provide default values server-side without duplicating the logic in multiple places (i.e., in both the withDefault() in the searchParamsParsers and the transform() in the schema)? Any suggestions on how to ensure the schema evaluates correctly when the query is missing or incomplete?

[franky47]

when the query is missing, the validation schema is not evaluated

Indeed, if there is no query value, there is nothing to parse, and so the schema is not called.

One issue I see with your approach is defining default values that are time-based: the default values for searchParamsParsers will be invalid (at most) 24 hours from the time it gets initialised.

For this, you may want to keep your state nullable, and apply the defaults dynamically (via nullish coalescing) after parsing, so that you always get fresh defaults.