Use default value on error

I’d like to validate a value, and if that validation fails I don’t care about the error - I want to use a default value.

const strZ = z.string().default('hello world');
strZ.parse() // 'hello world'
strZ.parse('hi') // 'hi'
strZ.parse(23) // error, but I want 'hello world'

I can’t figure out a clean way to do this without writing my own wrapper around zod. Is this supported already by the library?

[altitudems]

Looks like this might be a good workaround:

const withFallback = <T>(schema: z.ZodType<T>, fallback: T) =>
  z.preprocess(
    (value) => {
      const parseResult = schema.safeParse(value);
      if (parseResult.success) return value;
      return fallback;
    },
    z.custom((v) => true)
  );

Example Usage

[0916dhkim]

Throwing in some ideas here:

type Parser<T> = (val: unknown) => T;
const customSchema = <T>(parser: Parser<T>) => z.unknown().transform(parser);

// Now you can define your own zod schema like this:
const catOrDogSchema = customSchema((val) => {
  if (val === "cat" || val === "dog") return val;
  throw new Error("Not cat or dog");
});
catOrDogSchema.parse("cat") // type: "cat" | "dog"

We can extend this and make withFallback helper:

const withFallback = <T>(schema: z.ZodType<T>, fallback: T) =>
  customSchema((val) => {
    const parsed = schema.safeParse(val);
    if (parsed.success) return parsed.data;
    return fallback;
  });

@matthewoates

const strZ = withFallback(z.string().default("hello world"), "fallback");
strZ.parse() // "hello world"
strZ.parse("hi") // "hi"
strZ.parse(23) // "fallback"

@zavarka

const urlQueryParams = z.object({
  // ...
  queryParam7: withFallback(
    z.string().nullish().refine(isUUID),
    null
  ),
});
urlQueryParams.parse({
  // ...
  queryParam7: 13 // INVALID
}); // but it parses successfully. { queryParam7: null }

This works, but it would be nice if zod provided corresponding methods like z.custom() and z.string().fallback().