Provide a way to make default interop with null values

[remcohaszing]

I have some schemas where I want to accept null as input, and turn that into a default value. I’m generating JSON schemas and documentation from the zod schemas, so the actual .default('default') call is important to set metadata.

Let’s say this is the base schema. I would like this script to log default twice.

const schema = z
  .enum(['default', 'custom'])
  .nullish()
  .default('default')

console.log(schema.parse(null))
console.log(schema.parse(undefined))

I could do the following:

const schema = z
  .enum(['default', 'custom'])
  .nullish()
  .default('default')
  .transform((input, ctx) => input == null ? 'devault', input)

console.log(schema.parse(null))
console.log(schema.parse(undefined))

This works ok. However, this is error prone. I made a typo in the returned value from transform. I also need to maintain the default value twice.

I see two solutions.

1. .default() can accept a second argument. This is a function which returns whether or not the default value should be used.

   const schema = z
     .enum(['default', 'custom'])
     .nullish()
     .default('default', (input) => input == null)

2. Provide more context to the .transform() context, notably a default value.

   const schema = z
     .enum(['default', 'custom'])
     .nullish()
     .default('default')
     .transform((input, ctx) => input == null ? ctx.default, input)

   This is especially useful, because it makes the transform function reusable.

   function setDefault<T>(input: T | null | undefined, ctx: z.RefinementCtx): T {
     return input == null ? z.default : input
   }
   
   const schema = z
     .enum(['default', 'custom'])
     .nullish()
     .default('default')
     .transform(setDefault)
   
   const anotherSchema = z
     .enum(['another default', 'another custom'])
     .nullish()
     .default('another default')
     .transform(setDefault)