zod-array-ignore-unknown.ts

import { z, ZodTypeAny, ZodUnion } from 'zod'

/**
 * Zod helper for parsing arrays and ignore items not specified in the schema
 *
 * @param zodUnion - union of known types
 *
 * @example
 * const binaryArraySchema = arrayIgnoreUnknown(z.union([z.literal('0'), z.literal('1')]))
 * type BinaryArray = z.TypeOf<typeof binaryArraySchema>
 *
 * const binaryArray: BinaryArray = binaryArraySchema.parse(['0', '1', '2', '0'])
 * console.log(binaryArray) // ['0', '1', '0']
 */
export function zodArrayIgnoreUnknown<T extends [ZodTypeAny, ...ZodTypeAny[]]>(
  zodUnion: ZodUnion<T>,
) {
  const isKnownItem = (item: unknown) => zodUnion.safeParse(item).success

  return z.preprocess((val) => toSafeArray(val).filter(isKnownItem), z.array(zodUnion))
}

function toSafeArray(item: unknown): Array<unknown> {
  if (isArray(item)) {
    return item
  }
  return [item]
}

function isArray<T>(item: unknown): item is Array<T> {
  return Array.isArray(item)
}

@tintin10q If I remember correctly the ZodUnion requires at least two inputs. Because of that I used the same constraint

Thanks, the preprocess solution is working with recursive schema.

function makeFilteredArraySchema<T extends ZodSchema>(schema: T) {
  return z.preprocess((val) => {
    const array = Array.isArray(val) ? val : [val]
    return array.filter((item: unknown) => schema.safeParse(item).success)
  }, z.array(schema))
}

Usage

const baseCategorySchema = z.object({
  name: z.string(),
})

type Category = z.infer<typeof baseCategorySchema> & {
  subcategories: Category[]
}

const categorySchema: z.ZodType<Category, z.ZodTypeDef, unknown> = baseCategorySchema.extend({
  subcategories: z.lazy(() => makeFilteredArraySchema(categorySchema)),
})

categorySchema.parse({
  name: 'People',
  subcategories: [
    {
      name: 'Politicians',
      subcategories: [
        {
          name: 'Presidents',
          subcategories: [],
        },
        {
          name: 123,
          subcategories: [],
        },
      ],
    },
    {
      name: 456,
      subcategories: [],
    },
  ],
})