nullish vs optional vs nullable in Zod

.optional()

• Type: T | undefined
• Accepts: undefined, missing key in objects, any value passing the base schema T
• Rejects: null (unless separately allowed)
• Shorthand for "allow the value to be absent"

.nullable()

• Type: T | null
• Accepts: explicit null, any value passing T
• Rejects: undefined, missing key
• Purpose: value must be supplied but may be null

.nullish()

• Type: T | null | undefined
• Accepts: null, undefined, missing key, any value passing T
• Internally the same as .nullable().optional()

Interaction summary

• Missing key → allowed only by .optional() or .nullish()
• Explicit null → allowed only by .nullable() or .nullish()
• Both null and missing/undefined → .nullish()

Default values

• .default(val) inserts val when the value is undefined; it does not act on null, so combine with .nullable() or .nullish() according to needs

Object helpers

• z.object({...}).partial() marks every key as .optional() (still disallows null)
• z.object({...}).passthrough() or .strict() do not change nullish handling; only the property schema modifiers above do