Superstruct allows you to constrain existing structs with further validation. This doesn't change the type of the struct, but simply introduces extra validation logic. This can be useful for example when ensuring that a string matches a specific RegExp.


empty enforces that a string, array, map, or set is empty.
🤖 Technically this is the same as using size of zero, but "empty" feels slightly nicer and will give a slightly easier to read error.


max(number(), 0)
max enforces that a number struct is less than a threshold.
🤖 If you need an exclusive maxmimum you can pass { exclusive: true } as the third argument, like max(number(), 0, { exclusive: true }) for negative numbers.


min(number(), 9000)
min enforces that a number struct is greater than a threshold.
🤖 If you need an exclusive minimum you can pass { exclusive: true } as the third argument, like min(number(), 0, { exclusive: true }) for positive numbers.


nonempty enforces that a string, array, map, or set is not empty. This does the opposite of empty.


pattern(string(), /\d+/)
pattern enforces that a string struct also matches a supplied RegExp.


size(string(), 1, 100)
size(array(number()), 0)
size(number(), 93, Infinity)
'a string of text'
size enforces that a number, string, array, map, or set struct also is within a certain min and max size (or length).
🤖 The max argument is optional and defaults to whatever you pass for min, which makes specifying exact sizes easy (just omit the max).

Custom Refinements

You can also define your own custom refinments that are specific to your application's requirements, like so:
import { number, refine } from 'superstruct'
const Positive = refine(number(), 'positive', (value) => value >= 0)
This allows you to define more fine-grained runtime validation, while still preserving the number type at compile time.
🤖 The value provided to the refiner function is guaranteed to be the provided struct's type. This means you can layer additional validation on top of even complex structs with minimal hassle.
If you'd like to customize the error message that will be returned/thrown by your struct when a value doesn't pass your refinement's validation, you can return a string instead of a boolean inside the refiner.
const DateRange = refine(
startDate: number(),
endDate: number(),
(value) => {
if (value.startDate < value.endDate) {
return true
// Returning a string indicates that validation failed and the provided
// string should be used as a custom error message.
return (
`Expected 'startDate' to be less than 'endDate' on type 'DateRange', ` +
`but received ${JSON.stringify(value)}`