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Custom modifiers (refinements)

@typeofweb/schema is almost infinitely extensible thanks to the refine function. It can be used to:

  • create custom validators
  • modify or adjust validators which are built-in
  • parse data before validating


refine takes two parameters both of which are functions. First of them is a "refinement function" which is where the implementation details go. Such a function is given value: unknown as its first parameter and t: RefinementToolkit as second:

refine((value, t) => {
// โ€ฆ

value is, well, the value passed to the validator, and t is an object which contains four functions: right, left, nextValid, and nextNotValid, all of which are described in detail below.

Second parameter of refine is another function which is used for stringifying the validator for error purposes:

(value, t) => {
// โ€ฆ
() => 'XXX',

Such a validator would throw Invalid type! Expected XXX but got โ€ฆ! in case it's fed with invalid data.


As mentioned in the previous paragraph, RefinementToolkit consists of 3 functions: right, left, and next. They all play a different role and are equally useful when creating custom validators or modifiers:

  • right(value) โ€“ ends validation with a success and value
  • left(value) โ€“ ends validation with an error
  • nextValid(value) โ€“ continues validation if there are other validators after it; success otherwise
  • nextNotValid(value) โ€“ continues validation if there are other validators after it; fail otherwise

In most of the cases you'll want to use left and nextValid.

Let's look at a few short examples of where each of these functions comes handy:

export const optional = refine((value, t) =>
value === undefined ? t.right(undefined) : t.nextNotValid(value),

const presentOrFuture = refine((value: Date, t) =>
value.getTime() >= ? t.nextValid(value) : t.left(value),

optional is a built-in validator which either succeeds instantly if given value is equal to undefined (t.right) or passes it along to the next validator. In case there are no other validators in the pipeline, validation fails because the value is clearly not undefined.

presentOrFuture is a validator meant to be used after the date validator. It's role is to determine if given date is present or in the future, and if it is so, it should pass the date to another validator in line (t.nextValid). However, if given date is in the past, we expect this validator to short circuit and finish with an error (t.left).

t.nextValid and t.left are most commonly used. Use t.right sparingly only if you're certain no further validation will ever be required on given value. t.nextNotValid is meant for refinements which are supposed to extend other validators โ€“ such as optional, nullable or nil.

Custom validator

As a matter of fact, all built-in validators are now implemented using the refine function. Let's look at the number implementation:

export const number = refine(
(value, t) => {
const parsedValue = parseNumber(value);
if (typeof parsedValue !== 'number' || Number.isNaN(parsedValue)) {
return t.left(value);
return t.nextValid(parsedValue);
() => typeToPrint('number'),

function parseNumber(value: unknown) {
if (typeof value === 'string') {
if (value.trim() === '') {
return value;
return Number(value);
return value;

You can create your own validators in the same manner.

Modify validators

Parse data

Another common scenario is allowing timestamps where dates are expected. We can create a refinement which takes a timestamp and converts it to a Date instance before further validation is executed:

const allowTimestamps = refine((value, t) =>
typeof value === 'number' ? t.nextValid(new Date(value)) : t.nextValid(value),

ฮป(date, allowTimestamps, validate)(1231231231231); // new Date(1231231231231) Tue Jan 06 2009 09:40:31 GMT+0100