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This is preview documentation for the Refine library.
Refine will initially be bundled as part of the Recoil Sync package when released. Source is available here.

Refine is a type-refinement / validator combinator library for mixed / unknown values in Flow or TypeScript.

Getting Started

Refine is currently bundled as part of the Recoil Sync package.

To get started learning about Refine, check out the documentation on the core concepts of Utilities and Checkers.

Why would I want to use Refine?

Type Refinement Example

Coerce unknown types to a strongly typed variable. assertion() will throw if the input doesn't match the expected type while coercion() will return null.

const myObjectChecker = object({
numberProperty: number(),
stringProperty: optional(string()),
arrayProperty: array(number()),

const myObjectAssertion = assertion(myObjectChecker);
const myObject: CheckerReturnType<myObjectChecker> = myObjectAssertion({
numberProperty: 123,
stringProperty: 'hello',
arrayProperty: [1, 2, 3],

Backward Compatible Example

Using match() and asType() you can upgrade from previous types to the latest version.

const myChecker: Checker<{str: string}> = match(
object({str: string()}),
asType(string(), str => ({str: str})),
asType(number(), num => ({str: String(num)})),

const obj1: {str: string} = coercion(myChecker({str: 'hello'}));
const obj2: {str: string} = coercion(myChecker('hello'));
const obj3: {str: string} = coercion(myChecker(123));

JSON Parser Example

Refine wraps JSON to provide a built-in strongly typed parser.

const myParser = jsonParser(
array(object({num: number()}))

const result = myParser('[{"num": 1}, {"num": 2}]');

if (result != null) {
// we can now access values in num typesafe way
assert(result[0].num === 1);
} else {
// value failed to match parser spec

Usage in Recoil Sync

The Recoil Sync library leverages Refine for type refinement, input validation, and upgrading types for backward compatibility. See the recoil-sync docs for more details.