Getting started

Install

fast-check and juniper are peer dependencies, so install all three:

npm

npm i --save-dev juniper-arbitrary fast-check juniper

pnpm

pnpm add --save-dev juniper-arbitrary fast-check juniper

yarn

yarn add --dev juniper-arbitrary fast-check juniper

TIP

juniper-arbitrary is ESM only, and requires Node.js 22.13 or later.

Make an arbitrary

Pass a schema to makeArbitrary, and use the result anywhere fast-check takes an arbitrary:

import * as fc from 'fast-check';
import { arraySchema, stringSchema } from 'juniper';
import { makeArbitrary } from 'juniper-arbitrary';

const tags = arraySchema(stringSchema().minLength(1)).uniqueItems(true);

fc.assert(
    fc.property(makeArbitrary(tags), value => {
        // Sorting keeps every tag
        return value.toSorted().length === value.length;
    })
);

Failures shrink to smaller values that are still valid, and replay from fast-check's seed.

Check a validator

A schema's arbitrary is a quick way to test anything else that reads the schema, such as a validator or a rendered form of the schema:

import * as fc from 'fast-check';
import { numberSchema } from 'juniper';
import { makeArbitrary } from 'juniper-arbitrary';
import { makeValidator } from 'juniper-validator';

const score = numberSchema().minimum(0).maximum(100);
const validator = makeValidator(score);

fc.assert(fc.property(makeArbitrary(score), value => validator.is(value)));

Next steps

• How values are generated: what is aimed at, what is filtered, and what is deliberately limited.
• API: makeArbitrary and its errors.