Yup By Example

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Yup By Example is a random data generator driven from Yup schemas. For those practicing TDD, a rich and potentially shared schema increases the burden of managing test data. One solution is to create a common, hard coded set of test data, but this is almost certainly a bad idea. Not only does it lead to brittle tests, but also means that the tests come to depend on something that’s often hidden away, instead of the salient values being front and centre.

Instead, by generating random sets of test data, and explicitly overwriting just the key values, the tests will be more robust and communicate far more clearly. However, maintaining random test data generators is complex and onerous. If only it could be automatically generated from the same schema used for validation. This is where Yup By Example comes in.

Table Of Contents

TL;DR

1. Define the schema

const yupByExample = require('yup-by-example');
const { Schema, object, string, number, date, ...yup } = require('yup');

// This must be done before you build any schema that uses yup-by-example
yup.addMethod(Schema, 'example', yupByExample);

// Add .example() everywhere you want an example
const userSchema = object({
  name: string().required().example(),
  age: number().positive().integer().max(120)required().example(),
  email: string().email()required().example(),
  website: string().url().nullable().example(),
  createdOn: date().default(() => new Date()).example(),
}).example();

module.exports = { userSchema }

2. Generate test data

const { TestDataFactory } = require('yup-by-example');
const schema = require('../src/userSchema');

describe('User', () => {

  beforeEach() {
    TestDataFactory.init();
  }

  it('should create a valid user', async () => {
    const user = await TestDataFactory.generateValid(schema);
    console.log(user);
    //...
  })
})

3. Profit!

{
  "name": "GpxiKtlnEDBXpSX",
  "age": 75,
  "email": "vor@bigedac.cl",
  "website": "http://uloero.lr/et",
  "createdOn": "2848-04-15T06:10:26.256Z"
}

You can use the included and custom test data generators for even more realitic test data. You can even create arrays of test data too. See the example schema for more details.

Breaking changes

v4.0.0

As of yup v1.0.0 adding yupByExample to yup.mixed no longer works. Instead use

yup.addMethod(Schema, 'example', yupByExample);

Installation

npm i yup-by-example --save-dev

Generators

Default Generator

By default, Yup By Example will use the metadata type property or schema type to automatically generate valid test data for the following rules

type rules
string length, min, max, email, url, oneOf
number min, max, lessThan, morethan, positive, negative, integer, oneOf
boolean oneOf
date min, max, oneOf
array of, length, min, max, oneOf
object shape, oneOf

However for more nuanced validation and to make your data more realistic you can use one of Yup By Example’s in built generators or even write your own cusotm generator (see below).

Function Generator (fn)

A generator which uses an inline function to return test data. The function must be supplied as the second argument, e.g.

string().example({ generator: 'fn' }, () => {
  const randomOctet = () => Math.floor(Math.random() * 256);
  const ipAddress = Array.from({ length: 4 }, randomOctet).join('.');
  return ipAddress;
})

The inline function will passed an object with the following parameters

name notes
id Used to namespace session properties
session The test data session
chance An instance of Chance

Chance Generator (chance)

A generator which delegates to the Chance library. e.g.

string().example({ generator: 'chance' }, {
  method: 'name',
  params: {
    middle_initial: true
  },
});

Relative Date Generator (rel-date)

Sometimes you don’t need a random date, but an offset one.

date().example({ generator: 'rel-date' }, { days: 1 }),

By default, the generated dates will be reletive to when you initialised the TestDataFactory. You can override this as follows…

const { TestDataFactory } = require('yup-by-example');
TestDataFactory.init({ now: new Date('2000-01-01T00:00:00.000Z') });

rel-date uses date-fns add behind the scenes, and can be used to adjust the years, months, weeks, days, hours, minutes and seconds.

Literal Generator (literal)

The Literal generator lets you specify literal examples.

string().example({ generator: 'literal' }, 'Frank');

Custom Generator

For even greater flexibility, you can write a custom generator. This is a object exposing a generate function, which will be passed an object with the following parameters

name notes
id the generator id (used to namespace session properties)
params the generator parameters
session The test data session
chance An instance of Chance
now The timestamp when the TestDataFactory was initialised
schema The schema as supplied by yup
value The value as supplied by yup
originalValue The originalValue as supplied by yup

The generator must also be registered with the TestDataFactory. e.g.

const NiNumberGenerator = {
  generate: ({ chance }) => {
    const start = chance.string({ length: 2 });
    const middle = chance.integer({ min: 100000, max 999999 });
    const end = chance.string({ length: 1 });
    return `${start}${middle}${end}`.toUpperCase();
  }
}

TestDataFactory.init().addGenerator('ni-number', NiNumberGenerator);

string().example({ generator: 'ni-number' });

Generating Test Data

After defining the schema, there are two ways of generating test data.

  1. TestDataFactory.generateValid(schema: Schema, options?: Options): Promise<any>
  2. TestDataFactory.generate(schema: Schema, options?: Options): Promise<any>

Use the former when you want to generate valid test data and the latter when you need to generate a partial or invalid document.

You can optionally pass yup validation options as the second parameter.

Advanced Usage

Test Sessions

When generating test data, you often don’t want it to be completely random, instead it is often dependent on data previously generated in your test. You can communicate this information across examples by storing state in the session passed to the generator. The session exposes the following methods

You also can pre-initialise the session with values that your test generators will refer to, e.g.

const user = object().shape({
  name: string().example(),
}).example();

// Here we have given the generator an id of 'users'
const users = array.of(user).example({ id: 'users' });

const { TestDataFactory } = require('yup-by-example');
const userSchema = require('../src/userSchema');

describe('User', () => {

  beforeEach() {
    TestDataFactory.init();
  }

  it('should create users', async () => {
    // Here we stash a property in the session
    TestDataFactory.session.setProperty('users.length', 4);

    // Because the array generator checks for `${id}.length` it will generate exactly 4 users
    const users = await TestDataFactory.generateValid(schemas.users);

    // ...
  })
})

You can reset the session at any point by calling TestDataFactory.init(), however the session is shared, and so may prevent concurrent tests.

Intercepting Generated Values

Whenever a generate returns a value, before yielding it, the TestDataFactory will emit the event from the current session, allowing you to read and even modify the value. The event name will be one of:

This can be especially useful when adjusting values inside arrays

const { Schema, object, array, ...yup } = require('yup');
const yupByExample = require('yup-by-example');

yup.addMethod(Schema, 'example', yupByExample);

const user = object().meta({ type: 'user' }).shape({
  dob: date().example({ id: 'dob' generator: 'rel-date' }),
}).example();

const users = array().of(user).example();

const { TestDataFactory } = require('yup-by-example');
const schemas = require('../src/schemas');

describe('User', () => {

  beforeEach() {
    TestDataFactory.init();
  }

  it('should create users', async () => {
    let userIndex = 0;

    TestDataFactory.session.on('user', event => {
      userIndex++;
    })

    // Adjusts the generated dob based on the user index
    TestDataFactory.session.on('dob', event => {
      event.value = dateFns.add(event.value, { days: userIndex })
    })

    const users = await TestDataFactory.generateValid(schemas.users);
    // ...
  })
})

Random Seed Value

When you create random test data, it can be useful to repeatedly get the same “random” values for debugging purposes. When you instanciate the TestDataFactory you can pass the desired seed into the constructor.

TestDataFactory.init({ seed: 42 })

Now repeated runs should generate exactly the same data sets.

Caveats

Troubleshooting

TypeError: The value of field could not be cast to a value that satisfies the schema type: "object".

If you see this error you have probably neglected to add all the necessary .example() calls to your schema. Another possibilitiy is that some of your schemas were built using either stub example implementation, or a test data factory instantiated in a previous test.

TypeError: string(...).example is not a function
TypeError: number(...).example is not a function
TypeError: object(...).example is not a function
etc...

You forgot to call yup.addMethod(Schema, 'example', yupByExample)

For other problems try enabling debug

DEBUG yup-by-example:* npm t