Data modeling and relation library for testing JavaScript applications.

Motivation

When testing API interactions you often need to mock data. Instead of keeping a hard-coded set of fixtures, this library provides you with must-have tools for data-driven API mocking:

• An intuitive interface to model data
• The ability to create relationships between models
• The ability to query data in a manner similar to an actual database

Getting started

Install

$ npm install @mswjs/data --save-dev

Describe data

With this library, you're modeling data using the factory function. That function accepts an object where each key represents a model name and the values are model definitions. A model definition is an object where the keys represent model properties and the values are value getters.

// src/mocks/db.js
import { factory, primaryKey } from '@mswjs/data'

export const db = factory({
  // Create a "user" model,
  user: {
    // ...with these properties and value getters.
    id: primaryKey(() => 'abc-123'),
    firstName: () => 'John',
    lastName: () => 'Maverick',
  },
})

See the Recipes for more tips and tricks on data modeling.

Throughout this document native JavaScript constructors (i.e. String, Number) will be used as values getters for the models, as they both create a value and define its type. In practice, you may consider using value generators or tools like faker for value getters.

Using the primary key

Each model must have a primary key. That is a single key that can be used to reference an entity of that model. Think of it as an ID column for a particular table in a database.

Declare a primary key by using the primaryKey helper function:

import { factory, primaryKey } from '@mswjs/data'

factory({
  user: {
    id: primaryKey(String),
  },
})

In the example above the id is the primary key for the user model. This means that whenever a user is created it must have the id property that equals a unique String.

Integrate with mocks

Although this library can be used standalone, it's specifically fine-tuned to integrate with Mock Service Worker to compose an unrivaled API mocking experience for both testing and development of your JavaScript applications.

// src/mocks/browser.js
import { setupWorker, rest } from 'msw'
import { db } from './db'

setupWorker(
  // Mock a user creation operation.
  rest.post('/user', (req, res, ctx) => {
    const { firstName, lastName } = req.body

    const user = db.user.create({
      firstName,
      lastName,
    })

    return res(ctx.json(user))
  }),

  // Retrieve a single user from the database by ID.
  rest.get('/user/:userId', (req, res, ctx) => {
    const user = db.user.findFirst({
      where: {
        id: {
          equals: req.params.userId,
        },
      },
    })

    if (!user) {
      return res(ctx.status(404))
    }

    return res(ctx.json(user))
  }),
)

Recipes

• Model methods
• Querying data
• Strict mode
• Model relationships
• Pagination

Model methods

Each model has the following methods:

• create()
• findFirst()
• findMany()
• count()
• getAll()
• update()
• updateMany()
• delete()
• deleteMany()
• toHandlers()

create

Creates an entity for the model.

const user = db.user.create()

When called without arguments, .create() will populate the entity properties using the getter functions you've specified in the model definition.

You can also provide a partial initial values when creating an entity:

const user = db.user.create({
  firstName: 'John',
})

Note that all model properties are optional, including relational properties.

findFirst

Returns the first entity that satisfies the given query.

const user = db.user.findFirst({
  where: {
    id: {
      equals: 'abc-123',
    },
  },
})

findMany

Returns all the entities that satisfy the given query.

const users = db.user.findMany({
  where: {
    followersCount: {
      gte: 1000,
    },
  },
})

count

Returns the number of records for the given model.

db.user.create()
db.user.create()

db.user.count() // 2

Can accept an optional query argument to filter the records before counting them.

db.user.count({
  where: {
    role: {
      equals: 'reader',
    },
  },
})

getAll

Returns all the entities of the given model.

const allUsers = db.user.getAll()

update

Updates the first entity that matches the query.

const updatedUser = db.user.update({
  // Query for the entity to modify.
  where: {
    id: {
      equals: 'abc-123',
    },
  },
  // Provide partial next data to be
  // merged with the existing properties.
  data: {
    // Specify the exact next value.
    firstName: 'John',

    // Alternatively, derive the next value from
    // the previous one and the unmodified entity.
    role: (prevRole, user) => reformatRole(prevRole),
  },
})

updateMany

Updates multiple entities that match the query.

const updatedUsers = db.user.updateMany({
  // Query for the entity to modify.
  where: {
    id: {
      in: ['abc-123', 'def-456'],
    },
  },
  // Provide partial next data to be
  // merged with the existing properties.
  data: {
    firstName: (firstName) => firstName.toUpperCase(),
  },
})

delete

Deletes the entity that satisfies the given query.

const deletedUser = db.user.delete({
  where: {
    followersCount: {
      equals: 0,
    },
  },
})

deleteMany

Deletes multiple entities that match the query.

const deletedUsers = db.user.deleteMany({
  where: {
    followersCount: {
      lt: 10,
    },
  },
})

toHandlers

Generates request handlers for the given model to use with Mock Service Worker. All generated handlers are automatically connected to the respective model methods, enabling you to perform CRUD operations against your mocked database.

REST handlers

import { factory, primaryKey } from '@mswjs/data'
import { setupWorker } from 'msw'

const db = factory({
  user: {
    id: primaryKey(String),
    firstName: String,
  },
})

const worker = setupWorker(...db.user.toHandlers('rest'))

worker.start()

The following request handlers are generated and connected to the respective database operations:

• GET /users, returns all users (supports pagination).
• GET /users/:id (where "id" is your model's primary key), returns a user by primary key.
• POST /users, creates a new user.
• PUT /users/:id, updates an existing user by primary key.
• DELETE /users/:id, deletes an existing user by primary key.

The "/user" part of the route is derived from your model name. For example, if you have a "post" model defined in your factory, then the generated handlers will be /posts, /posts/:id, etc.

With the handlers generated and MSW configured, you can start querying the database:

// Create a new user in the database.
fetch('/users', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    id: 'abc-123',
    firstName: 'John',
  }),
})

GraphQL handlers

setupWorker(...db.user.toHandlers('graphql'))

The following GraphQL queries and mutations are generated:

• user(where: UserQueryInput): User, returns a user matching the query.
• users(where: UserQueryInput, cursor: ID, skip: Int, take: Int): [User!], returns all users matching the query (supports pagination).
• createUser(data: UserInput!): User!, creates a new user.
• updateUser(where: UserQueryInput!, data: UserInput!): User!, updates a user.
• updateUsers(where: UserQueryInput!, data: UserInput!): [User!], updates multiple users.
• deleteUser(where: UserQueryInput!): User!, deletes a user.
• deleteUsers(where: UserQueryInput!): [User!], deletes multiple users.

Notice how some operation names contain the plural model name to emphasize that they work on a collection of entities.

The "User" part of operation names is derived from your model name. For example, if you have a "post" model defined in your factory, then the generated handlers will have operations like post, createPost, updatePosts, etc.

With the handlers generated and MSW configured, you can start querying the database:

import { gql, useQuery } from '@apollo/client'

const CREATE_USER = gql`
  query CreateUser($initialValues: UserInput!) {
    createUser(data: $initialValues) {
      firstName
    }
  }
`

useQuery(CREATE_USER, {
  variables: {
    initialValues: {
      firstName: 'John',
    },
  },
})

Note that GraphQL queries must be named in order to be intercepted.

Scoping handlers

The .toHandlers() method supports an optional second baseUrl argument to scope the generated handlers to a given endpoint:

db.user.toHandlers('rest', 'https://example.com')
db.user.toHandlers('graphql', 'https://example.com/graphql')

Querying data

This library supports querying of the seeded data similar to how one would query a SQL database. The data is queried based on its properties. A query you construct depends on the value type you are querying.

String operators

• equals
• notEquals
• contains
• notContains
• in
• notIn

Number operators

• equals
• notEquals
• gt
• gte
• lt
• lte
• between
• notBetween

Boolean operators

• equals
• notEquals

Query example

const db = factory({
  post: {
    id: String,
    likes: Number,
    isDraft: Boolean,
  },
})

// Returns the list of `post` entities
// that satisfy the given query.
const popularPosts = db.post.findMany({
  where: {
    likes: {
      gte: 1000,
    },
    isDraft: {
      equals: false,
    },
  },
})

Strict mode

When querying or updating the entities you can supply the strict: boolean property on the query. When supplied, if a query operation fails (i.e. no entity found), the library will throw an exception.

import { factory, primaryKey } from '@mswjs/data'

const db = factory({
  user: {
    id: primaryKey(String),
  },
})

db.user.create({ id: 'abc-123' })

// This will throw an exception, because there are
// no "user" entities matching this query.
db.user.findFirst({
  where: {
    id: {
      equals: 'def-456',
    },
  },
  strict: true,
})

Model relationships

• One-to-One
• One-to-Many
• Many-to-One
• Unique relationships

Defining relationships is a way for one model to reference another. Models are flat by design, so if you wish to have a property that equals an object or an array, you create a relationship of the proper type to another existing model.

One-to-One

import { factory, primaryKey, oneOf } from '@mswjs/data'

const db = factory({
  user: {
    id: primaryKey(String),
    firstName: String,
  },
  post: {
    id: primaryKey(String),
    title: String,
    // The "post.author" references a "user" model.
    author: oneOf('user'),
  },
})

const user = db.user.create({ firstName: 'John' })
const post = db.post.create({
  title: 'My journey',
  // Use a "user" entity as the actual value of this post's author.
  author: user,
})

post.author.firstName // "John"

One-to-Many

import { factory, primaryKey, manyOf } from '@mswjs/data'

const db = factory({
  user: {
    id: primaryKey(String),
    // "user.posts" is a list of the "post" entities.
    posts: manyOf('post'),
  },
  post: {
    id: primaryKey(String),
    title: String,
  },
})

const posts = [
  db.post.create({ title: 'First' }),
  db.post.create({ title: 'Second' }),
]

const user = db.user.create({
  // Assign the list of existing posts to this user.
  posts,
})

user.posts // [{ title: "First" }, { title: "Second" }]

Many-to-One

import { factory, primaryKey, oneOf } from '@mswjs/data'

const db = factory({
  country: {
    name: primaryKey(String),
  },
  user: {
    id: primaryKey(String),
    country: oneOf('country'),
  },
  car: {
    serialNumber: primaryKey(String),
    country: oneOf('country'),
  },
})

const usa = db.country.create({ name: 'The United States of America' })

// Create a "user" and a "car" with the same country.
db.user.create({ country: usa })
db.car.create({ country: usa })

Unique relationships

Both oneOf and manyOf relationships may be marked as unique. A unique relationship is where a referenced entity cannot be assigned to another entity more than once.

In the example below we define the "user" and "invitation" models, and design their relationship so that one invitation cannot be assigned to multiple users.

import { factory, primaryKey, oneOf } from '@mswjs/data'

const db = factory({
  user: {
    id: primaryKey(String),
    invitation: oneOf('invitation', { unique: true }),
  },
  invitation: {
    id: primaryKey(String),
  },
})

const invitation = db.invitation.create()

const john = db.user.create({ invitation })

// Assigning the invitation already used by "john"
// will throw an exception when creating this entity.
const karl = db.user.create({ invitation })

Pagination

This library supports offset-based and cursor-based pagination of the findMany method results.

Offset-based pagination

const db = factory({
  post: {
    id: primaryKey(String),
    category: String,
  },
})

db.post.findMany({
  where: {
    category: {
      equals: 'Science',
    },
  },
  take: 15,
  skip: 10,
})

Cursor-based pagination

The cursor option of the findMany query expects a primary key value of a model to start the pagination from.

const db = factory({
  post: {
    // The `id` primary key will be used as a cursor.
    id: primaryKey(String),
    category: String,
  },
})

const firstPage = db.post.findMany({
  where: {
    category: {
      equals: 'Science',
    },
  },
  take: 15,
  cursor: null,
})

const secondPage = db.post.findMany({
  where: {
    category: {
      equals: 'Science',
    },
  },
  take: 15,
  // The second page will start from the last post
  // of the `firstPage`.
  cursor: firstPage[firstPage.length - 1].id,
})

Database utilities

drop

Drops the given database, deleting all its entities.

import { factory, drop } from '@mswjs/data'

const db = factory({...})

drop(db)

Usage with faker

Libraries like faker can help you generate fake data for your models.

import { seed, datatype, name } from 'faker'
import { factory, primaryKey } from '@mswjs/data'

// (Optional) Seed `faker` to ensure reproducible
// random values of model properties.
seed(123)

factory({
  user: {
    id: primaryKey(datatype.uuid),
    firstName: name.firstName,
  },
})

Honorable mentions

• Prisma for inspiring the querying client.
• Lenz Weber and Matt Sutkowski for great help with the TypeScript support.