How to use faker.seed()

The Faker version is v.9.8.0.

I couldn't quite understand after reading the Official Docs,
so I'll explain in detail how to use faker.seed() based on what I learned from using it.

Basic Concept of faker.seed()

faker.seed() is a feature for generating reproducible random data. When you set the same seed value, the same data will be generated every time.

Basic Usage

import { faker } from '@faker-js/faker'

// Set seed (specify a number)
faker.seed(123)

// Data generated after this will be the same every time
console.log(faker.person.firstName()) // Same name every time
console.log(faker.location.city())    // Same city name every time

Practical Example in Test Files

Let's add the seed feature to the React Component.

import '@testing-library/jest-dom'
import { render } from '@testing-library/react'
import { faker } from '@faker-js/faker'

import { Combobox } from './Combobox'

// Set seed to generate reproducible data
faker.seed(12345) // Set a fixed number

const facilityPrefixes = ['Tokyo', 'Osaka', 'Nagoya', 'Fukuoka', 'Sapporo', 'Kobe', 'Yokohama']
const facilitySuffixes = ['Center', 'Building', 'Tower', 'Plaza', 'Park', 'Hall', 'Facility']

const generateFacility = (index: number) => {
  const prefix = faker.helpers.arrayElement(facilityPrefixes)
  const suffix = faker.helpers.arrayElement(facilitySuffixes)
  const name = `${prefix}${suffix}${index + 1}`
  return { value: name, label: name }
}

// Same data is generated every time
export const facilities = Array.from({ length: 30 }, (_, index) => 
  generateFacility(index)
)

test('renders', () => {
  const { getByRole } = render(<Combobox />)
  expect(getByRole('combobox')).toBeVisible()
})

test('generates consistent facility data', () => {
  // Since the seed is the same, the same data is generated every time
  console.log('First facility:', facilities[0])
  // Example: { value: 'TokyoCenter1', label: 'TokyoCenter1' }
  
  expect(facilities).toHaveLength(30)
  expect(facilities[0].value).toBe('TokyoCenter1') // Same value every time
})

Detailed Usage of Seed

1. Numeric Seed

faker.seed(123)           // Single number
faker.seed([123, 456])    // Array of multiple numbers

2. Reset Seed for Each Test

describe('Combobox tests', () => {
  beforeEach(() => {
    faker.seed(123) // Reset seed before each test
  })

  test('test 1', () => {
    // Starts with the same data every time
  })

  test('test 2', () => {
    // Starts with the same data every time
  })
})

3. Conditional Seed Setting

// Fixed seed in development, true random in production
if (process.env.NODE_ENV === 'test') {
  faker.seed(123)
}

4. Dynamic Seed (Date-based)

// Date-based seed (changes daily)
const today = new Date().toDateString()
const seed = today.split('').reduce((acc, char) => acc + char.charCodeAt(0), 0)
faker.seed(seed)

Actual Usage Patterns

Pattern 1: Fixed Data for Testing

// Test-only fixtures
export const createTestFacilities = () => {
  faker.seed(999) // Fixed seed
  return Array.from({ length: 10 }, (_, i) => ({
    value: `Test Facility${i + 1}`,
    label: `Test Facility${i + 1}`
  }))
}

Pattern 2: Supporting Snapshot Tests

test('facility list snapshot', () => {
  faker.seed(123) // Fixed seed
  const facilities = generateFacilities(5)
  expect(facilities).toMatchSnapshot() // Same result every time
})

Pattern 3: Seed Value Management

const SEEDS = {
  FACILITIES: 123,
  USERS: 456,
  PRODUCTS: 789
} as const

// Separate seeds by purpose
export const generateFacilities = () => {
  faker.seed(SEEDS.FACILITIES)
  // Generate facility data
}

export const generateUsers = () => {
  faker.seed(SEEDS.USERS)
  // Generate user data
}

Important Notes

1. Seed is global: Once set, it affects all subsequent faker calls
2. Order matters: Even with the same seed, changing the order of faker method calls will change the results
3. Version dependent: If the Faker.js version changes, the same seed may produce different results

Using seeds improves test reproducibility and makes debugging easier!