feat: add initial seed parameter to constructors

src/faker.ts

@@ -125,6 +125,10 @@ export class Faker extends SimpleFaker {
    * Specify this only if you want to use it to achieve a specific goal,
    * such as sharing the same random generator with other instances/tools.
    * Defaults to faker's Mersenne Twister based pseudo random number generator.
+   * @param options.seed The initial seed to use.
+   * The seed can be used to generate reproducible values.
+   * Refer to the `seed()` method for more information.
+   * Defaults to a random seed.
    *
    * @example
    * import { Faker, es } from '@faker-js/faker';
@@ -157,8 +161,18 @@ export class Faker extends SimpleFaker {
      * @default generateMersenne53Randomizer()
      */
     randomizer?: Randomizer;
+
+    /**
+     * The initial seed to use.
+     * The seed can be used to generate reproducible values.
+     *
+     * Refer to the `seed()` method for more information.
+     *
+     * @default randomSeed()
+     */
+    seed?: number;
   }) {
-    super({ randomizer: options.randomizer });
+    super({ randomizer: options.randomizer, seed: options.seed });
 
     let { locale } = options;
 

src/internal/seed.ts

@@ -0,0 +1,8 @@
+/**
+ * Generates a random seed.
+ *
+ * @internal
+ */
+export function randomSeed(): number {
+  return Math.ceil(Math.random() * Number.MAX_SAFE_INTEGER);
+}

src/simple-faker.ts

@@ -1,3 +1,4 @@
+import { randomSeed } from './internal/seed';
 import { DatatypeModule } from './modules/datatype';
 import { SimpleDateModule } from './modules/date';
 import { SimpleHelpersModule } from './modules/helpers';
@@ -97,6 +98,10 @@ export class SimpleFaker {
    * Specify this only if you want to use it to achieve a specific goal,
    * such as sharing the same random generator with other instances/tools.
    * Defaults to faker's Mersenne Twister based pseudo random number generator.
+   * @param options.seed The initial seed to use.
+   * The seed can be used to generate reproducible values.
+   * Refer to the `seed()` method for more information.
+   * Defaults to a random seed.
    *
    * @example
    * import { SimpleFaker } from '@faker-js/faker';
@@ -120,11 +125,25 @@ export class SimpleFaker {
        * @default generateMersenne53Randomizer()
        */
       randomizer?: Randomizer;
+
+      /**
+       * The initial seed to use.
+       * The seed can be used to generate reproducible values.
+       *
+       * Refer to the `seed()` method for more information.
+       *
+       * @default randomSeed()
+       */
+      seed?: number;
     } = {}
   ) {
-    const { randomizer = generateMersenne53Randomizer() } = options;
+    const { randomizer, seed } = options;
+
+    if (randomizer != null && seed != null) {
+      randomizer.seed(seed);
+    }
 
-    this._randomizer = randomizer;
+    this._randomizer = randomizer ?? generateMersenne53Randomizer(seed);
   }
 
   /**
@@ -247,9 +266,7 @@ export class SimpleFaker {
    * @since 6.0.0
    */
   seed(seed?: number | number[]): number | number[];
-  seed(
-    seed: number | number[] = Math.ceil(Math.random() * Number.MAX_SAFE_INTEGER)
-  ): number | number[] {
+  seed(seed: number | number[] = randomSeed()): number | number[] {
     this._randomizer.seed(seed);
 
     return seed;

src/utils/mersenne.ts

@@ -1,14 +1,19 @@
 import { MersenneTwister19937 } from '../internal/mersenne';
+import { randomSeed } from '../internal/seed';
 import type { Randomizer } from '../randomizer';
 
 /**
  * Generates a MersenneTwister19937 randomizer with 32 bits of precision.
  * This is the default randomizer used by faker prior to v9.0.
+ *
+ * @param seed The initial seed to use. Defaults to a random number.
  */
-export function generateMersenne32Randomizer(): Randomizer {
+export function generateMersenne32Randomizer(
+  seed: number = randomSeed()
+): Randomizer {
   const twister = new MersenneTwister19937();
 
-  twister.initGenrand(Math.ceil(Math.random() * Number.MAX_SAFE_INTEGER));
+  twister.initGenrand(seed);
 
   return {
     next(): number {
@@ -27,11 +32,15 @@ export function generateMersenne32Randomizer(): Randomizer {
 /**
  * Generates a MersenneTwister19937 randomizer with 53 bits of precision.
  * This is the default randomizer used by faker starting with v9.0.
+ *
+ * @param seed The initial seed to use. Defaults to a random number.
  */
-export function generateMersenne53Randomizer(): Randomizer {
+export function generateMersenne53Randomizer(
+  seed: number = randomSeed()
+): Randomizer {
   const twister = new MersenneTwister19937();
 
-  twister.initGenrand(Math.ceil(Math.random() * Number.MAX_SAFE_INTEGER));
+  twister.initGenrand(seed);
 
   return {
     next(): number {

test/faker.spec.ts

@@ -1,18 +1,10 @@
 import type { MockInstance } from 'vitest';
 import { describe, expect, it, vi } from 'vitest';
-import { Faker, faker } from '../src';
+import { Faker, faker, generateMersenne32Randomizer } from '../src';
 import { FakerError } from '../src/errors/faker-error';
 import { keys } from '../src/internal/keys';
 
 describe('faker', () => {
-  it('should throw error if no locales passed', () => {
-    expect(() => new Faker({ locale: [] })).toThrow(
-      new FakerError(
-        'The locale option must contain at least one locale definition.'
-      )
-    );
-  });
-
   it('should not log anything on startup', async () => {
     const spies: MockInstance[] = keys(console)
       .filter((key) => typeof console[key] === 'function')
@@ -69,19 +61,70 @@ describe('faker', () => {
     });
   });
 
-  describe('randomizer', () => {
-    it('should be possible to provide a custom Randomizer', () => {
-      const customFaker = new Faker({
-        locale: {},
-        randomizer: {
-          next: () => 0,
-          seed: () => void 0,
-        },
+  describe('constructor()', () => {
+    describe('locale', () => {
+      it('should throw error if no locales passed', () => {
+        expect(() => new Faker({ locale: [] })).toThrow(
+          new FakerError(
+            'The locale option must contain at least one locale definition.'
+          )
+        );
+      });
+    });
+
+    describe('randomizer', () => {
+      it('should be possible to provide a custom Randomizer', () => {
+        const customFaker = new Faker({
+          locale: {},
+          randomizer: {
+            next: () => 0,
+            seed: () => void 0,
+          },
+        });
+
+        expect(customFaker.number.int()).toBe(0);
+        expect(customFaker.number.int()).toBe(0);
+        expect(customFaker.number.int()).toBe(0);
+      });
+    });
+
+    describe('seed', () => {
+      it('should be possible to provide an initial seed', () => {
+        const customFaker = new Faker({
+          locale: {},
+          seed: 12345,
+        });
+
+        expect(customFaker.number.int()).toBe(8373237378417847);
+        expect(customFaker.number.int()).toBe(2849657659447330);
+        expect(customFaker.number.int()).toBe(1656593383470774);
+
+        customFaker.seed(12345);
+
+        expect(customFaker.number.int()).toBe(8373237378417847);
+        expect(customFaker.number.int()).toBe(2849657659447330);
+        expect(customFaker.number.int()).toBe(1656593383470774);
       });
+    });
 
-      expect(customFaker.number.int()).toBe(0);
-      expect(customFaker.number.int()).toBe(0);
-      expect(customFaker.number.int()).toBe(0);
+    describe('randomizer+seed', () => {
+      it('should take apply both the randomizer and seed', () => {
+        const customFaker = new Faker({
+          locale: {},
+          randomizer: generateMersenne32Randomizer(67890),
+          seed: 12345,
+        });
+
+        expect(customFaker.number.int()).toBe(8373237322874880);
+        expect(customFaker.number.int()).toBe(8017800868134912);
+        expect(customFaker.number.int()).toBe(2849657711493120);
+
+        customFaker.seed(12345); // Retry with the expected seed
+
+        expect(customFaker.number.int()).toBe(8373237322874880);
+        expect(customFaker.number.int()).toBe(8017800868134912);
+        expect(customFaker.number.int()).toBe(2849657711493120);
+      });
     });
   });
 

test/internal/seed.spec.ts

@@ -0,0 +1,11 @@
+import { describe, expect, it } from 'vitest';
+import { randomSeed } from '../../src/internal/seed';
+
+describe('seed', () => {
+  it('should generate a random seed', () => {
+    const actual = randomSeed();
+
+    expect(actual).toBeTypeOf('number');
+    expect(actual).not.toBe(randomSeed());
+  });
+});

test/simple-faker.spec.ts

@@ -1,6 +1,6 @@
 import type { MockInstance } from 'vitest';
 import { describe, expect, it, vi } from 'vitest';
-import { SimpleFaker, simpleFaker } from '../src';
+import { generateMersenne32Randomizer, SimpleFaker, simpleFaker } from '../src';
 import { keys } from '../src/internal/keys';
 
 describe('simpleFaker', () => {
@@ -20,6 +20,60 @@ describe('simpleFaker', () => {
     }
   });
 
+  describe('constructor()', () => {
+    describe('randomizer', () => {
+      it('should be possible to provide a custom Randomizer', () => {
+        const customFaker = new SimpleFaker({
+          randomizer: {
+            next: () => 0,
+            seed: () => void 0,
+          },
+        });
+
+        expect(customFaker.number.int()).toBe(0);
+        expect(customFaker.number.int()).toBe(0);
+        expect(customFaker.number.int()).toBe(0);
+      });
+    });
+
+    describe('seed', () => {
+      it('should be possible to provide an initial seed', () => {
+        const customFaker = new SimpleFaker({
+          seed: 12345,
+        });
+
+        expect(customFaker.number.int()).toBe(8373237378417847);
+        expect(customFaker.number.int()).toBe(2849657659447330);
+        expect(customFaker.number.int()).toBe(1656593383470774);
+
+        customFaker.seed(12345); // Retry with the expected seed
+
+        expect(customFaker.number.int()).toBe(8373237378417847);
+        expect(customFaker.number.int()).toBe(2849657659447330);
+        expect(customFaker.number.int()).toBe(1656593383470774);
+      });
+    });
+
+    describe('randomizer+seed', () => {
+      it('should take apply both the randomizer and seed', () => {
+        const customFaker = new SimpleFaker({
+          randomizer: generateMersenne32Randomizer(67890),
+          seed: 12345,
+        });
+
+        expect(customFaker.number.int()).toBe(8373237322874880);
+        expect(customFaker.number.int()).toBe(8017800868134912);
+        expect(customFaker.number.int()).toBe(2849657711493120);
+
+        customFaker.seed(12345); // Retry with the expected seed
+
+        expect(customFaker.number.int()).toBe(8373237322874880);
+        expect(customFaker.number.int()).toBe(8017800868134912);
+        expect(customFaker.number.int()).toBe(2849657711493120);
+      });
+    });
+  });
+
   // This is only here for coverage
   // The actual test is in mersenne.spec.ts
   describe('seed()', () => {