diff --git a/js-fp3.tests/.gitignore b/js-fp3.tests/.gitignore
new file mode 100644
index 0000000..872d5f6
--- /dev/null
+++ b/js-fp3.tests/.gitignore
@@ -0,0 +1,143 @@
+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+lerna-debug.log*
+
+# Diagnostic reports (https://nodejs.org/api/report.html)
+report.[0-9]*.[0-9]*.[0-9]*.[0-9]*.json
+
+# Runtime data
+pids
+*.pid
+*.seed
+*.pid.lock
+
+# Directory for instrumented libs generated by jscoverage/JSCover
+lib-cov
+
+# Coverage directory used by tools like istanbul
+coverage
+*.lcov
+
+# nyc test coverage
+.nyc_output
+
+# Grunt intermediate storage (https://gruntjs.com/creating-plugins#storing-task-files)
+.grunt
+
+# Bower dependency directory (https://bower.io/)
+bower_components
+
+# node-waf configuration
+.lock-wscript
+
+# Compiled binary addons (https://nodejs.org/api/addons.html)
+build/Release
+
+# Dependency directories
+node_modules/
+jspm_packages/
+
+# Snowpack dependency directory (https://snowpack.dev/)
+web_modules/
+
+# TypeScript cache
+*.tsbuildinfo
+
+# Optional npm cache directory
+.npm
+
+# Optional eslint cache
+.eslintcache
+
+# Optional stylelint cache
+.stylelintcache
+
+# Optional REPL history
+.node_repl_history
+
+# Output of 'npm pack'
+*.tgz
+
+# Yarn Integrity file
+.yarn-integrity
+
+# dotenv environment variable files
+.env
+.env.*
+!.env.example
+
+# parcel-bundler cache (https://parceljs.org/)
+.cache
+.parcel-cache
+
+# Next.js build output
+.next
+out
+
+# Nuxt.js build / generate output
+.nuxt
+dist
+.output
+
+# Gatsby files
+.cache/
+# Comment in the public line in if your project uses Gatsby and not Next.js
+# https://nextjs.org/blog/next-9-1#public-directory-support
+# public
+
+# vuepress build output
+.vuepress/dist
+
+# vuepress v2.x temp directory
+.temp
+
+# Sveltekit cache directory
+.svelte-kit/
+
+# vitepress build output
+**/.vitepress/dist
+
+# vitepress cache directory
+**/.vitepress/cache
+
+# Docusaurus cache and generated files
+.docusaurus
+
+# Serverless directories
+.serverless/
+
+# FuseBox cache
+.fusebox/
+
+# DynamoDB Local files
+.dynamodb/
+
+# Firebase cache directory
+.firebase/
+
+# TernJS port file
+.tern-port
+
+# Stores VSCode versions used for testing VSCode extensions
+.vscode-test
+
+# pnpm
+.pnpm-store
+
+# yarn v3
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions
+
+# Vite files
+vite.config.js.timestamp-*
+vite.config.ts.timestamp-*
+.vite/
diff --git a/js-fp3.tests/account.test.js b/js-fp3.tests/account.test.js
new file mode 100644
index 0000000..751cf15
--- /dev/null
+++ b/js-fp3.tests/account.test.js
@@ -0,0 +1,54 @@
+import { describe, it, expect } from 'vitest';
+import { openAccount, withdraw } from '../js-fp3/domain/account.js';
+import { createMoney } from '../js-fp3/domain/money.js';
+
+describe('account domain', () => {
+  it('opening an account with a negative balance returns openingBalanceMustBeNonNegative', () => {
+    const result = openAccount('id-1', createMoney(-1));
+
+    expect(result.error.type).toBe('OpeningBalanceMustBeNonNegative');
+  });
+
+  it('withdrawing a zero amount returns amountMustBePositive', () => {
+    const account = openAccount('id-1', createMoney(100)).value;
+
+    const result = withdraw(account, createMoney(0));
+
+    expect(result.error.type).toBe('AmountMustBePositive');
+  });
+
+  it('withdrawing a negative amount returns amountMustBePositive', () => {
+    const account = openAccount('id-1', createMoney(100)).value;
+
+    const result = withdraw(account, createMoney(-10));
+
+    expect(result.error.type).toBe('AmountMustBePositive');
+  });
+
+  it('withdrawing more than the balance returns insufficientBalance', () => {
+    const account = openAccount('id-1', createMoney(100)).value;
+
+    const result = withdraw(account, createMoney(101));
+
+    expect(result.error.type).toBe('InsufficientBalance');
+  });
+
+  it('insufficientBalance error reports the current balance and attempted amount', () => {
+    const account = openAccount('id-1', createMoney(100)).value;
+
+    const error = withdraw(account, createMoney(150)).error;
+
+    expect(error.type).toBe('InsufficientBalance');
+    expect(error.balance.amount).toBe(100);
+    expect(error.amount.amount).toBe(150);
+  });
+
+  it('successive withdrawals are each applied to the running balance', () => {
+    const account = openAccount('id-1', createMoney(300)).value;
+
+    const updatedAccount1 = withdraw(account, createMoney(100)).value;
+    const updatedAccount2 = withdraw(updatedAccount1, createMoney(100)).value;
+
+    expect(updatedAccount2.balance.amount).toBe(100);
+  });
+});
diff --git a/js-fp3.tests/package-lock.json b/js-fp3.tests/package-lock.json
new file mode 100644
index 0000000..4bb3807
--- /dev/null
+++ b/js-fp3.tests/package-lock.json
@@ -0,0 +1,610 @@
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+      "name": "js-fp3-tests",
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+      "devDependencies": {
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+        "darwin"
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+        "@vitest/pretty-format": "^2.1.9",
+        "@vitest/runner": "2.1.9",
+        "@vitest/snapshot": "2.1.9",
+        "@vitest/spy": "2.1.9",
+        "@vitest/utils": "2.1.9",
+        "chai": "^5.1.2",
+        "debug": "^4.3.7",
+        "expect-type": "^1.1.0",
+        "magic-string": "^0.30.12",
+        "pathe": "^1.1.2",
+        "std-env": "^3.8.0",
+        "tinybench": "^2.9.0",
+        "tinyexec": "^0.3.1",
+        "tinypool": "^1.0.1",
+        "tinyrainbow": "^1.2.0",
+        "vite": "^5.0.0",
+        "vite-node": "2.1.9",
+        "why-is-node-running": "^2.3.0"
+      },
+      "bin": {
+        "vitest": "vitest.mjs"
+      },
+      "engines": {
+        "node": "^18.0.0 || >=20.0.0"
+      },
+      "funding": {
+        "url": "https://opencollective.com/vitest"
+      },
+      "peerDependencies": {
+        "@edge-runtime/vm": "*",
+        "@types/node": "^18.0.0 || >=20.0.0",
+        "@vitest/browser": "2.1.9",
+        "@vitest/ui": "2.1.9",
+        "happy-dom": "*",
+        "jsdom": "*"
+      },
+      "peerDependenciesMeta": {
+        "@edge-runtime/vm": {
+          "optional": true
+        },
+        "@types/node": {
+          "optional": true
+        },
+        "@vitest/browser": {
+          "optional": true
+        },
+        "@vitest/ui": {
+          "optional": true
+        },
+        "happy-dom": {
+          "optional": true
+        },
+        "jsdom": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/why-is-node-running": {
+      "version": "2.3.0",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "siginfo": "^2.0.0",
+        "stackback": "0.0.2"
+      },
+      "bin": {
+        "why-is-node-running": "cli.js"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    }
+  }
+}
diff --git a/js-fp3.tests/package.json b/js-fp3.tests/package.json
new file mode 100644
index 0000000..f72f3d3
--- /dev/null
+++ b/js-fp3.tests/package.json
@@ -0,0 +1,13 @@
+{
+  "name": "js-fp3-tests",
+  "version": "1.0.0",
+  "description": "Vitest tests for js-fp3",
+  "type": "module",
+  "scripts": {
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "devDependencies": {
+    "vitest": "^2.0.0"
+  }
+}
diff --git a/js-fp3.tests/patterns.test.js b/js-fp3.tests/patterns.test.js
new file mode 100644
index 0000000..4182c41
--- /dev/null
+++ b/js-fp3.tests/patterns.test.js
@@ -0,0 +1,82 @@
+import { describe, it, expect, vi } from 'vitest';
+import { match, when, any } from '../js-fp3/library/patterns.js';
+
+describe('when', () => {
+  it('returns a two-element tuple of [predicate, handler]', () => {
+    const predicate = () => true;
+    const handler = () => 'result';
+
+    const branch = when(predicate, handler);
+
+    expect(branch).toEqual([predicate, handler]);
+  });
+});
+
+describe('any', () => {
+  it('returns true for any value', () => {
+    expect(any(0)).toBe(true);
+    expect(any(null)).toBe(true);
+    expect(any('string')).toBe(true);
+    expect(any({ x: 1 })).toBe(true);
+  });
+});
+
+describe('match', () => {
+  it('executes the handler of the first matching branch', () => {
+    const result = match(
+      when((v) => v === 1, () => 'one'),
+      when((v) => v === 2, () => 'two'),
+      when(any, () => 'other')
+    )(1);
+
+    expect(result).toBe('one');
+  });
+
+  it('falls through to the next branch when the first does not match', () => {
+    const result = match(
+      when((v) => v === 1, () => 'one'),
+      when((v) => v === 2, () => 'two'),
+      when(any, () => 'other')
+    )(2);
+
+    expect(result).toBe('two');
+  });
+
+  it('uses the any catch-all when no earlier branch matches', () => {
+    const result = match(
+      when((v) => v === 1, () => 'one'),
+      when(any, () => 'catch-all')
+    )(99);
+
+    expect(result).toBe('catch-all');
+  });
+
+  it('passes the value to the handler', () => {
+    const result = match(
+      when(any, (v) => v * 2)
+    )(7);
+
+    expect(result).toBe(14);
+  });
+
+  it('throws when no branch matches', () => {
+    expect(() =>
+      match(
+        when((v) => v === 1, () => 'one')
+      )(99)
+    ).toThrow('No matching pattern');
+  });
+
+  it('does not call handlers for branches that were not selected', () => {
+    const skipped = vi.fn(() => 'skipped');
+    const selected = vi.fn(() => 'selected');
+
+    match(
+      when((v) => v === 1, selected),
+      when(any, skipped)
+    )(1);
+
+    expect(selected).toHaveBeenCalledOnce();
+    expect(skipped).not.toHaveBeenCalled();
+  });
+});
diff --git a/js-fp3.tests/result.test.js b/js-fp3.tests/result.test.js
new file mode 100644
index 0000000..e8479ab
--- /dev/null
+++ b/js-fp3.tests/result.test.js
@@ -0,0 +1,203 @@
+import { describe, it, expect, vi } from 'vitest';
+import {
+  ok,
+  fail,
+  catchResult,
+  bind,
+  map,
+  tap,
+  tapError,
+  mapError,
+  match,
+  switch_,
+  pipe,
+} from '../js-fp3/library/result.js';
+
+describe('ok / fail construction', () => {
+  it('ok is success', () => {
+    const result = ok(42);
+
+    expect(result.isSuccess).toBe(true);
+    expect(result.isFailure).toBe(false);
+  });
+
+  it('fail is failure', () => {
+    const result = fail('oops');
+
+    expect(result.isFailure).toBe(true);
+    expect(result.isSuccess).toBe(false);
+  });
+
+  it('ok exposes value', () => {
+    const result = ok(42);
+
+    expect(result.value).toBe(42);
+  });
+
+  it('fail exposes error', () => {
+    const result = fail('oops');
+
+    expect(result.error).toBe('oops');
+  });
+});
+
+describe('match', () => {
+  it('calls onSuccess for ok', () => {
+    const result = ok(10);
+
+    const output = match((v) => `value:${v}`, (e) => `error:${e}`)(result);
+
+    expect(output).toBe('value:10');
+  });
+
+  it('calls onFailure for fail', () => {
+    const result = fail('bad');
+
+    const output = match((v) => `value:${v}`, (e) => `error:${e}`)(result);
+
+    expect(output).toBe('error:bad');
+  });
+});
+
+describe('switch_', () => {
+  it('calls onSuccess for ok', () => {
+    const result = ok(7);
+    let called = false;
+
+    switch_(() => { called = true; }, () => {})(result);
+
+    expect(called).toBe(true);
+  });
+
+  it('calls onFailure for fail', () => {
+    const result = fail('err');
+    let called = false;
+
+    switch_(() => {}, () => { called = true; })(result);
+
+    expect(called).toBe(true);
+  });
+});
+
+describe('catchResult', () => {
+  it('returns ok when no exception is thrown', () => {
+    const result = catchResult(() => 99, (ex) => ex.message);
+
+    expect(result.isSuccess).toBe(true);
+    expect(result.value).toBe(99);
+  });
+
+  it('returns fail when an exception is thrown', () => {
+    const result = catchResult(
+      () => { throw new Error('boom'); },
+      (ex) => ex.message
+    );
+
+    expect(result.isFailure).toBe(true);
+    expect(result.error).toBe('boom');
+  });
+});
+
+describe('bind', () => {
+  it('chains to the next result on success', () => {
+    const result = bind((v) => ok(`got ${v}`))(ok(5));
+
+    expect(result.value).toBe('got 5');
+  });
+
+  it('short-circuits on failure without calling the binder', () => {
+    const binder = vi.fn((v) => ok(`got ${v}`));
+    const result = bind(binder)(fail('nope'));
+
+    expect(binder).not.toHaveBeenCalled();
+    expect(result.error).toBe('nope');
+  });
+
+  it('propagates the inner failure when the binder returns fail', () => {
+    const result = bind(() => fail('inner fail'))(ok(5));
+
+    expect(result.error).toBe('inner fail');
+  });
+});
+
+describe('map', () => {
+  it('transforms the value on success', () => {
+    const result = map((v) => v * 2)(ok(3));
+
+    expect(result.value).toBe(6);
+  });
+
+  it('does not call the mapper on failure', () => {
+    const mapper = vi.fn((v) => v * 2);
+    const result = map(mapper)(fail('err'));
+
+    expect(mapper).not.toHaveBeenCalled();
+    expect(result.error).toBe('err');
+  });
+});
+
+describe('tap', () => {
+  it('calls the action and returns the original result on success', () => {
+    let seen = -1;
+    const result = tap((v) => { seen = v; })(ok(8));
+
+    expect(seen).toBe(8);
+    expect(result.value).toBe(8);
+  });
+
+  it('does not call the action on failure', () => {
+    const action = vi.fn();
+    tap(action)(fail('err'));
+
+    expect(action).not.toHaveBeenCalled();
+  });
+});
+
+describe('tapError', () => {
+  it('calls the action and returns the original result on failure', () => {
+    let seen = '';
+    const result = tapError((e) => { seen = e; })(fail('bad'));
+
+    expect(seen).toBe('bad');
+    expect(result.error).toBe('bad');
+  });
+
+  it('does not call the action on success', () => {
+    const action = vi.fn();
+    tapError(action)(ok(1));
+
+    expect(action).not.toHaveBeenCalled();
+  });
+});
+
+describe('mapError', () => {
+  it('transforms the error on failure', () => {
+    const result = mapError((e) => e.length)(fail('oops'));
+
+    expect(result.error).toBe(4);
+  });
+
+  it('does not call the mapper on success', () => {
+    const mapper = vi.fn((e) => e.length);
+    const result = mapError(mapper)(ok(1));
+
+    expect(mapper).not.toHaveBeenCalled();
+    expect(result.value).toBe(1);
+  });
+});
+
+describe('pipe', () => {
+  it('applies functions left to right starting from the initial result', () => {
+    const result = pipe(ok(2), map((v) => v + 1), map((v) => v * 3));
+
+    expect(result.value).toBe(9);
+  });
+
+  it('short-circuits on the first failure', () => {
+    const mapper = vi.fn((v) => v * 2);
+    const result = pipe(fail('stop'), map(mapper));
+
+    expect(mapper).not.toHaveBeenCalled();
+    expect(result.error).toBe('stop');
+  });
+});
diff --git a/js-fp3.tests/withdrawMoney.test.js b/js-fp3.tests/withdrawMoney.test.js
new file mode 100644
index 0000000..c3f6f78
--- /dev/null
+++ b/js-fp3.tests/withdrawMoney.test.js
@@ -0,0 +1,86 @@
+import { describe, it, expect } from 'vitest';
+import { createWithdrawMoney } from '../js-fp3/application/accountApp.js';
+import { openAccount } from '../js-fp3/domain/account.js';
+import { createMoney } from '../js-fp3/domain/money.js';
+import { createInMemoryRepository } from '../js-fp3/infrastructure/repository.js';
+
+const buildHandler = () => {
+  const repo = createInMemoryRepository();
+  const withdraw = createWithdrawMoney(repo).value;
+  return { withdraw, repo };
+};
+
+describe('withdrawMoney application use case', () => {
+  it('withdrawing from an account reduces its balance by the withdrawn amount', () => {
+    const { withdraw, repo } = buildHandler();
+    const accountId = 'acc-1';
+    repo.saveAccount(openAccount(accountId, createMoney(200)).value);
+
+    withdraw(accountId, 75);
+
+    const account = repo.loadAccount(accountId).value;
+    expect(account.balance.amount).toBe(125);
+  });
+
+  it('withdrawing the entire balance leaves the account at zero', () => {
+    const { withdraw, repo } = buildHandler();
+    const accountId = 'acc-2';
+    repo.saveAccount(openAccount(accountId, createMoney(100)).value);
+
+    withdraw(accountId, 100);
+
+    const account = repo.loadAccount(accountId).value;
+    expect(account.balance.amount).toBe(0);
+  });
+
+  it('withdrawing from a non-existent account returns accountNotFound', () => {
+    const { withdraw } = buildHandler();
+
+    const result = withdraw('no-such-id', 50);
+
+    expect(result.error.type).toBe('AccountNotFound');
+  });
+
+  it('withdrawing more than the available balance returns insufficientBalance', () => {
+    const { withdraw, repo } = buildHandler();
+    const accountId = 'acc-3';
+    repo.saveAccount(openAccount(accountId, createMoney(50)).value);
+
+    const result = withdraw(accountId, 100);
+
+    expect(result.error.type).toBe('InsufficientBalance');
+  });
+
+  it('accountNotFound error reports the requested account id', () => {
+    const { withdraw } = buildHandler();
+    const accountId = 'missing-acc';
+
+    const error = withdraw(accountId, 50).error;
+
+    expect(error.type).toBe('AccountNotFound');
+    expect(error.accountId).toBe(accountId);
+  });
+
+  it('insufficientBalance error reports the current balance and attempted amount', () => {
+    const { withdraw, repo } = buildHandler();
+    const accountId = 'acc-4';
+    repo.saveAccount(openAccount(accountId, createMoney(50)).value);
+
+    const error = withdraw(accountId, 120).error;
+
+    expect(error.type).toBe('InsufficientBalance');
+    expect(error.balance.amount).toBe(50);
+    expect(error.amount.amount).toBe(120);
+  });
+
+  it('after a failed withdrawal the balance is unchanged', () => {
+    const { withdraw, repo } = buildHandler();
+    const accountId = 'acc-5';
+    repo.saveAccount(openAccount(accountId, createMoney(50)).value);
+
+    withdraw(accountId, 999);
+
+    const account = repo.loadAccount(accountId).value;
+    expect(account.balance.amount).toBe(50);
+  });
+});