diff --git a/src/arrays.test.ts b/src/arrays.test.ts
new file mode 100644
index 0000000..53bf689
--- /dev/null
+++ b/src/arrays.test.ts
@@ -0,0 +1,112 @@
+import { describe, it, expect } from 'vitest';
+import {
+  chunk,
+  compact,
+  uniq,
+  uniqBy,
+  flatten,
+  flattenDeep,
+  difference,
+  intersection,
+  groupBy,
+  partition,
+  sample,
+  shuffle,
+} from '../src/arrays';
+
+describe('chunk', () => {
+  it('splits array into chunks of given size', () => {
+    expect(chunk([1, 2, 3, 4, 5], 2)).toEqual([[1, 2], [3, 4], [5]]);
+  });
+  it('handles size larger than array', () => {
+    expect(chunk([1, 2], 5)).toEqual([[1, 2]]);
+  });
+  it('handles empty array', () => {
+    expect(chunk([], 3)).toEqual([]);
+  });
+});
+
+describe('compact', () => {
+  it('removes falsy values', () => {
+    expect(compact([0, 1, false, 2, '', 3])).toEqual([1, 2, 3]);
+  });
+  it('handles all falsy', () => {
+    expect(compact([false, null, undefined, 0, ''])).toEqual([]);
+  });
+});
+
+describe('uniq', () => {
+  it('removes duplicates', () => {
+    expect(uniq([1, 2, 2, 3, 3, 3])).toEqual([1, 2, 3]);
+  });
+  it('handles empty array', () => {
+    expect(uniq([])).toEqual([]);
+  });
+});
+
+describe('uniqBy', () => {
+  it('removes duplicates by key', () => {
+    const arr = [{ id: 1 }, { id: 2 }, { id: 1 }];
+    expect(uniqBy(arr, 'id')).toEqual([{ id: 1 }, { id: 2 }]);
+  });
+});
+
+describe('flatten', () => {
+  it('flattens one level', () => {
+    expect(flatten([[1, 2], [3, 4]])).toEqual([1, 2, 3, 4]);
+  });
+});
+
+describe('flattenDeep', () => {
+  it('flattens all levels', () => {
+    expect(flattenDeep([1, [2, [3, [4]]]])).toEqual([1, 2, 3, 4]);
+  });
+});
+
+describe('difference', () => {
+  it('returns items not in second array', () => {
+    expect(difference([1, 2, 3, 4], [2, 4])).toEqual([1, 3]);
+  });
+});
+
+describe('intersection', () => {
+  it('returns common items', () => {
+    expect(intersection([1, 2, 3], [2, 3, 4])).toEqual([2, 3]);
+  });
+});
+
+describe('groupBy', () => {
+  it('groups by key', () => {
+    const arr = [{ type: 'a' }, { type: 'b' }, { type: 'a' }];
+    const result = groupBy(arr, 'type');
+    expect(result.a).toHaveLength(2);
+    expect(result.b).toHaveLength(1);
+  });
+});
+
+describe('partition', () => {
+  it('splits by predicate', () => {
+    const [evens, odds] = partition([1, 2, 3, 4], n => n % 2 === 0);
+    expect(evens).toEqual([2, 4]);
+    expect(odds).toEqual([1, 3]);
+  });
+});
+
+describe('sample', () => {
+  it('returns element from array', () => {
+    const arr = [1, 2, 3];
+    const result = sample(arr);
+    expect(arr).toContain(result);
+  });
+  it('handles empty array', () => {
+    expect(sample([])).toBeUndefined();
+  });
+});
+
+describe('shuffle', () => {
+  it('returns array with same elements', () => {
+    const arr = [1, 2, 3, 4, 5];
+    const result = shuffle(arr);
+    expect(result.sort()).toEqual([1, 2, 3, 4, 5]);
+  });
+});
diff --git a/src/dates.test.ts b/src/dates.test.ts
new file mode 100644
index 0000000..4ed3190
--- /dev/null
+++ b/src/dates.test.ts
@@ -0,0 +1,81 @@
+import { describe, it, expect } from 'vitest';
+import {
+  formatDate,
+  formatRelative,
+  isToday,
+  isYesterday,
+  daysBetween,
+  addDays,
+} from '../src/dates';
+
+describe('formatDate', () => {
+  it('formats a date string', () => {
+    const result = formatDate('2026-05-27T10:00:00Z');
+    expect(result).toBeTruthy();
+    expect(typeof result).toBe('string');
+  });
+  it('handles invalid date', () => {
+    expect(formatDate('invalid')).toBeFalsy();
+  });
+});
+
+describe('formatRelative', () => {
+  it('returns relative time string', () => {
+    const result = formatRelative(new Date().toISOString());
+    expect(result).toBeTruthy();
+    expect(typeof result).toBe('string');
+  });
+  it('handles past dates', () => {
+    const past = new Date(Date.now() - 86400000).toISOString();
+    const result = formatRelative(past);
+    expect(result).toBeTruthy();
+  });
+  it('handles future dates', () => {
+    const future = new Date(Date.now() + 86400000).toISOString();
+    const result = formatRelative(future);
+    expect(result).toBeTruthy();
+  });
+});
+
+describe('isToday', () => {
+  it('returns true for today', () => {
+    expect(isToday(new Date().toISOString())).toBe(true);
+  });
+  it('returns false for yesterday', () => {
+    const yesterday = new Date(Date.now() - 86400000).toISOString();
+    expect(isToday(yesterday)).toBe(false);
+  });
+});
+
+describe('isYesterday', () => {
+  it('returns true for yesterday', () => {
+    const yesterday = new Date(Date.now() - 86400000).toISOString();
+    expect(isYesterday(yesterday)).toBe(true);
+  });
+  it('returns false for today', () => {
+    expect(isYesterday(new Date().toISOString())).toBe(false);
+  });
+});
+
+describe('daysBetween', () => {
+  it('calculates days between dates', () => {
+    const start = '2026-05-01T00:00:00Z';
+    const end = '2026-05-05T00:00:00Z';
+    expect(daysBetween(start, end)).toBe(4);
+  });
+  it('returns 0 for same date', () => {
+    const date = '2026-05-01T00:00:00Z';
+    expect(daysBetween(date, date)).toBe(0);
+  });
+});
+
+describe('addDays', () => {
+  it('adds days to date', () => {
+    const result = addDays('2026-05-01T00:00:00Z', 5);
+    expect(result.includes('2026-05-06')).toBe(true);
+  });
+  it('subtracts days with negative', () => {
+    const result = addDays('2026-05-01T00:00:00Z', -5);
+    expect(result.includes('2026-04-26')).toBe(true);
+  });
+});
diff --git a/src/money.test.ts b/src/money.test.ts
new file mode 100644
index 0000000..66bffe5
--- /dev/null
+++ b/src/money.test.ts
@@ -0,0 +1,80 @@
+import { describe, it, expect } from 'vitest';
+import {
+  formatMoney,
+  parseMoney,
+  toCents,
+  fromCents,
+  addMoney,
+  subtractMoney,
+  multiplyMoney,
+  divideMoney,
+} from '../src/money';
+
+describe('formatMoney', () => {
+  it('formats cents to dollars', () => {
+    expect(formatMoney(1234)).toBe('$12.34');
+  });
+  it('handles zero', () => {
+    expect(formatMoney(0)).toBe('$0.00');
+  });
+  it('handles negative', () => {
+    expect(formatMoney(-500)).toBe('-$5.00');
+  });
+});
+
+describe('parseMoney', () => {
+  it('parses dollar string to cents', () => {
+    expect(parseMoney('$12.34')).toBe(1234);
+  });
+  it('parses without dollar sign', () => {
+    expect(parseMoney('12.34')).toBe(1234);
+  });
+  it('handles invalid input', () => {
+    expect(parseMoney('abc')).toBe(0);
+  });
+});
+
+describe('toCents', () => {
+  it('converts dollars to cents', () => {
+    expect(toCents(12.34)).toBe(1234);
+  });
+  it('handles whole dollars', () => {
+    expect(toCents(100)).toBe(10000);
+  });
+});
+
+describe('fromCents', () => {
+  it('converts cents to dollars', () => {
+    expect(fromCents(1234)).toBe(12.34);
+  });
+  it('handles zero', () => {
+    expect(fromCents(0)).toBe(0);
+  });
+});
+
+describe('addMoney', () => {
+  it('adds two amounts in cents', () => {
+    expect(addMoney(1000, 500)).toBe(1500);
+  });
+});
+
+describe('subtractMoney', () => {
+  it('subtracts two amounts in cents', () => {
+    expect(subtractMoney(1000, 300)).toBe(700);
+  });
+});
+
+describe('multiplyMoney', () => {
+  it('multiplies cents by factor', () => {
+    expect(multiplyMoney(1000, 3)).toBe(3000);
+  });
+});
+
+describe('divideMoney', () => {
+  it('divides cents by factor', () => {
+    expect(divideMoney(1000, 4)).toBe(250);
+  });
+  it('handles division by zero', () => {
+    expect(divideMoney(1000, 0)).toBe(0);
+  });
+});