diff --git a/src/arrays.test.ts b/src/arrays.test.ts
new file mode 100644
index 0000000..960c093
--- /dev/null
+++ b/src/arrays.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, it } from "vitest";
+import { chunk, dedupe, groupBy } from "./arrays";
+
+describe("chunk", () => {
+  it("splits arrays into fixed-size chunks", () => {
+    expect(chunk([1, 2, 3, 4, 5], 2)).toEqual([[1, 2], [3, 4], [5]]);
+  });
+
+  it("returns an empty array for non-positive chunk sizes", () => {
+    expect(chunk([1, 2, 3], 0)).toEqual([]);
+    expect(chunk([1, 2, 3], -1)).toEqual([]);
+  });
+});
+
+describe("dedupe", () => {
+  it("keeps first occurrences in insertion order", () => {
+    expect(dedupe(["a", "b", "a", "c", "b"])).toEqual(["a", "b", "c"]);
+  });
+});
+
+describe("groupBy", () => {
+  it("groups values by the key returned from the callback", () => {
+    const grouped = groupBy(
+      [
+        { type: "fruit", name: "apple" },
+        { type: "veg", name: "carrot" },
+        { type: "fruit", name: "banana" },
+      ],
+      (item) => item.type,
+    );
+
+    expect(grouped).toEqual({
+      fruit: [
+        { type: "fruit", name: "apple" },
+        { type: "fruit", name: "banana" },
+      ],
+      veg: [{ type: "veg", name: "carrot" }],
+    });
+  });
+});
diff --git a/src/dates.test.ts b/src/dates.test.ts
new file mode 100644
index 0000000..732eb06
--- /dev/null
+++ b/src/dates.test.ts
@@ -0,0 +1,41 @@
+import { describe, expect, it } from "vitest";
+import { addDays, daysBetween, isWeekend, startOfDayUTC } from "./dates";
+
+describe("daysBetween", () => {
+  it("returns rounded absolute day differences", () => {
+    const start = new Date("2026-01-01T00:00:00.000Z");
+    const end = new Date("2026-01-03T12:00:00.000Z");
+
+    expect(daysBetween(start, end)).toBe(3);
+    expect(daysBetween(end, start)).toBe(3);
+  });
+});
+
+describe("isWeekend", () => {
+  it("detects Saturday and Sunday as weekends", () => {
+    expect(isWeekend(new Date("2026-05-30T12:00:00.000Z"))).toBe(true);
+    expect(isWeekend(new Date("2026-05-31T12:00:00.000Z"))).toBe(true);
+  });
+
+  it("detects weekdays as non-weekends", () => {
+    expect(isWeekend(new Date("2026-05-29T12:00:00.000Z"))).toBe(false);
+  });
+});
+
+describe("addDays", () => {
+  it("returns a new date advanced by the requested number of days", () => {
+    const original = new Date("2026-01-10T00:00:00.000Z");
+    const result = addDays(original, 5);
+
+    expect(result.toISOString()).toBe("2026-01-15T00:00:00.000Z");
+    expect(original.toISOString()).toBe("2026-01-10T00:00:00.000Z");
+  });
+});
+
+describe("startOfDayUTC", () => {
+  it("returns midnight UTC for the given date", () => {
+    const result = startOfDayUTC(new Date("2026-05-31T19:45:30.000Z"));
+
+    expect(result.toISOString()).toBe("2026-05-31T00:00:00.000Z");
+  });
+});
diff --git a/src/money.test.ts b/src/money.test.ts
new file mode 100644
index 0000000..a40f2c2
--- /dev/null
+++ b/src/money.test.ts
@@ -0,0 +1,41 @@
+import { describe, expect, it } from "vitest";
+import { formatCents, netPayoutCents, platformFeeCents, splitEvenly } from "./money";
+
+describe("formatCents", () => {
+  it("formats USD and EUR with currency symbols", () => {
+    expect(formatCents(1234)).toBe("$12.34");
+    expect(formatCents(1234, "EUR")).toBe("€12.34");
+  });
+
+  it("prefixes unsupported currencies with the currency code", () => {
+    expect(formatCents(1234, "GBP")).toBe("GBP 12.34");
+  });
+});
+
+describe("platformFeeCents", () => {
+  it("returns zero when gross amount or basis points are non-positive", () => {
+    expect(platformFeeCents(0, 250)).toBe(0);
+    expect(platformFeeCents(1000, 0)).toBe(0);
+  });
+
+  it("rounds percentage fees to the nearest cent", () => {
+    expect(platformFeeCents(999, 250)).toBe(25);
+  });
+});
+
+describe("netPayoutCents", () => {
+  it("subtracts the calculated platform fee", () => {
+    expect(netPayoutCents(1000, 250)).toBe(975);
+  });
+});
+
+describe("splitEvenly", () => {
+  it("splits cents evenly and distributes the remainder first", () => {
+    expect(splitEvenly(10, 3)).toEqual([4, 3, 3]);
+  });
+
+  it("returns an empty list for non-positive recipient counts", () => {
+    expect(splitEvenly(10, 0)).toEqual([]);
+    expect(splitEvenly(10, -1)).toEqual([]);
+  });
+});
diff --git a/src/strings.test.ts b/src/strings.test.ts
index daeb1cc..6148c10 100644
--- a/src/strings.test.ts
+++ b/src/strings.test.ts
@@ -1,7 +1,49 @@
-import { describe, it, expect } from "vitest";
-import { slugify } from "./strings";
+import { describe, expect, it } from "vitest";
+import { escapeHtml, slugify, titleCase, truncate } from "./strings";
+
 describe("slugify", () => {
   it("lowercases and dashifies basic input", () => {
     expect(slugify("Hello World")).toBe("hello-world");
   });
+
+  it("removes punctuation and collapses duplicate dashes", () => {
+    expect(slugify("  Hello, --- Tiny Utils!!!  ")).toBe("hello-tiny-utils");
+  });
+});
+
+describe("truncate", () => {
+  it("returns an empty string for non-positive limits", () => {
+    expect(truncate("hello", 0)).toBe("");
+    expect(truncate("hello", -1)).toBe("");
+  });
+
+  it("returns short input unchanged", () => {
+    expect(truncate("hello", 5)).toBe("hello");
+  });
+
+  it("adds a suffix when input exceeds the limit", () => {
+    expect(truncate("hello world", 8, "...")).toBe("hello...");
+  });
+
+  it("handles suffixes longer than the limit", () => {
+    expect(truncate("hello", 2, "...")).toBe("...");
+  });
+});
+
+describe("titleCase", () => {
+  it("capitalizes each word and lowercases the rest", () => {
+    expect(titleCase("hELLO tiny UTILS")).toBe("Hello Tiny Utils");
+  });
+
+  it("preserves spacing behavior from split and join", () => {
+    expect(titleCase("hello   world")).toBe("Hello World");
+  });
+});
+
+describe("escapeHtml", () => {
+  it("escapes HTML-sensitive characters", () => {
+    expect(escapeHtml(`Tom & "Jerry" <cat>'s>`)).toBe(
+      "Tom &amp; &quot;Jerry&quot; &lt;cat&gt;&#39;s&gt;",
+    );
+  });
 });