diff --git a/src/json.test.ts b/src/json.test.ts
new file mode 100644
index 0000000..24678ba
--- /dev/null
+++ b/src/json.test.ts
@@ -0,0 +1,35 @@
+import { expect, test } from "vitest";
+
+import { jsonStringify } from "./json";
+
+const values: any[] = [
+  1,
+  "hello, world",
+  '"and"',
+  ['"and"'],
+  {},
+  [true, false],
+  { foo: 1, bar: "charlie" },
+  [1, 2, 3],
+  { foo: [1, 2, 3], bar: { foo: 1, bar: "charlie" } }
+  // 100n, // @todo returns "100" which is correct but we need to adapt the test
+];
+
+test("jsonStringify re-parse", async () => {
+  for (let v of values) {
+    expect(JSON.parse(jsonStringify(v))).toEqual(v);
+  }
+});
+
+test("jsonStringify order", () => {
+  expect(jsonStringify({ a: 1, b: "bar" })).toBe(
+    jsonStringify({ b: "bar", a: 1 })
+  );
+  expect(jsonStringify({ a: { c: 100, z: 1 }, b: "bar" })).toBe(
+    jsonStringify({ b: "bar", a: { z: 1, c: 100 } })
+  );
+});
+
+test("jsonStringify undefined", () => {
+  expect(jsonStringify({ a: 1, b: undefined })).toBe(jsonStringify({ a: 1 }));
+});
diff --git a/src/json.ts b/src/json.ts
index 6481691..06920da 100644
--- a/src/json.ts
+++ b/src/json.ts
@@ -7,7 +7,10 @@ const errorCell = new Error("cell");
  * @param obj
  * @returns
  */
-export const jsonStringify = <T>(obj: T) => {
+export const jsonStringify = <T>(
+  obj: T,
+  options: { skipNull?: boolean } = {}
+) => {
   if (obj instanceof Cell) throw errorCell;
   let out = "";
   const aux = <T>(v: T) => {
@@ -24,13 +27,17 @@ export const jsonStringify = <T>(obj: T) => {
     }
     switch (typeof v) {
       case "object": {
+        if (v === null) {
+          out += "null";
+          break;
+        }
         out += "{";
         let first = true;
         // sort objects alphabetically
         for (const [k, elt] of Object.entries(v).sort(([k1, _v1], [k2, _v2]) =>
           k1 < k2 ? -1 : k1 > k2 ? 1 : 0
         )) {
-          if (elt === undefined || elt === null) continue;
+          if (elt === undefined || (options.skipNull && elt === null)) continue;
           if (!first) out += ",";
           out += JSON.stringify(k);
           out += ":";
@@ -46,9 +53,6 @@ export const jsonStringify = <T>(obj: T) => {
       case "bigint":
         out += `"${v.toString()}"`;
         break;
-      // case "boolean":
-      //   out += v ? "true" : "false";
-      //   break;
       default:
         out += JSON.stringify(v);
     }
diff --git a/src/utils.test.ts b/src/utils.test.ts
new file mode 100644
index 0000000..b72ed6b
--- /dev/null
+++ b/src/utils.test.ts
@@ -0,0 +1,27 @@
+import { describe, it, expect } from "vitest";
+
+import { intersection } from "./utils";
+
+describe("intersection function", () => {
+  it("returns the correct intersection of two arrays with common elements", () => {
+    expect(intersection([1, 2, 3], [2, 3, 4])).toEqual([2, 3]);
+  });
+
+  it("returns an empty array when there are no common elements", () => {
+    expect(intersection([1, 2, 3], [4, 5, 6])).toEqual([]);
+  });
+
+  it("handles identical arrays correctly", () => {
+    expect(intersection([1, 2, 3], [1, 2, 3])).toEqual([1, 2, 3]);
+  });
+
+  it("returns an empty array when one or both input arrays are empty", () => {
+    expect(intersection([], [1, 2, 3])).toEqual([]);
+    expect(intersection([1, 2, 3], [])).toEqual([]);
+    expect(intersection([], [])).toEqual([]);
+  });
+
+  it("correctly handles duplicates in the input arrays", () => {
+    expect(intersection([1, 2, 2, 3], [2, 2, 3, 4])).toEqual([2, 2, 3]);
+  });
+});
diff --git a/vitest.config.mjs b/vitest.config.mjs
index d37dfd5..86a7857 100644
--- a/vitest.config.mjs
+++ b/vitest.config.mjs
@@ -3,5 +3,8 @@ import { defineConfig } from "vite";
 export default defineConfig({
   test: {
     environment: "happy-dom",
+    coverage: {
+      exclude: ["src/index.ts", "src/types.ts"],
+    },
   },
 });