diff --git a/src/functions.test.ts b/src/functions.test.ts
new file mode 100644
index 0000000..f1e99b7
--- /dev/null
+++ b/src/functions.test.ts
@@ -0,0 +1,211 @@
+import { describe, test } from "vitest"
+import type { FunctionDef, ObjectProp, Prop } from "./functions.js"
+import {
+	formatFunctionDefinitions,
+	formatObjectProperties,
+	formatType,
+	isAnyOfProp,
+} from "./functions.js"
+
+describe("isAnyOfProp", () => {
+	test("true", ({ expect }) => {
+		const result = isAnyOfProp({ anyOf: [] })
+		expect(result).toBe(true)
+	})
+
+	test("false", ({ expect }) => {
+		const result = isAnyOfProp({ type: "null" })
+		expect(result).toBe(false)
+	})
+})
+
+describe("formatFunctionDefinitions", () => {
+	test("empty array", ({ expect }) => {
+		const result = formatFunctionDefinitions([])
+		expect(result).toBe(`namespace functions {
+
+} // namespace functions`)
+	})
+
+	test("single function with no parameters", ({ expect }) => {
+		const functions: FunctionDef[] = [
+			{
+				name: "testFunction",
+				description: "A test function",
+				parameters: { type: "object" },
+			},
+		]
+
+		const result = formatFunctionDefinitions(functions)
+		expect(result).toBe(`namespace functions {
+
+// A test function
+type testFunction = () => any;
+
+} // namespace functions`)
+	})
+
+	test("single function with parameters", ({ expect }) => {
+		const functions: FunctionDef[] = [
+			{
+				name: "testFunction",
+				description: "A test function",
+				parameters: {
+					type: "object",
+					properties: {
+						param1: { type: "string" },
+						param2: { type: "number" },
+					},
+					required: ["param1"],
+				},
+			},
+		]
+
+		const result = formatFunctionDefinitions(functions)
+		expect(result).toBe(`namespace functions {
+
+// A test function
+type testFunction = (_: {
+param1: string,
+param2?: number,
+}) => any;
+
+} // namespace functions`)
+	})
+
+	test("multiple functions", ({ expect }) => {
+		const functions: FunctionDef[] = [
+			{
+				name: "functionOne",
+				description: "First function",
+				parameters: { type: "object" },
+			},
+			{
+				name: "functionTwo",
+				description: "Second function",
+				parameters: {
+					type: "object",
+					properties: { param1: { type: "boolean" } },
+				},
+			},
+		]
+
+		const result = formatFunctionDefinitions(functions)
+		expect(result).toBe(`namespace functions {
+
+// First function
+type functionOne = () => any;
+
+// Second function
+type functionTwo = (_: {
+param1?: boolean,
+}) => any;
+
+} // namespace functions`)
+	})
+})
+
+describe("formatObjectProperties", () => {
+	test("empty object", ({ expect }) => {
+		const obj: ObjectProp = { type: "object" }
+		const result = formatObjectProperties(obj, 0)
+		expect(result).toBe("")
+	})
+
+	test("single required property", ({ expect }) => {
+		const obj: ObjectProp = {
+			type: "object",
+			properties: { prop1: { type: "string" } },
+			required: ["prop1"],
+		}
+
+		const result = formatObjectProperties(obj, 0)
+		expect(result).toBe("prop1: string,")
+	})
+
+	test("multiple properties with optional ones", ({ expect }) => {
+		const obj: ObjectProp = {
+			type: "object",
+			properties: { prop1: { type: "string" }, prop2: { type: "number" } },
+			required: ["prop1"],
+		}
+
+		const result = formatObjectProperties(obj, 0)
+		expect(result).toBe("prop1: string,\nprop2?: number,")
+	})
+
+	test("nested objects", ({ expect }) => {
+		const obj: ObjectProp = {
+			type: "object",
+			properties: {
+				prop1: { type: "object", properties: { subProp: { type: "boolean" } } },
+			},
+			required: ["prop1"],
+		}
+
+		const result = formatObjectProperties(obj, 0)
+		expect(result).toBe("prop1: {\n  subProp?: boolean,\n},")
+	})
+})
+
+describe("formatType", () => {
+	test("string", ({ expect }) => {
+		const param: Prop = { type: "string" }
+		const result = formatType(param, 0)
+
+		expect(result).toBe("string")
+	})
+
+	test("number", ({ expect }) => {
+		const param: Prop = { type: "number" }
+		const result = formatType(param, 0)
+
+		expect(result).toBe("number")
+	})
+
+	test("integer", ({ expect }) => {
+		const param: Prop = { type: "integer" }
+		const result = formatType(param, 0)
+
+		expect(result).toBe("number")
+	})
+
+	test("boolean", ({ expect }) => {
+		const param: Prop = { type: "boolean" }
+		const result = formatType(param, 0)
+
+		expect(result).toBe("boolean")
+	})
+
+	test("null", ({ expect }) => {
+		const param: Prop = { type: "null" }
+		const result = formatType(param, 0)
+
+		expect(result).toBe("null")
+	})
+
+	test("object", ({ expect }) => {
+		const param: Prop = {
+			type: "object",
+			properties: { prop1: { type: "string" } },
+			required: ["prop1"],
+		}
+
+		const result = formatType(param, 0)
+		expect(result).toBe("{\n  prop1: string,\n}")
+	})
+
+	test("array", ({ expect }) => {
+		const param: Prop = { type: "array", items: { type: "string" } }
+		const result = formatType(param, 0)
+
+		expect(result).toBe("string[]")
+	})
+
+	test("anyOf", ({ expect }) => {
+		const param: Prop = { anyOf: [{ type: "string" }, { type: "number" }] }
+
+		const result = formatType(param, 0)
+		expect(result).toBe("string | number")
+	})
+})