diff --git a/spec/FieldsArray.ts b/spec/FieldsArray.ts
new file mode 100644
index 0000000..00bd2c0
--- /dev/null
+++ b/spec/FieldsArray.ts
@@ -0,0 +1,112 @@
+import { PKPass } from "../lib";
+import FieldsArray from "../lib/FieldsArray";
+import * as Messages from "../lib/messages";
+
+describe("FieldsArray", () => {
+	let fa: FieldsArray;
+	let frozen = false;
+	let pool: Set<string>;
+
+	beforeEach(() => {
+		frozen = false;
+		pool = new Set<string>();
+		fa = new FieldsArray(
+			{
+				get isFrozen() {
+					return frozen;
+				},
+			} as PKPass /** Fake pass. This is okay for testing */,
+			pool,
+		);
+	});
+
+	it("should extend an array", () => {
+		expect(fa instanceof Array).toBe(true);
+	});
+
+	describe("push", () => {
+		it("should prevent adding new fields if pass is frozen", () => {
+			frozen = true;
+
+			expect(() => fa.push({ key: "t1", value: "v1" })).toThrowError(
+				Error,
+				Messages.BUNDLE.CLOSED,
+			);
+		});
+
+		it("should allow adding fields", () => {
+			expect(fa.push({ key: "t1", value: "v1" })).toBe(1);
+			expect(fa.length).toBe(1);
+			expect(fa[0]).toEqual({ key: "t1", value: "v1" });
+		});
+
+		it("should add the key to the pool", () => {
+			fa.push({ key: "t1", value: "v1" });
+
+			expect(pool.has("t1")).toBe(true);
+		});
+
+		it("should log a warning if key already exists and omit that object", () => {
+			fa.push({ key: "t1", value: "v1" });
+
+			console.warn = jasmine.createSpy("log");
+
+			fa.push({ key: "t1", value: "v1" });
+
+			expect(console.warn).toHaveBeenCalledWith(
+				Messages.FIELDS.REPEATED_KEY.replace("%s", "t1"),
+			);
+
+			expect(fa.length).toBe(1);
+		});
+	});
+
+	describe("pop", () => {
+		beforeEach(() => {
+			fa.push({ key: "t1", value: "v1" });
+		});
+
+		it("should prevent popping out fields if pass is frozen", () => {
+			frozen = true;
+
+			expect(() => fa.pop()).toThrowError(Error, Messages.BUNDLE.CLOSED);
+		});
+
+		it("should popping out fields", () => {
+			expect(fa.pop()).toEqual({ key: "t1", value: "v1" });
+			expect(fa.length).toBe(0);
+			expect(fa[0]).toBeUndefined();
+		});
+
+		it("should remove the key from the pool", () => {
+			expect(pool.has("t1")).toBe(true);
+
+			fa.pop();
+
+			expect(pool.has("t1")).toBe(false);
+		});
+	});
+
+	describe("splice", () => {
+		beforeEach(() => {
+			fa.push({ key: "t1", value: "v1" });
+		});
+
+		it("should prevent splicing fields if pass is frozen", () => {
+			frozen = true;
+
+			expect(() =>
+				fa.splice(0, 1, { key: "k1", value: "v1" }),
+			).toThrowError(Error, Messages.BUNDLE.CLOSED);
+		});
+
+		it("should remove the key from the pool", () => {
+			expect(pool.has("t1")).toBe(true);
+
+			fa.splice(0, 1, { key: "k1", value: "v2" });
+
+			expect(pool.has("t1")).toBe(false);
+			expect(pool.has("k1")).toBe(true);
+		});
+	});
+});