diff --git a/src/js/builtins.d.ts b/src/js/builtins.d.ts
index 2c07c8aa0e350d..b86948f2c4d64b 100644
--- a/src/js/builtins.d.ts
+++ b/src/js/builtins.d.ts
@@ -161,8 +161,26 @@ declare function $toPropertyKey(x: any): PropertyKey;
  * `$toObject(this, "Class.prototype.method requires that |this| not be null or undefined");`
  */
 declare function $toObject(object: any, errorMessage?: string): object;
+/**
+ * ## References
+ * - [WebKit - `emit_intrinsic_newArrayWithSize`](https://github.com/oven-sh/WebKit/blob/e1a802a2287edfe7f4046a9dd8307c8b59f5d816/Source/JavaScriptCore/bytecompiler/NodesCodegen.cpp#L2317)
+ */
 declare function $newArrayWithSize<T>(size: number): T[];
-declare function $newArrayWithSpecies(): TODO;
+/**
+ * Optimized path for creating a new array storing objects with the same homogenous Structure
+ * as {@link array}.
+ *
+ * @param size the initial size of the new array
+ * @param array the array whose shape we want to copy
+ *
+ * @returns a new array
+ *
+ * ## References
+ * - [WebKit - `emit_intrinsic_newArrayWithSpecies`](https://github.com/oven-sh/WebKit/blob/e1a802a2287edfe7f4046a9dd8307c8b59f5d816/Source/JavaScriptCore/bytecompiler/NodesCodegen.cpp#L2328)
+ * - [WebKit - #4909](https://github.com/WebKit/WebKit/pull/4909)
+ * - [WebKit Bugzilla - Related Issue/Ticket](https://bugs.webkit.org/show_bug.cgi?id=245797)
+ */
+declare function $newArrayWithSpecies<T>(size: number, array: T[]): T[];
 declare function $newPromise(): TODO;
 declare function $createPromise(): TODO;
 declare const $iterationKindKey: TODO;
diff --git a/src/js/builtins/StreamInternals.ts b/src/js/builtins/StreamInternals.ts
index a81ff3ca6f7d47..73b85878b84718 100644
--- a/src/js/builtins/StreamInternals.ts
+++ b/src/js/builtins/StreamInternals.ts
@@ -89,8 +89,8 @@ export function validateAndNormalizeQueuingStrategy(size, highWaterMark) {
 
 $linkTimeConstant;
 export function createFIFO() {
-  const Denqueue = require("internal/fifo");
-  return new Denqueue();
+  const Dequeue = require("internal/fifo");
+  return new Dequeue();
 }
 
 export function newQueue() {
diff --git a/src/js/internal-for-testing.ts b/src/js/internal-for-testing.ts
index 893ff59006ff8f..fb9d0391e1680c 100644
--- a/src/js/internal-for-testing.ts
+++ b/src/js/internal-for-testing.ts
@@ -151,3 +151,4 @@ export const bindgen = $zig("bindgen_test.zig", "getBindgenTestFunctions") as {
 };
 
 export const noOpForTesting = $cpp("NoOpForTesting.cpp", "createNoOpForTesting");
+export const Dequeue = require("internal/fifo");
diff --git a/src/js/internal/fifo.ts b/src/js/internal/fifo.ts
index a7438aa3bdc698..c9ead667060e13 100644
--- a/src/js/internal/fifo.ts
+++ b/src/js/internal/fifo.ts
@@ -1,5 +1,10 @@
 var slice = Array.prototype.slice;
-class Denqueue {
+class Dequeue<T> {
+  _head: number;
+  _tail: number;
+  _capacityMask: number;
+  _list: (T | undefined)[];
+
   constructor() {
     this._head = 0;
     this._tail = 0;
@@ -8,26 +13,21 @@ class Denqueue {
     this._list = $newArrayWithSize(4);
   }
 
-  _head;
-  _tail;
-  _capacityMask;
-  _list;
-
-  size() {
+  size(): number {
     if (this._head === this._tail) return 0;
     if (this._head < this._tail) return this._tail - this._head;
     else return this._capacityMask + 1 - (this._head - this._tail);
   }
 
-  isEmpty() {
+  isEmpty(): boolean {
     return this.size() == 0;
   }
 
-  isNotEmpty() {
+  isNotEmpty(): boolean {
     return this.size() > 0;
   }
 
-  shift() {
+  shift(): T | undefined {
     var { _head: head, _tail, _list, _capacityMask } = this;
     if (head === _tail) return undefined;
     var item = _list[head];
@@ -37,24 +37,21 @@ class Denqueue {
     return item;
   }
 
-  peek() {
+  peek(): T | undefined {
     if (this._head === this._tail) return undefined;
     return this._list[this._head];
   }
 
-  push(item) {
+  push(item: T): void {
     var tail = this._tail;
     $putByValDirect(this._list, tail, item);
     this._tail = (tail + 1) & this._capacityMask;
     if (this._tail === this._head) {
       this._growArray();
     }
-    // if (this._capacity && this.size() > this._capacity) {
-    // this.shift();
-    // }
   }
 
-  toArray(fullCopy) {
+  toArray(fullCopy: boolean): T[] {
     var list = this._list;
     var len = $toLength(list.length);
 
@@ -66,19 +63,19 @@ class Denqueue {
       var j = 0;
       for (var i = _head; i < len; i++) $putByValDirect(array, j++, list[i]);
       for (var i = 0; i < _tail; i++) $putByValDirect(array, j++, list[i]);
-      return array;
+      return array as T[];
     } else {
       return slice.$call(list, this._head, this._tail);
     }
   }
 
-  clear() {
+  clear(): void {
     this._head = 0;
     this._tail = 0;
     this._list.fill(undefined);
   }
 
-  _growArray() {
+  private _growArray(): void {
     if (this._head) {
       // copy existing data, head to end, then beginning to tail.
       this._list = this.toArray(true);
@@ -92,10 +89,10 @@ class Denqueue {
     this._capacityMask = (this._capacityMask << 1) | 1;
   }
 
-  _shrinkArray() {
+  private _shrinkArray(): void {
     this._list.length >>>= 1;
     this._capacityMask >>>= 1;
   }
 }
 
-export default Denqueue;
+export default Dequeue;
diff --git a/test/internal/fifo.test.ts b/test/internal/fifo.test.ts
new file mode 100644
index 00000000000000..69ff6867ff9c33
--- /dev/null
+++ b/test/internal/fifo.test.ts
@@ -0,0 +1,235 @@
+import { Dequeue } from "bun:internal-for-testing";
+import { describe, expect, test, it, beforeAll, beforeEach } from "bun:test";
+
+/**
+ * Implements the same API as {@link Dequeue} but uses a simple list as the
+ * backing store.
+ *
+ * Used to check expected behavior.
+ */
+class DequeueList<T> {
+  private _list: T[];
+
+  constructor() {
+    this._list = [];
+  }
+
+  size(): number {
+    return this._list.length;
+  }
+
+  isEmpty(): boolean {
+    return this.size() == 0;
+  }
+
+  isNotEmpty(): boolean {
+    return this.size() > 0;
+  }
+
+  shift(): T | undefined {
+    return this._list.shift();
+  }
+
+  peek(): T | undefined {
+    return this._list[0];
+  }
+
+  push(item: T): void {
+    this._list.push(item);
+  }
+
+  toArray(fullCopy: boolean): T[] {
+    return fullCopy ? this._list.slice() : this._list;
+  }
+
+  clear(): void {
+    this._list = [];
+  }
+}
+
+describe("Given an empty queue", () => {
+  let queue: Dequeue<number>;
+
+  beforeEach(() => {
+    queue = new Dequeue();
+  });
+
+  it("has a size of 0", () => {
+    expect(queue.size()).toBe(0);
+  });
+
+  it("is empty", () => {
+    expect(queue.isEmpty()).toBe(true);
+    expect(queue.isNotEmpty()).toBe(false);
+  });
+
+  it("shift() returns undefined", () => {
+    expect(queue.shift()).toBe(undefined);
+    expect(queue.size()).toBe(0);
+  });
+
+  it("has an initial capacity of 4", () => {
+    expect(queue._list.length).toBe(4);
+    expect(queue._capacityMask).toBe(3);
+  });
+
+  it("toArray() returns an empty array", () => {
+    expect(queue.toArray()).toEqual([]);
+  });
+
+  describe("When an element is pushed", () => {
+    beforeEach(() => {
+      queue.push(42);
+    });
+
+    it("has a size of 1", () => {
+      expect(queue.size()).toBe(1);
+    });
+
+    it("can be peeked without removing it", () => {
+      expect(queue.peek()).toBe(42);
+      expect(queue.size()).toBe(1);
+    });
+
+    it("is not empty", () => {
+      expect(queue.isEmpty()).toBe(false);
+      expect(queue.isNotEmpty()).toBe(true);
+    });
+
+    it("can be shifted out", () => {
+      const el = queue.shift();
+      expect(el).toBe(42);
+      expect(queue.size()).toBe(0);
+      expect(queue.isEmpty()).toBe(true);
+    });
+  }); // </When an element is pushed>
+}); // </Given an empty queue>
+
+describe("grow boundary conditions", () => {
+  describe.each([3, 4, 16])("when %d items are pushed", n => {
+    let queue: Dequeue<number>;
+
+    beforeEach(() => {
+      queue = new Dequeue();
+      for (let i = 0; i < n; i++) {
+        queue.push(i);
+      }
+    });
+
+    it(`has a size of ${n}`, () => {
+      expect(queue.size()).toBe(n);
+    });
+
+    it("is not empty", () => {
+      expect(queue.isEmpty()).toBe(false);
+      expect(queue.isNotEmpty()).toBe(true);
+    });
+
+    it(`can shift() ${n} times`, () => {
+      for (let i = 0; i < n; i++) {
+        expect(queue.peek()).toBe(i);
+        expect(queue.shift()).toBe(i);
+      }
+      expect(queue.size()).toBe(0);
+      expect(queue.shift()).toBe(undefined);
+    });
+
+    it("toArray() returns [0..n-1]", () => {
+      // same as repeated push() but only allocates once
+      var expected = new Array<number>(n);
+      for (let i = 0; i < n; i++) {
+        expected[i] = i;
+      }
+      expect(queue.toArray()).toEqual(expected);
+    });
+  });
+}); // </grow boundary conditions>
+
+describe("adding and removing items", () => {
+  let queue: Dequeue<number>;
+  let expected: DequeueList<number>;
+
+  describe("when 10k items are pushed", () => {
+    beforeEach(() => {
+      queue = new Dequeue();
+      expected = new DequeueList();
+
+      for (let i = 0; i < 10_000; i++) {
+        queue.push(i);
+        expected.push(i);
+      }
+    });
+
+    it("has a size of 10000", () => {
+      expect(queue.size()).toBe(10_000);
+      expect(expected.size()).toBe(10_000);
+    });
+
+    describe("when 10 items are shifted", () => {
+      beforeEach(() => {
+        for (let i = 0; i < 10; i++) {
+          expect(queue.shift()).toBe(expected.shift());
+        }
+      });
+
+      it("has a size of 9990", () => {
+        expect(queue.size()).toBe(9990);
+        expect(expected.size()).toBe(9990);
+      });
+    });
+
+    describe("when 1k items are pushed, then removed", () => {
+      beforeEach(() => {
+        for (let i = 0; i < 1_000; i++) {
+          queue.push(i);
+          expected.push(i);
+        }
+        expect(queue.size()).toBe(1_000);
+
+        while (queue.isNotEmpty()) {
+          expect(queue.shift()).toBe(expected.shift());
+        }
+        it("when new items are added, the backing list is resized", () => {
+          for (let i = 0; i < 10_000; i++) {
+            queue.push(i);
+            expected.push(i);
+            expect(queue.size()).toBe(expected.size());
+            expect(queue.peek()).toBe(expected.peek());
+            expect(queue.isEmpty()).toBeFalse();
+            expect(queue.isNotEmpty()).toBeTrue();
+          }
+        });
+      });
+    });
+  }); // </when 10k items are pushed>
+
+  it("pushing and shifting a lot of items affects the size and backing list correctly", () => {
+    queue = new Dequeue();
+    expected = new DequeueList();
+
+    for (let i = 0; i < 15_000; i++) {
+      queue.push(i);
+      expected.push(i);
+      expect(queue.size()).toBe(expected.size());
+      expect(queue.peek()).toBe(expected.peek());
+      expect(queue.isEmpty()).toBeFalse();
+      expect(queue.isNotEmpty()).toBeTrue();
+    }
+
+    // shift() shrinks the backing array when tail > 10,000 and the list is
+    // shrunk too far (tail <= list.length >>> 2)
+    for (let i = 0; i < 10_000; i++) {
+      expect(queue.shift()).toBe(expected.shift());
+      expect(queue.size()).toBe(expected.size());
+    }
+
+    for (let i = 0; i < 5_000; i++) {
+      queue.push(i);
+      expected.push(i);
+      expect(queue.size()).toBe(expected.size());
+      expect(queue.peek()).toBe(expected.peek());
+      expect(queue.isEmpty()).toBeFalse();
+      expect(queue.isNotEmpty()).toBeTrue();
+    }
+  }); // </pushing a lot of items affects the size and backing list correctly>
+});