feat(curriculum): add countdown lab (#64878)

client/i18n/locales/english/intro.json

@@ -6187,6 +6187,12 @@
           "In this lecture, you will learn about recursion and the call stack."
         ]
       },
+      "lab-countdown": {
+        "title": "Build a Countdown",
+        "intro": [
+          "For this lab, you will build a countdown function that returns an array of numbers counting down from given number to 1."
+        ]
+      },
       "workshop-decimal-to-binary-converter": {
         "title": "Build a Decimal to Binary Converter",
         "intro": [

client/src/pages/learn/javascript-v9/lab-countdown/index.md

@@ -0,0 +1,9 @@
+---
+title: Introduction to the Build a Countdown
+block: lab-countdown
+superBlock: javascript-v9
+---
+
+## Introduction to the Build a Countdown
+
+For this lab, you will build a countdown function that returns an array of numbers counting down from given number to 1.

curriculum/challenges/english/blocks/lab-countdown/5cd9a70215d3c4e65518328f.md

@@ -0,0 +1,106 @@
+---
+id: 5cd9a70215d3c4e65518328f
+title: Build a Countdown
+challengeType: 26
+dashedName: build-a-countdown
+---
+
+# --description--
+
+In this lab, you will build a `countdown` function that takes a starting number and generates a countdown list.
+
+When you call `countdown(n)`, it should return an array that starts at `n` and counts down by `1` until it reaches `1`.
+
+For example:
+
+- `countdown(5)` should return `[5, 4, 3, 2, 1]`
+- `countdown(1)` should return `[1]`
+
+If the starting number is less than `1`, there is nothing to countdown, so return an empty array:
+
+- `countdown(0)` should return `[]`
+- `countdown(-3)` should return `[]`
+
+To complete the lab, you must build the result using recursion and you must not use loops (`for`, `while`) or array-iteration methods (`forEach`, `map`, `filter`, `reduce`). Each call should produce its own result array (don't use globals to store the countdown).
+
+**Objective:** Fulfill the user stories below and get all the tests to pass to complete the lab.
+
+**User Stories:**
+
+1. You should create a function named `countdown`.
+2. The `countdown` function should take a single argument `n` (a number).
+3. If `n` is less than `1`, `countdown` should return an empty array.
+4. Otherwise, `countdown` should return an array containing the integers from `n` down to `1` in descending order.
+5. You must use recursion (the function should call itself) and must not use loops of any kind (`for`, `while`, or higher-order methods like `forEach`, `map`, `filter`, `reduce`).
+6. Each recursive call should use a smaller value than the previous call (for example, `n - 1`) so the function reaches its base case.
+7. The solution should not use global variables to store or cache the result array.
+8. Calling `countdown` multiple times with different inputs should always return the correct, independent result.
+
+# --hints--
+
+You should have a function named `countdown`.
+
+```js
+assert.isFunction(countdown);
+```
+
+`countdown(-1)` should return an empty array.
+
+```js
+assert.isEmpty(countdown(-1));
+```
+
+`countdown(10)` should return `[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]`
+
+```js
+assert.deepStrictEqual(countdown(10), [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]);
+```
+
+`countdown(5)` should return `[5, 4, 3, 2, 1]`
+
+```js
+assert.deepStrictEqual(countdown(5), [5, 4, 3, 2, 1]);
+```
+
+Your code should not rely on any kind of loops (`for`, `while` or higher order functions such as `forEach`, `map`, `filter`, and `reduce`).
+
+```js
+assert(
+  !__helpers.removeJSComments(code).match(/for|while|forEach|map|filter|reduce/g)
+);
+```
+
+You should use recursion to solve this problem.
+
+```js
+assert(
+  countdown.toString().match(/countdown\s*\(.+\)/)
+);
+```
+
+Global variables should not be used to cache the array.
+
+```js
+countdown(1)
+assert.deepStrictEqual(countdown(5), [5, 4, 3, 2, 1]);
+```
+
+# --seed--
+
+## --seed-contents--
+
+```js
+
+```
+
+# --solutions--
+
+```js
+function countdown(n) {
+  if (n < 1) {
+    return [];
+  }
+
+  return [n, ...countdown(n - 1)];
+}
+```

curriculum/structure/blocks/lab-countdown.json

@@ -0,0 +1,12 @@
+{
+  "name": "Build a Countdown",
+  "isUpcomingChange": false,
+  "dashedName": "lab-countdown",
+  "helpCategory": "JavaScript",
+  "blockLayout": "link",
+  "challengeOrder": [
+    { "id": "5cd9a70215d3c4e65518328f", "title": "Build a Countdown" }
+  ],
+  "blockLabel": "lab",
+  "usesMultifileEditor": true
+}

curriculum/structure/superblocks/javascript-v9.json

@@ -286,6 +286,7 @@
           "dashedName": "recursion",
           "blocks": [
             "lecture-understanding-recursion-and-the-call-stack",
+            "lab-countdown",
             "workshop-decimal-to-binary-converter",
             "lab-permutation-generator",
             "review-recursion",