diff --git a/Sprint-1/fix/median.js b/Sprint-1/fix/median.js
index 5c5b796e1..53b578809 100644
--- a/Sprint-1/fix/median.js
+++ b/Sprint-1/fix/median.js
@@ -6,9 +6,28 @@
 // or contains values that aren't numbers (the function is expected to throw - see the tests).
 
 function calculateMedian(list) {
-  const middleIndex = Math.floor(list.length / 2);
-  const median = list.splice(middleIndex, 1)[0];
-  return median;
+  if (!Array.isArray(list)) {
+    throw new Error("calculateMedian requires an array of numbers");
+  }
+
+  if (list.length === 0) {
+    throw new Error("calculateMedian requires a non-empty array");
+  }
+
+  for (const item of list) {
+    if (typeof item !== "number") {
+      throw new Error("calculateMedian requires an array of numbers");
+    }
+  }
+
+  const sorted = list.slice().sort((a, b) => a - b);
+  const middleIndex = Math.floor(sorted.length / 2);
+
+  if (sorted.length % 2 === 0) {
+    return (sorted[middleIndex - 1] + sorted[middleIndex]) / 2;
+  }
+
+  return sorted[middleIndex];
 }
 
 module.exports = calculateMedian;
diff --git a/Sprint-1/implement/dedupe.js b/Sprint-1/implement/dedupe.js
index 781e8718a..47c5ef797 100644
--- a/Sprint-1/implement/dedupe.js
+++ b/Sprint-1/implement/dedupe.js
@@ -1 +1,5 @@
-function dedupe() {}
+function dedupe(arr) {
+  return arr.filter((item, index) => arr.indexOf(item) === index);
+}
+
+module.exports = dedupe;
diff --git a/Sprint-1/implement/dedupe.test.js b/Sprint-1/implement/dedupe.test.js
index d7c8e3d8e..eddd4e9d4 100644
--- a/Sprint-1/implement/dedupe.test.js
+++ b/Sprint-1/implement/dedupe.test.js
@@ -16,13 +16,37 @@ E.g. dedupe([1, 2, 1]) returns [1, 2]
 // Given an empty array
 // When passed to the dedupe function
 // Then it should return an empty array
-test.todo("given an empty array, it returns an empty array");
+test("Given an empty array, should return an empty array", () => {
+  expect(dedupe([])).toEqual([]);
+});
 
 // Given an array with no duplicates
 // When passed to the dedupe function
 // Then it should return a copy of the original array
+test("Given an array with no duplicates, should return a copy of the original array", () => {
+  const original = [5, 1, 2, 3, 8];
+  const result = dedupe(original);
+
+  expect(result).toEqual([5, 1, 2, 3, 8]);
+  expect(result).not.toBe(original);
+});
 
 // Given an array of strings or numbers
 // When passed to the dedupe function
-// Then it should return a new array with duplicates removed while preserving the 
+// Then it should return a new array with duplicates removed while preserving the
 // first occurrence of each element from the original array.
+test("Given an array of strings, removes duplicates and keeps first occurrence", () => {
+  expect(dedupe(["car", "plane", "car", "train", "plane"])).toEqual([
+    "car",
+    "plane",
+    "train",
+  ]);
+});
+
+test("Given an array of numbers, removes duplicates and keeps first occurrence", () => {
+  expect(dedupe([5, 1, 1, 2, 3, 2, 5, 8])).toEqual([5, 1, 2, 3, 8]);
+});
+
+test("Given [1, 2, 1], returns [1, 2]", () => {
+  expect(dedupe([1, 2, 1])).toEqual([1, 2]);
+});
diff --git a/Sprint-1/implement/describe-median.js b/Sprint-1/implement/describe-median.js
index 3381cfc0a..f6fdd5660 100644
--- a/Sprint-1/implement/describe-median.js
+++ b/Sprint-1/implement/describe-median.js
@@ -20,6 +20,13 @@ function calculateMedian(list) {
 }
 
 // Implement this function. See describe-median.test.js for the acceptance criteria.
-function describeMedian(list) {}
+function describeMedian(list) {
+  try {
+    const median = calculateMedian(list);
+    return `The median is ${median}`;
+  } catch (error) {
+    return `Could not calculate a median: ${error.message}`;
+  }
+}
 
 module.exports = describeMedian;
diff --git a/Sprint-1/implement/describe-median.test.js b/Sprint-1/implement/describe-median.test.js
index e7d18a264..f04bc5929 100644
--- a/Sprint-1/implement/describe-median.test.js
+++ b/Sprint-1/implement/describe-median.test.js
@@ -20,13 +20,31 @@ const describeMedian = require("./describe-median.js");
 // Given an array of numbers
 // When passed to describeMedian
 // Then it should return "The median is " followed by the median
-// Delete this test.todo and replace it with a test.
-test.todo('given [1, 2, 3], returns "The median is 2"');
+test('Given an array of numbers, returns "The median is " followed by the median', () => {
+  expect(describeMedian([5, 6, 7])).toEqual("The median is 6");
+});
 
 // Given an empty array
 // When passed to describeMedian
 // Then it should return "Could not calculate a median: calculateMedian requires a non-empty array"
+test("Given an empty array, returns could-not-calculate message", () => {
+  expect(describeMedian([])).toEqual(
+    "Could not calculate a median: calculateMedian requires a non-empty array"
+  );
+});
 
 // Given something that isn't an array of numbers, e.g. "banana"
 // When passed to describeMedian
 // Then it should return "Could not calculate a median: calculateMedian requires an array of numbers"
+test("Given non-numbers, returns could-not-calculate message", () => {
+  expect(describeMedian(["banana"])).toEqual(
+    "Could not calculate a median: calculateMedian requires an array of numbers"
+  );
+});
+
+test("Given a string, returns could-not-calculate message", () => {
+  expect(describeMedian("banana")).toEqual(
+    "Could not calculate a median: calculateMedian requires an array of numbers"
+  );
+});
+
diff --git a/Sprint-1/implement/max.js b/Sprint-1/implement/max.js
index 6dd76378e..ddf6b202d 100644
--- a/Sprint-1/implement/max.js
+++ b/Sprint-1/implement/max.js
@@ -1,4 +1,26 @@
+
 function findMax(elements) {
+  if (!Array.isArray(elements)) {
+    throw new Error("findMax requires an array of numbers");
+  }
+
+  for (const item of elements) {
+    if (typeof item !== "number" || Number.isNaN(item)) { 
+      throw new Error("findMax requires an array of numbers");
+    }
+  }
+
+  if (elements.length === 0) {
+    return -Infinity;
+  }
+
+  let max = elements[0];
+  for (const item of elements) {
+    if (item > max) {
+      max = item;
+    }
+  }
+  return max;
 }
 
 module.exports = findMax;
diff --git a/Sprint-1/implement/max.test.js b/Sprint-1/implement/max.test.js
index ff3fbb5e5..0485062c7 100644
--- a/Sprint-1/implement/max.test.js
+++ b/Sprint-1/implement/max.test.js
@@ -15,29 +15,69 @@ const findMax = require("./max.js");
 // Given an empty array
 // When passed to the max function
 // Then it should return -Infinity
-// Delete this test.todo and replace it with a test.
-test.todo("given an empty array, returns -Infinity");
+test("given an empty array, returns -Infinity", () => {
+  expect(findMax([])).toEqual(-Infinity);
+});
 
 // Given an array with one number
 // When passed to the max function
 // Then it should return that number
+test("Given an array with one number, should return thant number", () => {
+  expect(findMax([10])).toEqual(10);
+});
 
 // Given an array with both positive and negative numbers
 // When passed to the max function
 // Then it should return the largest number overall
+test("Given an array with both positive and negative numbers, should return the largest number overall", () => {
+  expect(findMax([30, -7, 1, 45, -70, 10, -10])).toEqual(45);
+});
 
 // Given an array with just negative numbers
 // When passed to the max function
 // Then it should return the closest one to zero
+test("Given an array with just negative numbers, should return the closest one to zero", () => {
+  expect(findMax([-15, -10, -7, -20, -70])).toEqual(-7);
+});
 
 // Given an array with decimal numbers
 // When passed to the max function
 // Then it should return the largest decimal number
+test("Given an array with decimal numbers, should return the largest decimal number", () => {
+  expect(findMax([3.5, 2.7, 1.9, 5.1])).toEqual(5.1);
+});
 
 // Given an array containing a value that isn't a number
 // When passed to the max function
 // Then it should throw Error("findMax requires an array of numbers")
-
+test("Throws an error when array contains a value that isn't a number", () => {
+  expect(() => findMax(["plane", 10, "car", 60, 10])).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
 // Given something that isn't an array at all, such as "hey", 42 or no argument
 // When passed to the max function
 // Then it should throw Error("findMax requires an array of numbers")
+test("Throws an error when the input is not an array at all", () => {
+  expect(() => findMax("hey")).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
+
+test("Throws an error when given a number", () => {
+  expect(() => findMax(36)).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
+
+test("Throws an error when called with no argument", () => {
+  expect(() => findMax()).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
+
+test("Throws an error when array contains NaN", () => {
+  expect(() => findMax([1, NaN, 3])).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
diff --git a/Sprint-1/implement/mean.js b/Sprint-1/implement/mean.js
index 36909ea1f..a048fdf08 100644
--- a/Sprint-1/implement/mean.js
+++ b/Sprint-1/implement/mean.js
@@ -1,3 +1,24 @@
-function calculateMean(list) {}
+function calculateMean(list) {
+  if (!Array.isArray(list)) {
+    throw new Error("calculateMean requires an array of numbers");
+  }
+
+  if (list.length === 0) {
+    throw new Error("calculateMean requires a non-empty array");
+  }
+
+  for (const item of list) {
+    if (typeof item !== "number" || !Number.isFinite(item)) {
+      throw new Error("calculateMean requires an array of numbers");
+    }
+  }
+
+  let sum = 0;
+  for (const num of list) {
+    sum += num;
+  }
+  return sum / list.length;
+}
 
 module.exports = calculateMean;
+
diff --git a/Sprint-1/implement/mean.test.js b/Sprint-1/implement/mean.test.js
index 847b88676..388c0fa73 100644
--- a/Sprint-1/implement/mean.test.js
+++ b/Sprint-1/implement/mean.test.js
@@ -22,29 +22,77 @@ const calculateMean = require("./mean.js");
 // Given an array of numbers
 // When passed to calculateMean
 // Then it should return their mean
-// Delete this test.todo and replace it with a test.
-test.todo("given [1, 2, 6], returns 3");
+
+test("Given an array of numbers, should return their mean", () => {
+  expect(calculateMean([3, 7, 50,40])).toEqual(25);
+});
 
 // Given an array with a single number
 // When passed to calculateMean
 // Then it should return that number
+test("Given an array with a single number, should return that number", () => {
+  expect(calculateMean([10])).toEqual(10);
+});
 
 // Given an array containing negative or decimal numbers
 // When passed to calculateMean
 // Then it should return the correct mean
+test("Given an array containing negative numbers, should return the correct mean", () => {
+  expect(calculateMean([-3, -50, -7, -40])).toEqual(-25);
+});
+
+test("Given an array containing decimal numbers, should return the correct mean", () => {
+  expect(calculateMean([2.2, 1.9, 3.5, 4.1])).toBeCloseTo(2.925);
+});
+
+test("Given an array containing both negative and decimal numbers, should return the correct mean", () => {
+  expect(calculateMean([2.2, -1.9, 3.5, -4.1])).toBeCloseTo(-0.075);
+});
 
 // Given an empty array
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires a non-empty array")
+test("Throws an error when given an empty array", () => {
+  expect(() => calculateMean([])).toThrow(
+    new Error("calculateMean requires a non-empty array")
+  );
+});
 
 // Given a value that isn't an array, e.g. "banana", 42, null or {}
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires an array of numbers")
+test("Throws an error when given a value that isn't an array", () => {
+  expect(() => calculateMean(["banana"])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
 
 // Given no argument at all
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires an array of numbers")
+test("Throws an error when given no argument at all", () => {
+  expect(() => calculateMean()).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
 
 // Given an array containing a non-number value, e.g. [1, "2", 3]
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires an array of numbers")
+test("Throws an error when given an array containing a non-number value", () => {
+  expect(() => calculateMean([1, "2", 3])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
+
+test("Throws an error when array contains Infinity or -Infinity", () => {
+  expect(() => calculateMean([Infinity, 0, -Infinity])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
+
+test("Throws an error when array contains NaN", () => {
+  expect(() => calculateMean([0, NaN])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
diff --git a/Sprint-1/implement/sum.js b/Sprint-1/implement/sum.js
index 9062aafe3..a1474f1f0 100644
--- a/Sprint-1/implement/sum.js
+++ b/Sprint-1/implement/sum.js
@@ -1,4 +1,19 @@
 function sum(elements) {
+  if (!Array.isArray(elements)) {
+    throw new Error("sum requires an array of numbers");
+  }
+
+  for (const item of elements) {
+    if (typeof item !== "number") {
+      throw new Error("sum requires an array of numbers");
+    }
+  }
+
+  let total = 0;
+  for (const item of elements) {
+    total += item;
+  }
+  return total;
 }
 
 module.exports = sum;
diff --git a/Sprint-1/implement/sum.test.js b/Sprint-1/implement/sum.test.js
index a288e0f0a..ef591a15f 100644
--- a/Sprint-1/implement/sum.test.js
+++ b/Sprint-1/implement/sum.test.js
@@ -13,24 +13,46 @@ const sum = require("./sum.js");
 // Given an empty array
 // When passed to the sum function
 // Then it should return 0
-test.todo("given an empty array, returns 0")
+test("Given an empty array, should return 0", () => {
+  expect(sum([])).toEqual(0);
+});
 
 // Given an array with just one number
 // When passed to the sum function
 // Then it should return that number
+test("Given an array with just one number, should return that number", () => {
+  expect(sum([10])).toEqual(10);
+});
 
 // Given an array containing negative numbers
 // When passed to the sum function
 // Then it should still return the correct total sum
+test("Given an array containing negative numbers, should still return the correct total sum", () => {
+  expect(sum([-3, -7, -2])).toEqual(-12);
+});
 
 // Given an array with decimal/float numbers
 // When passed to the sum function
 // Then it should return the correct total sum
+test("Given an array with decimal/float numbers, should return the correct total sum", () => {
+  expect(sum([1.3, 2.7, 3.5])).toEqual(7.5);
+});
 
 // Given an array containing a value that isn't a number
 // When passed to the sum function
 // Then it should throw Error("sum requires an array of numbers")
+test("Throws an error when an array containing a value that isn't a number", () => {
+  expect(() => sum([15, "car", 11])).toThrow(
+    new Error("sum requires an array of numbers")
+  );
+});
 
 // Given something that isn't an array at all, such as "hey", 42 or no argument
 // When passed to the sum function
 // Then it should throw Error("sum requires an array of numbers")
+test("Throws an error when given something that isn't an array at all", () => {
+  expect(() => sum("string")).toThrow(
+    new Error("sum requires an array of numbers")
+  );
+});
+
diff --git a/Sprint-1/refactor/includes.js b/Sprint-1/refactor/includes.js
index 29dad81f0..07a0e6877 100644
--- a/Sprint-1/refactor/includes.js
+++ b/Sprint-1/refactor/includes.js
@@ -1,8 +1,7 @@
 // Refactor the implementation of includes to use a for...of loop
 
 function includes(list, target) {
-  for (let index = 0; index < list.length; index++) {
-    const element = list[index];
+  for (const element of list) {
     if (element === target) {
       return true;
     }
@@ -11,3 +10,4 @@ function includes(list, target) {
 }
 
 module.exports = includes;
+
diff --git a/Sprint-1/stretch/aoc-2018-day1/solution.js b/Sprint-1/stretch/aoc-2018-day1/solution.js
index e69de29bb..e45f52fcd 100644
--- a/Sprint-1/stretch/aoc-2018-day1/solution.js
+++ b/Sprint-1/stretch/aoc-2018-day1/solution.js
@@ -0,0 +1,44 @@
+
+import fs from "node:fs";
+
+//  Puzzle input
+// trim() removes leading/trailing whitespace so split("\n")
+// does not create empty lines that become 0 after Number()
+const changes = fs
+  .readFileSync(new URL("./input.txt", import.meta.url), "utf8")
+  .trim()
+  .split("\n")
+  .map(Number);
+
+// Part One
+function calculateFrequency(changes) {
+  let frequency = 0;
+
+  for (const change of changes) {
+    frequency += change;
+  }
+
+  return frequency;
+}
+
+// Part Two
+function findRepeatedFrequency(changes) {
+  let frequency = 0;
+  const visited = new Set([0]);
+
+  while (true) {
+    for (let change of changes) {
+      frequency += change;
+
+      if (visited.has(frequency)) {
+        return frequency;
+      }
+
+      visited.add(frequency);
+    }
+  }
+}
+
+// Display answers
+console.log("Part One:", calculateFrequency(changes));
+console.log("Part Two:", findRepeatedFrequency(changes));