diff --git a/Sprint-2/debug/address.js b/Sprint-2/debug/address.js
index 940a6af83..d4ae41a89 100644
--- a/Sprint-2/debug/address.js
+++ b/Sprint-2/debug/address.js
@@ -1,4 +1,5 @@
 // Predict and explain first...
+// Prediction: There will be an error because address[0] is trying to access an index instead of a property
 
 // This code should log out the houseNumber from the address object
 // but it isn't working...
@@ -12,4 +13,4 @@ const address = {
   postcode: "XYZ 123",
 };
 
-console.log(`My house number is ${address[0]}`);
+console.log(`My house number is ${address.houseNumber}`);
diff --git a/Sprint-2/debug/author.js b/Sprint-2/debug/author.js
index 8c2125977..afcdcadc9 100644
--- a/Sprint-2/debug/author.js
+++ b/Sprint-2/debug/author.js
@@ -1,4 +1,5 @@
 // Predict and explain first...
+// Prediction: The error is that it is trying to iterate over a non-array object.
 
 // This program attempts to log out all the property values in the object.
 // But it isn't working. Explain why first and then fix the problem
@@ -11,6 +12,11 @@ const author = {
   alive: true,
 };
 
-for (const value of author) {
-  console.log(value);
+for (const key in author) {
+  console.log(author[key]);
 }
+
+// for iterating in an object for...in can be used
+// and each iteration it returns the key
+// then each value can be accessed using the key with bracket notation
+
diff --git a/Sprint-2/debug/recipe.js b/Sprint-2/debug/recipe.js
index 6cbdd22cd..655960dfd 100644
--- a/Sprint-2/debug/recipe.js
+++ b/Sprint-2/debug/recipe.js
@@ -1,4 +1,5 @@
 // Predict and explain first...
+// Prediction: The error is that recipe is an object and it cannot be logged as a string.
 
 // This program should log out the title, how many it serves and the ingredients.
 // Each ingredient should be logged on a new line
@@ -12,4 +13,4 @@ const recipe = {
 
 console.log(`${recipe.title} serves ${recipe.serves}
   ingredients:
-${recipe}`);
+${recipe.ingredients.join(", ")}`);
diff --git a/Sprint-2/implement/contains.js b/Sprint-2/implement/contains.js
index cd779308a..ef9793875 100644
--- a/Sprint-2/implement/contains.js
+++ b/Sprint-2/implement/contains.js
@@ -1,3 +1,15 @@
-function contains() {}
+function contains(object, name) {
+  if (object == null || Object.keys(object).length === 0) {
+    return false;
+  }
+
+  for (let key in object) {
+    if (key === name) {
+      return true;
+    }
+  }
+
+  return false;
+}
 
 module.exports = contains;
diff --git a/Sprint-2/implement/contains.test.js b/Sprint-2/implement/contains.test.js
index 326bdb1f2..1999891f3 100644
--- a/Sprint-2/implement/contains.test.js
+++ b/Sprint-2/implement/contains.test.js
@@ -20,16 +20,29 @@ as the object doesn't contains a key of 'c'
 // Given an empty object
 // When passed to contains
 // Then it should return false
-test.todo("contains on empty object returns false");
+test("contains on empty object returns false", () =>
+  expect(contains({}, "key1")).toEqual(false));
 
 // Given an object with properties
 // When passed to contains with an existing property name
 // Then it should return true
+test("contains returns true when object contains the given property", () =>
+  expect(contains({ key1: "value1", key2: "value2" }, "key1")).toEqual(true));
 
 // Given an object with properties
 // When passed to contains with a non-existent property name
 // Then it should return false
+test("contains returns false when object does not contain the given property", () =>
+  expect(contains({ key1: "value1", key2: "value2" }, "key4")).toEqual(false));
 
 // Given invalid parameters like an array
 // When passed to contains
 // Then it should return false or throw an error
+it("contains returns false or throws an error if given parameter is not a valid object", () => {
+  expect(contains([], "key1")).toEqual(false);
+  expect(contains([1, 2], 1)).toEqual(false);
+  expect(contains("key1:value1", "1")).toEqual(false);
+  expect(contains(5235, "key1")).toEqual(false);
+  expect(contains(undefined, "key1")).toEqual(false);
+  expect(contains(null, "key1")).toEqual(false);
+});
diff --git a/Sprint-2/implement/lookup.js b/Sprint-2/implement/lookup.js
index a6746e07f..d7dd553eb 100644
--- a/Sprint-2/implement/lookup.js
+++ b/Sprint-2/implement/lookup.js
@@ -1,5 +1,13 @@
 function createLookup() {
-  // implementation here
+  const array = {};
+
+  for (const pair of countryCurrencyPairs) {
+    const country = pair[0];
+    const currency = pair[1];
+    array[country] = currency;
+  }
+
+  return array;
 }
 
 module.exports = createLookup;
diff --git a/Sprint-2/implement/lookup.test.js b/Sprint-2/implement/lookup.test.js
index 547e06c5a..ddf69b5d0 100644
--- a/Sprint-2/implement/lookup.test.js
+++ b/Sprint-2/implement/lookup.test.js
@@ -1,7 +1,5 @@
 const createLookup = require("./lookup.js");
 
-test.todo("creates a country currency code lookup for multiple codes");
-
 /*
 
 Create a lookup object of key value pairs from an array of code pairs
@@ -33,3 +31,24 @@ It should return:
    'CA': 'CAD'
  }
 */
+
+describe("createLookup", () => {
+  it("returns empty object if the parameter passed is not an array", () => {
+    expect(createLookup("US:USD")).toEqual({});
+    expect(createLookup(undefined)).toEqual({});
+  });
+
+  it("returns an object of country initials and currency code", () =>
+    expect(
+      createLookup([
+        ["US", "USD"],
+        ["CA", "CAD"],
+      ])
+    ).toEqual({
+      US: "USD",
+      CA: "CAD",
+    }));
+
+  it("returns an empty object if passed an empty array", () =>
+    expect(createLookup([])).toEqual({}));
+});
diff --git a/Sprint-2/implement/querystring.js b/Sprint-2/implement/querystring.js
index 45ec4e5f3..c4714832c 100644
--- a/Sprint-2/implement/querystring.js
+++ b/Sprint-2/implement/querystring.js
@@ -6,8 +6,28 @@ function parseQueryString(queryString) {
   const keyValuePairs = queryString.split("&");
 
   for (const pair of keyValuePairs) {
-    const [key, value] = pair.split("=");
-    queryParams[key] = value;
+    let key = "",
+      value = "";
+    let equalSignIndex = pair.indexOf("=");
+
+    if (equalSignIndex === -1) {
+      key = decodeURIComponent(pair);
+      value = "";
+    } else {
+      key = decodeURIComponent(pair.slice(0, equalSignIndex));
+      value = decodeURIComponent(pair.slice(equalSignIndex + 1));
+    }
+
+    if (key === "" && value === "") continue;
+    if (queryParams[key]) {
+      if (queryParams[key] === value) continue;
+      if (Array.isArray(queryParams[key])) {
+        queryParams[key].push(value);
+      } else {
+        const temp = queryParams[key];
+        queryParams[key] = [temp, value];
+      }
+    } else queryParams[key] = value;
   }
 
   return queryParams;
diff --git a/Sprint-2/implement/querystring.test.js b/Sprint-2/implement/querystring.test.js
index 3e218b789..fb873c6fd 100644
--- a/Sprint-2/implement/querystring.test.js
+++ b/Sprint-2/implement/querystring.test.js
@@ -3,10 +3,47 @@
 // Below is one test case for an edge case the implementation doesn't handle well.
 // Fix the implementation for this test, and try to think of as many other edge cases as possible - write tests and fix those too.
 
-const parseQueryString = require("./querystring.js")
+const parseQueryString = require("./querystring.js");
 
 test("parses querystring values containing =", () => {
   expect(parseQueryString("equation=x=y+1")).toEqual({
-    "equation": "x=y+1",
+    equation: "x=y+1",
   });
 });
+
+test("parses querystring values containing repetitive keys", () => {
+  expect(parseQueryString("a=1&b=6&a=2&a=3&b=7")).toEqual({
+    a: ["1", "2", "3"],
+    b: ["6", "7"],
+  });
+});
+
+test("parses querystring values containing repetitive keys with same values", () => {
+  expect(parseQueryString("a=1&b=6&a=2&a=3&b=6")).toEqual({
+    a: ["1", "2", "3"],
+    b: "6",
+  });
+});
+
+test("parses querystring values containing no key and value but only =", () => {
+  expect(parseQueryString("=&b=6&a=2&=&=")).toEqual({
+    a: "2",
+    b: "6",
+  });
+});
+
+test("parses querystring values missing = sign", () => {
+  expect(parseQueryString("id=5&name=mohsen&age")).toEqual({
+    id: "5",
+    name: "mohsen",
+    age: "",
+  });
+});
+
+test("parses querystring with encoded keys and values", () => {
+  expect(
+    parseQueryString(
+      "search=hello%20world&city=New%20York&search=happy%20coding"
+    )
+  ).toEqual({ search: ["hello world", "happy coding"], city: "New York" });
+});
diff --git a/Sprint-2/implement/tally.js b/Sprint-2/implement/tally.js
index f47321812..f2ba06655 100644
--- a/Sprint-2/implement/tally.js
+++ b/Sprint-2/implement/tally.js
@@ -1,3 +1,19 @@
-function tally() {}
+function tally(itemsList) {
+  const counts = {};
+
+  if (itemsList.length === 0) {
+    return {};
+  }
+
+  if (!Array.isArray(itemsList)) {
+    throw new Error("Input must be an array");
+  }
+
+  for (const item of itemsList) {
+    counts[item] = (counts[item] || 0) + 1;
+  }
+
+  return counts;
+}
 
 module.exports = tally;
diff --git a/Sprint-2/implement/tally.test.js b/Sprint-2/implement/tally.test.js
index 2ceffa8dd..1ba68ca2f 100644
--- a/Sprint-2/implement/tally.test.js
+++ b/Sprint-2/implement/tally.test.js
@@ -32,3 +32,21 @@ test.todo("tally on an empty array returns an empty object");
 // Given an invalid input like a string
 // When passed to tally
 // Then it should throw an error
+
+describe("tally", () => {
+  test("returns an empty object when given an empty array", () => {
+    expect(tally([])).toEqual({});
+  });
+
+  test("returns counts for each unique item", () => {
+    expect(tally(["a", "a", "b", "c"])).toEqual({
+      a: 2,
+      b: 1,
+      c: 1,
+    });
+  });
+
+  test("throws an error when input is not an array", () => {
+    expect(() => tally("abc")).toThrow();
+  });
+});
diff --git a/Sprint-2/interpret/invert.js b/Sprint-2/interpret/invert.js
index bb353fb1f..7498dbb9a 100644
--- a/Sprint-2/interpret/invert.js
+++ b/Sprint-2/interpret/invert.js
@@ -17,13 +17,65 @@ function invert(obj) {
 }
 
 // a) What is the current return value when invert is called with { a : 1 }
+// Answer: Object.entries({a: 1}) → [["a", 1]]
+//loop runs once:
+// key = "a", value = 1
+// invertedObj.key = value sets property "key" to 1
+{ key: 1 }
 
 // b) What is the current return value when invert is called with { a: 1, b: 2 }
+// Answer: Object.entries({a: 1, b: 2}) → [["a", 1], ["b", 2]]
+// loop:
+// first iteration sets { key: 1 }
+// second iteration overwrites it to { key: 2 }
+{ key: 2 }
 
 // c) What is the target return value when invert is called with {a : 1, b: 2}
+// Answer: “Invert” should swap keys and values, and object keys are strings, so numeric values become string keys.
+{ "1": "a", "2": "b" }
 
 // c) What does Object.entries return? Why is it needed in this program?
+// Answer: Object.entries(obj) returns an array of [key, value] pairs for an object’s own enumerable string-keyed properties.
+// It’s useful here because it lets you iterate an object as pairs.
+// So Object.entries is just a clean, safe way to get both key and value in one go.
 
 // d) Explain why the current return value is different from the target output
+// Answer: The output is wrong because the code has two main bugs: it uses dot notation (invertedObj.key), which creates a literal "key" property instead of using the variable key, 
+// and it doesn’t actually swap keys and values, it assigns the value back instead of inverting. 
+// As a result, each loop iteration overwrites the same "key" property, leaving only the last value.
 
 // e) Fix the implementation of invert (and write tests to prove it's fixed!)
+
+function invert(obj) {
+  const invertedObj = {};
+
+  for (const [key, value] of Object.entries(obj)) {
+    invertedObj[String(value)] = key; // ensure keys are strings
+  }
+
+  return invertedObj;
+}
+
+
+describe("invert", () => {
+  test("inverts a single key-value pair", () => {
+    expect(invert({ a: 1 })).toEqual({ "1": "a" });
+  });
+
+  test("inverts multiple key-value pairs", () => {
+    expect(invert({ a: 1, b: 2 })).toEqual({ "1": "a", "2": "b" });
+  });
+
+  test("stringifies numeric values as keys", () => {
+    const result = invert({ x: 10, y: 20 });
+    expect(result).toEqual({ "10": "x", "20": "y" });
+  });
+
+  test("later keys override earlier keys when values collide", () => {
+    expect(invert({ a: 1, b: 1 })).toEqual({ "1": "b" });
+  });
+
+  test("works with string values too", () => {
+    expect(invert({ a: "cat", b: "dog" })).toEqual({ cat: "a", dog: "b" });
+  });
+});