diff --git a/Sprint-2/debug/address.js b/Sprint-2/debug/address.js
index 940a6af83..9a130e510 100644
--- a/Sprint-2/debug/address.js
+++ b/Sprint-2/debug/address.js
@@ -4,6 +4,11 @@
 // but it isn't working...
 // Fix anything that isn't working
 
+/*
+When we test the file without fixing we get "My house number is [object Object]"
+We need to properly export the house number: adress.houseNumber.
+*/
+
 const address = {
   houseNumber: 42,
   street: "Imaginary Road",
@@ -12,4 +17,4 @@ 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..abbe421f5 100644
--- a/Sprint-2/debug/author.js
+++ b/Sprint-2/debug/author.js
@@ -1,7 +1,12 @@
 // Predict and explain first...
 
+/*
+TypeError: author is not iterable
+The way the code is written is that author is an 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
+*/
 
 const author = {
   firstName: "Zadie",
@@ -11,6 +16,7 @@ const author = {
   alive: true,
 };
 
-for (const value of author) {
+for (const value of Object.values(author)) {
+  // gives you an array of all the values
   console.log(value);
 }
diff --git a/Sprint-2/debug/recipe.js b/Sprint-2/debug/recipe.js
index 6cbdd22cd..fc2523a47 100644
--- a/Sprint-2/debug/recipe.js
+++ b/Sprint-2/debug/recipe.js
@@ -1,5 +1,13 @@
 // Predict and explain first...
 
+/*
+First of all the output will not give us the proper ingredients. We will again get [object Object].
+We can fix this by using the correct array object recipe.ingredients.
+
+Secondly we don't loop through the ingredients and create a new line \n after each ingredient.
+
+*/
+
 // This program should log out the title, how many it serves and the ingredients.
 // Each ingredient should be logged on a new line
 // How can you fix it?
@@ -12,4 +20,4 @@ const recipe = {
 
 console.log(`${recipe.title} serves ${recipe.serves}
   ingredients:
-${recipe}`);
+${recipe.ingredients.join("\n")}`); //You can do this by joining the array with \n (newline)
diff --git a/Sprint-2/implement/contains.js b/Sprint-2/implement/contains.js
index cd779308a..f64e04615 100644
--- a/Sprint-2/implement/contains.js
+++ b/Sprint-2/implement/contains.js
@@ -1,3 +1,25 @@
-function contains() {}
+function contains(objectToCheck, propertyName) {
+  if (Array.isArray(objectToCheck)) {
+    throw new Error("Invalid parameter, No arrays allowed");
+  }
+  if (typeof objectToCheck !== "object" || objectToCheck === null) {
+    return false;
+  }
+  return propertyName in objectToCheck;
+}
 
 module.exports = contains;
+
+/*
+If the first argument is an array
+    Throw an error: "Invalid parameter, No arrays allowed"
+
+If the first argument is not an object
+    Return false
+
+If the object has the property (second argument)
+    Return true
+
+Otherwise
+    Return false
+*/
diff --git a/Sprint-2/implement/contains.test.js b/Sprint-2/implement/contains.test.js
index 326bdb1f2..fc80af968 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({}, "a")).toBe(false);
+});
 
 // Given an object with properties
 // When passed to contains with an existing property name
 // Then it should return true
+test("Given an object with properties that are included, return true", () => {
+  expect(contains({ a: 1, b: 2 }, "a")).toBe(true);
+});
 
 // Given an object with properties
 // When passed to contains with a non-existent property name
 // Then it should return false
+test("Given an object with properties that are NON included, return false", () => {
+  expect(contains({ a: 1, b: 2 }, "c")).toBe(false);
+});
 
 // Given invalid parameters like an array
 // When passed to contains
 // Then it should return false or throw an error
+test("Given an object with invalid properties such as arrays, trow error", () => {
+  expect(() => contains([1, 2, 3])).toThrow(
+    "Invalid parameter, No arrays allowed"
+  );
+});
diff --git a/Sprint-2/implement/lookup.js b/Sprint-2/implement/lookup.js
index a6746e07f..a56f1ee16 100644
--- a/Sprint-2/implement/lookup.js
+++ b/Sprint-2/implement/lookup.js
@@ -1,5 +1,19 @@
-function createLookup() {
-  // implementation here
+function createLookup(countryCurrencyPairs) {
+  return Object.fromEntries(countryCurrencyPairs);
 }
 
 module.exports = createLookup;
+
+/*
+The Object.fromEntries() static method transforms a list of key-value pairs into an object.
+
+const entries = new Map([
+  ["foo", "bar"],
+  ["baz", 42],
+]);
+
+const obj = Object.fromEntries(entries);
+
+console.log(obj);
+// Expected output: Object { foo: "bar", baz: 42 }
+*/
diff --git a/Sprint-2/implement/lookup.test.js b/Sprint-2/implement/lookup.test.js
index 547e06c5a..754a2af7b 100644
--- a/Sprint-2/implement/lookup.test.js
+++ b/Sprint-2/implement/lookup.test.js
@@ -1,7 +1,25 @@
 const createLookup = require("./lookup.js");
 
-test.todo("creates a country currency code lookup for multiple codes");
-
+test("Given an array of code pairs, create a lookup object representing country code", () => {
+  expect(createLookup([["US", "USD"]])).toEqual({ US: "USD" });
+});
+
+test("Given an array of code pairs CA - CAD, create a lookup object representing country code", () => {
+  expect(createLookup([["CA", "CAD"]])).toEqual({ CA: "CAD" });
+});
+
+test("Given 2 arrays of code pairs, create a lookup object with 2 object pairs", () => {
+  expect(
+    createLookup([
+      ["US", "USD"],
+      ["CA", "CAD"],
+    ])
+  ).toEqual({ US: "USD", CA: "CAD" });
+});
+
+test("Given an empty array of code pairs, we should return an empty object", () => {
+  expect(createLookup([])).toEqual({});
+});
 /*
 
 Create a lookup object of key value pairs from an array of code pairs
diff --git a/Sprint-2/implement/querystring.js b/Sprint-2/implement/querystring.js
index 45ec4e5f3..da86793b8 100644
--- a/Sprint-2/implement/querystring.js
+++ b/Sprint-2/implement/querystring.js
@@ -6,8 +6,14 @@ function parseQueryString(queryString) {
   const keyValuePairs = queryString.split("&");
 
   for (const pair of keyValuePairs) {
-    const [key, value] = pair.split("=");
-    queryParams[key] = value;
+    const whereIs = pair.indexOf("=");
+    if (whereIs === -1) {
+      queryParams[pair] = "";
+    } else {
+      const key = pair.slice(0, whereIs);
+      const value = pair.slice(whereIs + 1);
+      queryParams[key] = value;
+    }
   }
 
   return queryParams;
diff --git a/Sprint-2/implement/querystring.test.js b/Sprint-2/implement/querystring.test.js
index 3e218b789..ae3b9f56d 100644
--- a/Sprint-2/implement/querystring.test.js
+++ b/Sprint-2/implement/querystring.test.js
@@ -3,10 +3,65 @@
 // 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 =", () => {
+test("parses a value that contains =", () => {
   expect(parseQueryString("equation=x=y+1")).toEqual({
-    "equation": "x=y+1",
+    equation: "x=y+1",
+  });
+});
+
+// Empty string edge case:
+test("returns an empty object for an empty string", () => {
+  expect(parseQueryString("")).toEqual({});
+});
+
+// Key with empty value:
+test("parses a key with an empty value", () => {
+  expect(parseQueryString("Key=")).toEqual({ Key: "" });
+});
+
+// Value with an empty key:
+test("parses a value with an empty key", () => {
+  expect(parseQueryString("=Value")).toEqual({ "": "Value" });
+});
+
+// When there's no = in the Querystring:
+test("parses a query string without = as an empty value", () => {
+  expect(parseQueryString("nothingToSeeHere")).toEqual({
+    nothingToSeeHere: "",
+  });
+});
+
+// Multiple pairs per Querystring:
+test("parses multiple key-value pairs", () => {
+  expect(parseQueryString("name=Ben&city=Amsterdam")).toEqual({
+    name: "Ben",
+    city: "Amsterdam",
+  });
+});
+
+// Multiple pairs with missing values:
+test("parses multiple pairs when one value is missing", () => {
+  expect(parseQueryString("name=Ben&city=&country=Netherlands")).toEqual({
+    name: "Ben",
+    city: "",
+    country: "Netherlands",
+  });
+});
+
+// Pairs with a missing key:
+test("parses multiple pairs when one key is missing", () => {
+  expect(parseQueryString("name=Ben&=Amsterdam")).toEqual({
+    name: "Ben",
+    "": "Amsterdam",
+  });
+});
+
+// Multiple pairs where one value contains =:
+test("parses multiple pairs when one value contains =", () => {
+  expect(parseQueryString("equation=x=y+1&answer=42")).toEqual({
+    equation: "x=y+1",
+    answer: "42",
   });
 });
diff --git a/Sprint-2/implement/tally.js b/Sprint-2/implement/tally.js
index f47321812..0477e1bbc 100644
--- a/Sprint-2/implement/tally.js
+++ b/Sprint-2/implement/tally.js
@@ -1,3 +1,26 @@
-function tally() {}
+function tally(arrayItems) {
+  if (!Array.isArray(arrayItems)) {
+    throw new Error("The input is not an Array");
+  }
+  // Createa an empty object to store the counted value appears in the array.
+  const counter = {};
+  for (const item of arrayItems) {
+    if (counter[item]) {
+      counter[item] += 1;
+    } else {
+      counter[item] = 1;
+    }
+  }
+  return counter;
+}
 
 module.exports = tally;
+
+/*
+If the input is not an array, throw an error.
+Make an empty object called counts.
+For each item in the array:
+If the item is already in counts, add 1 to its value.
+If the item is not in counts, set its value to 1.
+Return the counts object.
+*/
diff --git a/Sprint-2/implement/tally.test.js b/Sprint-2/implement/tally.test.js
index 2ceffa8dd..7a0a8931d 100644
--- a/Sprint-2/implement/tally.test.js
+++ b/Sprint-2/implement/tally.test.js
@@ -23,12 +23,38 @@ const tally = require("./tally.js");
 // Given an empty array
 // When passed to tally
 // Then it should return an empty object
-test.todo("tally on an empty array returns an empty object");
+
+test("tally on an empty array returns an empty object", () => {
+  expect(tally([])).toEqual({});
+});
 
 // Given an array with duplicate items
 // When passed to tally
 // Then it should return counts for each unique item
 
+test("Given an array of duplicate items, return counts for each unique item", () => {
+  expect(tally(["a", "a", "b", "b", "c"])).toEqual({ a: 2, b: 2, c: 1 });
+});
+
+// Test with 1 value in the array
+test("Given an array of duplicate items, return counts for each unique item", () => {
+  expect(tally(["a"])).toEqual({ a: 1 });
+});
+
 // Given an invalid input like a string
 // When passed to tally
 // Then it should throw an error
+
+test("Given an invalid input like string, trow error: The input is not an Array", () => {
+  expect(() => tally("Malakia not a string")).toThrow(
+    "The input is not an Array"
+  );
+});
+
+/*
+Remember: “toThrow needs a function, toEqual needs a value.
+If you want to test for an error, always wrap the function call in another function.
+
+expect(() => myFunc(badInput)).toThrow();
+
+*/
diff --git a/Sprint-2/interpret/invert.js b/Sprint-2/interpret/invert.js
index bb353fb1f..8dc55abfa 100644
--- a/Sprint-2/interpret/invert.js
+++ b/Sprint-2/interpret/invert.js
@@ -10,20 +10,37 @@ function invert(obj) {
   const invertedObj = {};
 
   for (const [key, value] of Object.entries(obj)) {
-    invertedObj.key = value;
+    // Use the old value as the "new" key, and the "old" key as the new value.
+    // invertedObj.key = value; => This always creates a property called "key" not the value of "key"
+    invertedObj[value] = key;
   }
 
   return invertedObj;
 }
 
 // a) What is the current return value when invert is called with { a : 1 }
+// Current return value: { key: 1 }
+// It used the word `key` as the property name.
 
 // b) What is the current return value when invert is called with { a: 1, b: 2 }
+// Current return value: { key: 2 }
+// The second loop overwrites the first one.
 
 // c) What is the target return value when invert is called with {a : 1, b: 2}
+// Target return value: { 1: "a", 2: "b" }
+// The old values become keys, and the old keys become values.
 
 // c) What does Object.entries return? Why is it needed in this program?
+// `Object.entries(obj)` returns an array of key-value pairs.
+// Example: `Object.entries({ a: 1, b: 2 })` becomes `[["a", 1], ["b", 2]]`.
+// We use it to get both the key and value while looping.
+// MDN: Object.entries()
+// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries
 
 // d) Explain why the current return value is different from the target output
+// `invertedObj.key` always makes a property called `key`.
+// We need `invertedObj[value]` so JavaScript uses the real value as the property name.
 
 // e) Fix the implementation of invert (and write tests to prove it's fixed!)
+
+module.exports = invert;
diff --git a/Sprint-2/interpret/invert.test.js b/Sprint-2/interpret/invert.test.js
new file mode 100644
index 000000000..4197069b4
--- /dev/null
+++ b/Sprint-2/interpret/invert.test.js
@@ -0,0 +1,29 @@
+const invert = require("./invert.js");
+
+// When the object has one key-value pair:
+test("swaps the key and value in an object with one property", () => {
+  expect(invert({ a: 1 })).toEqual({
+    1: "a",
+  });
+});
+
+// When the object has multiple key-value pairs:
+test("swaps all keys and values in an object with multiple properties", () => {
+  expect(invert({ a: 1, b: 2 })).toEqual({
+    1: "a",
+    2: "b",
+  });
+});
+
+// When the object is empty:
+test("returns an empty object when given an empty object", () => {
+  expect(invert({})).toEqual({});
+});
+
+// When the values are strings:
+test("uses the original string values as keys in the new object", () => {
+  expect(invert({ first: "Benzaki", second: "BenziMou" })).toEqual({
+    Benzaki: "first",
+    BenziMou: "second",
+  });
+});