diff --git a/Sprint-2/debug/address.js b/Sprint-2/debug/address.js
index 940a6af83..6ad482e39 100644
--- a/Sprint-2/debug/address.js
+++ b/Sprint-2/debug/address.js
@@ -1,5 +1,15 @@
 // Predict and explain first...
 
+/*
+prediction: My house number is undefined
+Reason: the code is trying to access the key 0 of the address object, which
+does not exist.
+
+Object properties aren't accessible by index. Author likely tried to treat it
+the same as an array. Arrays are a special type of object which allows indexing into (some)
+of its values.
+*/
+
 // This code should log out the houseNumber from the address object
 // but it isn't working...
 // Fix anything that isn't working
@@ -12,4 +22,4 @@ 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..509be84f2 100644
--- a/Sprint-2/debug/author.js
+++ b/Sprint-2/debug/author.js
@@ -1,4 +1,11 @@
 // Predict and explain first...
+/*
+Prediction: TypeError: author is not iterable
+
+Cause: Plain Js objects aren't iterable the way an array's elements are.
+To iterate over keys and/or values use the Object.keys(), Object.values(), 
+or Object.entries() methods to covert the data into an array first.
+*/
 
 // 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 +18,6 @@ const author = {
   alive: true,
 };
 
-for (const value of author) {
+for (const value of Object.values(author)) {
   console.log(value);
 }
diff --git a/Sprint-2/debug/recipe.js b/Sprint-2/debug/recipe.js
index 6cbdd22cd..894f7f472 100644
--- a/Sprint-2/debug/recipe.js
+++ b/Sprint-2/debug/recipe.js
@@ -1,5 +1,13 @@
 // Predict and explain first...
 
+/*
+Prediction: the ingredients: ${recipe} part will only print something like [Object object]
+
+Cause: The code is attempting to print the recipe object itself, rather than the ingredients.
+In Js, when printing objects, it doesn't print a string of key, pair values of its 
+properties. Should be printing the ingredients property, which is an array, which does print out its values.
+*/
+
 // 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}`);
diff --git a/Sprint-2/implement/contains.js b/Sprint-2/implement/contains.js
index cd779308a..22480ed6a 100644
--- a/Sprint-2/implement/contains.js
+++ b/Sprint-2/implement/contains.js
@@ -1,3 +1,12 @@
-function contains() {}
+function isObject(item) {
+  return typeof item === "object" && item !== null && !Array.isArray(item);
+}
+
+function contains(object, key) {
+  if (isObject(object)) {
+    return Object.keys(object).includes(key);
+  }
+  throw new TypeError("Item is not plain object, Date, or Map");
+}
 
 module.exports = contains;
diff --git a/Sprint-2/implement/contains.test.js b/Sprint-2/implement/contains.test.js
index 326bdb1f2..c994dfd0a 100644
--- a/Sprint-2/implement/contains.test.js
+++ b/Sprint-2/implement/contains.test.js
@@ -20,16 +20,27 @@ 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("given an empty object, should return false", () => {
+  expect(contains({}, "a")).toEqual(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 a property, and property name that exists, should return true", () => {
+  expect(contains({ a: 12, b: 2 }, "a")).toEqual(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, and property that does not exist in object, should return false", () => {
+  expect(contains({ a: 12, b: 2 }, "c")).toEqual(false);
+});
 
 // Given invalid parameters like an array
 // When passed to contains
 // Then it should return false or throw an error
+test("given invalid parameter, like an array, should return false or throw error", () => {
+  expect(() => contains(22, "a")).toThrow(TypeError);
+});
diff --git a/Sprint-2/implement/lookup.js b/Sprint-2/implement/lookup.js
index a6746e07f..43d15d403 100644
--- a/Sprint-2/implement/lookup.js
+++ b/Sprint-2/implement/lookup.js
@@ -1,5 +1,25 @@
-function createLookup() {
-  // implementation here
+// check if arry is 2d, and each element is a k,v pair and k,v both strings
+function is2dKeyValStringArray(arr) {
+  return (
+    Array.isArray(arr) &&
+    arr.every((element) => {
+      return (
+        Array.isArray(element) &&
+        element.length === 2 &&
+        typeof element[0] === "string" &&
+        typeof element[1] === "string"
+      );
+    })
+  );
+}
+
+function createLookup(countryAndCurrency) {
+  // bit lazy, should have tests for type, and content seperately,
+  // to throw more specific errors
+  if (!is2dKeyValStringArray(countryAndCurrency)) {
+    throw new Error("Input is not valid type and/or format");
+  }
+  return Object.fromEntries(countryAndCurrency);
 }
 
 module.exports = createLookup;
diff --git a/Sprint-2/implement/lookup.test.js b/Sprint-2/implement/lookup.test.js
index 547e06c5a..f6e2722d9 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 +31,57 @@ It should return:
    'CA': 'CAD'
  }
 */
+
+test("converts 2d array to object", () => {
+  expect(
+    createLookup([
+      ["a", "x"],
+      ["b", "y"],
+      ["c", "z"],
+    ])
+  ).toEqual({ a: "x", b: "y", c: "z" });
+});
+
+test("convert empty array, to empty object", () => {
+  expect(createLookup([])).toEqual({});
+});
+
+test("convert single 2d array, to object with one key", () => {
+  expect(createLookup([["a", "wow"]])).toEqual({ a: "wow" });
+});
+
+// this is the behaviour of Object.fromEntries(), I keep it for consistency
+test("when there are duplicate keys, later entries overwrite earlier entries", () => {
+  expect(
+    createLookup([
+      ["a", "ff"],
+      ["b", "gg"],
+      ["a", "hh"],
+    ])
+  ).toEqual({ b: "gg", a: "hh" });
+});
+
+test("when input is not an array throw error", () => {
+  expect(() => createLookup("asdf")).toThrow();
+  expect(() => createLookup(1)).toThrow();
+  expect(() => createLookup({ a: 12 })).toThrow();
+});
+
+test("when input is not a  2d array throw error", () => {
+  expect(() => createLookup(["a", "b"])).toThrow();
+  expect(() => createLookup([["a", "b"]])).not.toThrow();
+});
+
+test("when length of internal array not 1 < arr.length < 3", () => {
+  expect(() => createLookup(["a"])).toThrow();
+  expect(() => createLookup(["a", "b", "c"])).toThrow();
+});
+
+test("when non-string k, v pairs throw error", () => {
+  expect(() =>
+    createLookup([
+      [1, 2],
+      [3, 4],
+    ])
+  ).toThrow();
+});
diff --git a/Sprint-2/implement/querystring.js b/Sprint-2/implement/querystring.js
index 45ec4e5f3..bb42c66e5 100644
--- a/Sprint-2/implement/querystring.js
+++ b/Sprint-2/implement/querystring.js
@@ -1,13 +1,45 @@
 function parseQueryString(queryString) {
   const queryParams = {};
-  if (queryString.length === 0) {
-    return queryParams;
-  }
+  if (!queryString || typeof queryString !== "string") return queryParams;
+
+  if (queryString.trim() === "") return queryParams;
+
   const keyValuePairs = queryString.split("&");
 
   for (const pair of keyValuePairs) {
-    const [key, value] = pair.split("=");
-    queryParams[key] = value;
+    const firstEqualsIndex = pair.indexOf("=");
+    let key, value;
+
+    // Checks for pairs with key but no value like debug&verbose&name=Alice
+    // by keeping them, prevent loss of information
+    if (firstEqualsIndex === -1) {
+      key = pair.trim();
+      value = "";
+    } else {
+      // splits on first equal, but not any of the rest
+      key = pair.slice(0, firstEqualsIndex).trim();
+      value = pair.slice(firstEqualsIndex + 1).trim();
+    }
+
+    // skips next part if the ends up being an empty str after slice and trim
+    if (!key) continue;
+
+    /* use hasOwnProperty to prevent conflict with built-in/inherited properties
+    like "toString", if the query string has a "toString=value".
+
+    if a key already exists on queryParams, but is not an array, then create an array
+    and store the the old value and new value in the array
+    */
+    if (Object.prototype.hasOwnProperty.call(queryParams, key)) {
+      if (queryParams[key] === value || queryParams[key].includes(value))
+        continue;
+      if (!Array.isArray(queryParams[key])) {
+        queryParams[key] = [queryParams[key]];
+      }
+      queryParams[key].push(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..d4c1ab4aa 100644
--- a/Sprint-2/implement/querystring.test.js
+++ b/Sprint-2/implement/querystring.test.js
@@ -1,12 +1,83 @@
 // In the prep, we implemented a function to parse query strings.
 // Unfortunately, it contains several bugs!
 // 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.
+// 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");
 
+// empty string
+test("empty query string returns {}", () => {
+  expect(parseQueryString("")).toEqual({});
+});
+
+// null or undefined
+test("empty query string returns {}", () => {
+  expect(parseQueryString(null)).toEqual({});
+  expect(parseQueryString(undefined)).toEqual({});
+  expect(parseQueryString()).toEqual({});
+});
+
+// happy path
+// single valid kv pair
+test("a single k, v with no special chars return object with matching k, v pair", () => {
+  expect(parseQueryString("mykey=myvalue")).toEqual({ mykey: "myvalue" });
+});
+// mutiple valid kv pairs
+test("multiple k, v pairs with no special chars return object with matching k, v pairs", () => {
+  expect(parseQueryString("mykey=myvalue&otherkey=othervalue&third=3")).toEqual(
+    {
+      mykey: "myvalue",
+      otherkey: "othervalue",
+      third: "3",
+    }
+  );
+});
+
+// special characters in key, can't handle = in key, as that is fundamentally unparsable
+test("special character in key without URL encoding", () => {
+  expect(parseQueryString("my+key=myvalue")).toEqual({
+    "my+key": "myvalue",
+  });
+});
+// special characters in value
 test("parses querystring values containing =", () => {
   expect(parseQueryString("equation=x=y+1")).toEqual({
-    "equation": "x=y+1",
+    equation: "x=y+1",
+  });
+});
+
+// missing values
+test("key with no value return object with the key and value of empty string", () => {
+  expect(parseQueryString("foo=")).toEqual({ foo: "" });
+});
+
+//valueless flags
+test("when given valueless keys, keep the key with empty string as value", () => {
+  expect(parseQueryString("debug&verbose")).toEqual({ debug: "", verbose: "" });
+});
+
+// ignore keyless values
+test("ignores pairs with no key", () => {
+  expect(parseQueryString("=value")).toEqual({});
+});
+
+// duplicate keys with different values (array)
+test("collects duplicate keys into an array", () => {
+  expect(parseQueryString("color=red&color=blue")).toEqual({
+    color: ["red", "blue"],
+  });
+});
+
+// duplicate k,v pair
+test("if k,v already exists, or value array for a key already as value, ignore", () => {
+  expect(parseQueryString("color=red&color=red")).toEqual({ color: "red" });
+});
+
+// white spaces
+test("trims whitespace from keys and values", () => {
+  expect(parseQueryString("name= Alice &age= 30 ")).toEqual({
+    name: "Alice",
+    age: "30",
   });
 });
diff --git a/Sprint-2/implement/tally.js b/Sprint-2/implement/tally.js
index f47321812..94c934ef4 100644
--- a/Sprint-2/implement/tally.js
+++ b/Sprint-2/implement/tally.js
@@ -1,3 +1,14 @@
-function tally() {}
+function tally(list) {
+  if (!Array.isArray(list)) {
+    throw new TypeError("Invalid input: not an array");
+  }
+  return (itemCountMapping = list.reduce((acc, curr) => {
+    if (typeof curr !== "string" && curr !== "number") {
+      throw new Error(`Invalid element: ${curr} must be a string or number`);
+    }
+    acc[curr] = (acc[curr] ?? 0) + 1;
+    return acc;
+  }, {}));
+}
 
 module.exports = tally;
diff --git a/Sprint-2/implement/tally.test.js b/Sprint-2/implement/tally.test.js
index 2ceffa8dd..ea8652e24 100644
--- a/Sprint-2/implement/tally.test.js
+++ b/Sprint-2/implement/tally.test.js
@@ -23,12 +23,24 @@ 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("given empty array, returns 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 array with duplicate elements, returns correct count of each item", () => {
+  expect(tally(["a", "a", "b"])).toEqual({ a: 2, b: 1 });
+});
 
 // Given an invalid input like a string
 // When passed to tally
 // Then it should throw an error
+test("given invalid input like a string, throw error", () => {
+  expect(() => tally("asdf")).toThrow(TypeError);
+});
+
+test("elements of the list must be string or number, or throw error", () => {
+  expect(() => tally([{ a: 12 }, { b: "c" }, { a: 12 }])).toThrow();
+});
diff --git a/Sprint-2/interpret/invert.js b/Sprint-2/interpret/invert.js
index bb353fb1f..378d88567 100644
--- a/Sprint-2/interpret/invert.js
+++ b/Sprint-2/interpret/invert.js
@@ -7,23 +7,63 @@
 // E.g. invert({x : 10, y : 20}), target output: {"10": "x", "20": "y"}
 
 function invert(obj) {
+  // need obj == null because 'type of obj === null' does not work in Js
+  if (typeof obj !== "object" || obj === null || Array.isArray(obj)) {
+    throw new TypeError("Input must be a plain object");
+  }
+
   const invertedObj = {};
 
   for (const [key, value] of Object.entries(obj)) {
-    invertedObj.key = value;
+    // if a value is not a number or string, it can't be used as a key in the inverted version
+    if (typeof value !== "string" && typeof value !== "number") {
+      throw new TypeError(`Value ${JSON.stringify(value)} is not a valid key`);
+    }
+    invertedObj[String(value)] = String(key);
   }
 
   return invertedObj;
 }
 
+module.exports = invert;
 // a) What is the current return value when invert is called with { a : 1 }
+/* 
+It would return {key: 1}. The dot nation means that it uses the string "key" as the key.
+and the value for that key is the value from the object being iterated over.
+*/
 
 // b) What is the current return value when invert is called with { a: 1, b: 2 }
+/*
+It would return {key : 2}. First it would do as in a, and the overwrite the value of the key "key"
+from 1 to 2.
+*/
 
 // c) What is the target return value when invert is called with {a : 1, b: 2}
+/*
+The target is {"1": "a", "2": "b"}
+*/
 
 // c) What does Object.entries return? Why is it needed in this program?
+/*
+It returns a 2d array containing arrays of key value pairs from the object of the form:
+[[key1, value1], [key2, value2], [key3, value3]].
+
+It is needed because can't iterate over objects the same way as arrays. Objects are a bit more 
+complicated, because you have to decide if you want to iterate over the keys, values, or both.
+you also have to decide if you want to include inherited properties. 
 
+Object.entries() makes this assumption for you, by including only the object's own properties. 
+It also just makes this particular task easier by accessing the key and value together.
+If you wanted to include inherited properties you'd use (for ... in) loop.
+
+*/
 // d) Explain why the current return value is different from the target output
+/*
+The dot nation means that it uses the string "key" as the key.
+and the value for that key is the value from the object being iterated over. 
+So it just continuously overwrites that one "key" property of the to-be-returned object.
+
+It's also swapping key, value from the old object.
+*/
 
 // e) Fix the implementation of invert (and write tests to prove it's fixed!)
diff --git a/Sprint-2/interpret/invert.test.js b/Sprint-2/interpret/invert.test.js
new file mode 100644
index 000000000..34673bb2f
--- /dev/null
+++ b/Sprint-2/interpret/invert.test.js
@@ -0,0 +1,47 @@
+const invert = require("./invert.js");
+
+test("Inverts simple object with string key and string value", () => {
+  expect(invert({ a: "x", b: "y", c: "z" })).toEqual({
+    x: "a",
+    y: "b",
+    z: "c",
+  });
+});
+
+test("Returns empty object if input is an empty object", () => {
+  expect(invert({})).toEqual({});
+});
+
+test("Throws error for non-object inputs", () => {
+  expect(() => invert("asdf")).toThrow(TypeError);
+  expect(() => invert([1, "a", 2, "b"])).toThrow(TypeError);
+  expect(() => invert()).toThrow();
+  expect(() => invert(null)).toThrow();
+});
+
+test("Throws errors for non-string, non-numeric keys or values", () => {
+  expect(() => invert({ a: { b: 12 } })).toThrow(TypeError);
+});
+
+test("Duplicate values become one key, where the last key in original becomes its new value", () => {
+  expect(invert({ a: "x", b: "x" })).toEqual({ x: "b" });
+});
+
+test("numeric keys become string converted to value", () => {
+  expect(invert({ 1: "a", 2: "b" })).toEqual({ a: "1", b: "2" });
+});
+
+test("invert twice returns original if all values are unique", () => {
+  expect(invert(invert({ a: "x", b: "y", x: "z" }))).toEqual({
+    a: "x",
+    b: "y",
+    x: "z",
+  });
+});
+
+test("invert twice does not returns original if all values are not unique", () => {
+  expect(invert(invert({ a: "x", b: "y", c: "y" }))).toEqual({
+    a: "x",
+    c: "y",
+  });
+});
diff --git a/Sprint-2/package-lock.json b/Sprint-2/package-lock.json
index 9b4c725d6..ceda7296e 100644
--- a/Sprint-2/package-lock.json
+++ b/Sprint-2/package-lock.json
@@ -56,6 +56,7 @@
       "integrity": "sha512-Oixnb+DzmRT30qu9d3tJSQkxuygWm32DFykT4bRoORPa9hZ/L4KhVB/XiRm6KG+roIEM7DBQlmg27kw2HZkdZg==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "@ampproject/remapping": "^2.2.0",
         "@babel/code-frame": "^7.25.7",
@@ -1368,6 +1369,7 @@
         }
       ],
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "caniuse-lite": "^1.0.30001663",
         "electron-to-chromium": "^1.5.28",