Brussels | ITP-2026-1 | Nik | Sprint 2 | Week 8

Sprint-2/debug/address.js

@@ -1,4 +1,6 @@
 // Predict and explain first...
+// objects are not arrays and don`t have ordinal numbers (0, 1, 2)
+// their data is accessed by keys (names)
 
 // This code should log out the houseNumber from the address object
 // but it isn't working...
@@ -12,4 +14,4 @@ const address = {
   postcode: "XYZ 123",
 };
 
-console.log(`My house number is ${address[0]}`);
+console.log(`My house number is ${address.houseNumber}`);

Sprint-2/debug/author.js

@@ -1,4 +1,8 @@
 // Predict and explain first...
+// for...of loop
+// designed for arrays
+// doesn't work with objects
+// objects in JS are not iterable
 
 // 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 +15,6 @@ const author = {
   alive: true,
 };
 
-for (const value of author) {
-  console.log(value);
-}
+for (const value of Object.values(author)) {
+  console.log (value);
+}

Sprint-2/debug/recipe.js

@@ -1,4 +1,5 @@
 // Predict and explain first...
+// ${recipe} object inserted in console.log.
 
 // 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(`\n`)}`);

Sprint-2/implement/contains.js

@@ -1,3 +1,11 @@
-function contains() {}
+function contains(obj, key) {
+    // check if obj is actually an obj not null or array
+    if (!obj || typeof obj !== 'object' || Array.isArray(obj)) {
+        return false;
+    }
+
+    // use 'in' operator to check if key exists in object
+    return key in obj;
+}
 
 module.exports = contains;

Sprint-2/implement/contains.test.js

@@ -1,5 +1,28 @@
 const contains = require("./contains.js");
 
+test ("returns false for empty object", () => {
+    expect(contains({}, 'a')).toBe(false);
+});
+
+test("returns true when property exists", () => {
+    expect(contains({a: 1,  b: 2}, 'a')).toBe(true);
+    expect(contains({name: 'John'}, 'name')).toBe(true);
+});
+
+test("returns false when property doesn't exist", () => {
+    expect(contains({a: 1, b: 2}, 'c')).toBe(false);
+    expect(contains({x: 10}, 'y')).toBe(false);
+});
+
+test("returns false for invalid input (array)", () => {
+    expect(contains([1, 2, 3], '0')).toBe(false);
+});
+
+test("returns false for null or undefined", () => {
+    expect(contains(null, 'a')).toBe(false);
+    expect(contains(undefined, 'a')).toBe(false);
+});
+
 /*
 Implement a function called contains that checks an object contains a
 particular property
@@ -20,7 +43,6 @@ 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");
 
 // Given an object with properties
 // When passed to contains with an existing property name

Sprint-2/implement/lookup.js

@@ -1,5 +1,45 @@
-function createLookup() {
+/**
+ * creates a lookup object from an array of key-value pairs
+ * 
+ * @param {Array<Array<string>>} pairs - array of [key, value] pairs
+ * @returns {Object} - lookup object with keys mapped to values
+ * @throws {Typeerror} - if input is not a valid array of pairs
+ * 
+ * @example 
+ * createLookup([['US', 'USD'], ['CA', 'CAD']])
+ * // Returns: { US: 'USD', CA: 'CAD' }
+ */
+function createLookup(pairs) {
   // implementation here
+  // 1. validate input
+  if (!Array.isArray(pairs)) {
+    throw new TypeError('Input must be an array');
+  }
+
+  // 2. initialize empty result object
+  const lookup = {};
+
+  // 3. iterate through each pair
+  for (const pair of pairs) {
+    // 3a. validate each pair: must be an array with 2 elements
+    if (!Array.isArray(pair) || pair.length !== 2) {
+      throw new TypeError('Each pair must be an array of exactly 2 elements');
+    }
+
+    // 3b. extract key and value
+    const [key, value] = pair; // destructing assignment
+
+    // 3c. validate key must be a string
+    if (typeof key !== 'string') {
+      throw new TypeError('Keys must be stings');
+    }
+
+    // 3d. add to lookup object
+    lookup[key] = value;
+  }
+
+  // 4. return the constructed lookup object
+  return lookup;
 }
 
 module.exports = createLookup;

Sprint-2/implement/lookup.test.js

@@ -1,9 +1,59 @@
 const createLookup = require("./lookup.js");
 
-test.todo("creates a country currency code lookup for multiple codes");
+// happy path: valid input
+test("creates a lookup object from valid pairs", () => {
+  const pairs = [['US', 'USD'], ['CA', 'CAD']];
+  const result = createLookup(pairs);
 
-/*
+  expect(result).toEqual({
+    US: 'USD',
+    CA: 'CAD'
+  });
+});
 
+// empty array
+test("returns empty object for empty array", () => {
+  expect(createLookup([])).toEqual({});
+});
+
+// single pair
+test("handle single pair correctly", () => {
+  const pairs = [['UK', 'GBP']];
+  expect(createLookup(pairs)).toEqual({ UK: 'GBP' });
+});
+
+// duplicates keys (last value wins)
+test("handle duplicates keys by keeping last value", () => {
+  const pairs = [['US', 'USD'], ['US', 'EUR']];
+  expect(createLookup(pairs)).toEqual({ US: 'EUR' });
+});
+
+// error handling: not an array
+test("throws TypeError when input is not an array", () => {
+  expect(() => createLookup(null)).toThrow(TypeError);
+  expect(() => createLookup('invalid')).toThrow(TypeError);
+  expect(() => createLookup({a: 1})).toThrow(TypeError);
+});
+
+// error handling: invalid pair (not array)
+test("throws TypeError when pair is not an array", () => {
+  const pairs = [['US', 'USD'], 'invalid'];
+  expect(() => createLookup(pairs)).toThrow(TypeError);
+});
+
+// error handling: invalid pair (wrong length)
+test("throws TypeError when pair has wrong length", () => {
+  const pairs = [['US', 'USD'], ['CA']]; // missing value
+  expect(() => createLookup(pairs)).toThrow(TypeError);
+});
+
+// error handling: non-string key
+test("throws TypeError when key is not a string", () => {
+  const pairs = [[42, 'USD']]; // number as key
+  expect(() => createLookup(pairs)).toThrow(TypeError);
+});
+
+/**
 Create a lookup object of key value pairs from an array of code pairs
 
 Acceptance Criteria:

Sprint-2/implement/querystring.js

@@ -1,12 +1,26 @@
 function parseQueryString(queryString) {
   const queryParams = {};
-  if (queryString.length === 0) {
+  if (!queryString || queryString.length === 0) {
     return queryParams;
   }
+
   const keyValuePairs = queryString.split("&");
 
   for (const pair of keyValuePairs) {
-    const [key, value] = pair.split("=");
+    // divide only by the first sign '='
+    const firstEqualIndex = pair.indexOf("=");
+
+    // If there is no '=',
+    // consider it a key without a value (or skip it)
+    if (firstEqualIndex === -1) {
+      queryParams[pair] = "";
+      continue;
+    }
+
+    const key = pair.slice(0, firstEqualIndex);
+    const value = pair.slice(firstEqualIndex +1);
+
+    // const [key, value] = pair.split("=");
     queryParams[key] = value;
   }
 

Sprint-2/implement/querystring.test.js

@@ -10,3 +10,22 @@ test("parses querystring values containing =", () => {
     "equation": "x=y+1",
   });
 });
+
+test("handles keys without values", () => {
+  expect(parseQueryString("name")).toEqual({ name: ""});
+});
+
+test("handles empty values", () => {
+  expect(parseQueryString("name=")).toEqual({ name: "" });
+});
+
+test("parses simple query string", () => {
+  expect(parseQueryString("name=John&age=30")).toEqual({
+    name: "John",
+    age: "30",
+  });
+});
+
+test("returns empty objects for empty strings", () => {
+  expect(parseQueryString("")).toEqual({});
+});

Sprint-2/implement/tally.js

@@ -1,3 +1,14 @@
-function tally() {}
+function tally(items) {
+    if(!Array.isArray(items)) {
+        throw new TypeError("Input must be an array");
+    }
+
+    const counts = {};
+    for (const item of items) {
+        // for no key, initialize 0 and add 1
+        counts[item] = (counts[item] || 0) + 1;
+    }
+    return counts;
+}
 
 module.exports = tally;

Sprint-2/implement/tally.test.js

@@ -1,5 +1,20 @@
 const tally = require("./tally.js");
 
+test("tally on an empty array returns an empty object", () => {
+    expect(tally([])).toEqual({});
+});
+
+test("tally counts frequency of items in an array", () => {
+    expect(tally(['a'])).toEqual({ a: 1 });
+    expect(tally(['a', 'a', 'a'])).toEqual({ a: 3 });
+    expect(tally(['a', 'a', 'b', 'c'])).toEqual({ a:2, b:1, c:1});;
+});
+
+test("tally throws TypeError on invalid input", () => {
+    expect(() => tally("not an array")).toThrow(TypeError);
+    expect(() => tally(null)).toThrow(TypeError);
+});
+
 /**
  * tally array
  *
@@ -23,7 +38,6 @@ 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");
 
 // Given an array with duplicate items
 // When passed to tally

Sprint-2/interpret/invert.js

@@ -7,23 +7,54 @@
 // E.g. invert({x : 10, y : 20}), target output: {"10": "x", "20": "y"}
 
 function invert(obj) {
+  // 1. valiadate input 
+  if (!obj || typeof obj !== 'object' || Array.isArray(obj)) {
+    throw new TypeError('Input must be a plain object');
+  }
+
+  // 2. initialize empty results object
   const invertedObj = {};
 
+  // 3. iterate through key-value pairs
   for (const [key, value] of Object.entries(obj)) {
-    invertedObj.key = value;
+    // 4. swap: value becomes key, bey becomes value
+    invertedObj[value] = key;
   }
 
   return invertedObj;
 }
 
+module.exports = invert;
+
 // a) What is the current return value when invert is called with { a : 1 }
+// invert({ a: 1 })
+// Result: { key: 1 }
+// invertedObj.key = value → creates a "key" property (string), not a key variable
 
 // b) What is the current return value when invert is called with { a: 1, b: 2 }
+// invert({ a: 1, b: 2 })
+// Result: { key: 2 }
+// First iteration: { key: 1 }
+// Second iteration: { key: 2 } ← overwritten!
 
 // c) What is the target return value when invert is called with {a : 1, b: 2}
+// result:
+// { "1": "a", "2": "b" }
+// Values ​​become keys, keys become values
 
 // c) What does Object.entries return? Why is it needed in this program?
+// Object.entries({ a: 1, b: 2 })
+// returns: [["a", 1], ["b", 2]]
+// Why is this necessary?
+// To access both the key and the value simultaneously
+// Destructuring is used: const [key, value] = ["a", 1]
 
 // d) Explain why the current return value is different from the target output
+// 1. Dot notation instead of bracket notation:
+// invertedObj.key = value; // ❌ Creates a "key" property (string)
+// invertedObj[key] = value; // ✅ Uses the value of the key variable
+// 2. Incorrect key-value order:
+// invertedObj[key] = value; // ❌ Key remains the key
+// invertedObj[value] = key; // ✅ Value becomes the key
 
 // e) Fix the implementation of invert (and write tests to prove it's fixed!)

Sprint-2/interpret/invert.test.js

@@ -0,0 +1,29 @@
+const invert = require("./invert.js");
+
+// happy path
+test("inverts a single key-value pair", () => {
+    expect(invert({ a: 1})).toEqual({ "1": "a" });
+});
+
+test("inverts with string values", () => {
+    expect(invert({ x: "hello", y: "world" })).toEqual({
+        hello: "x",
+        world: "y"
+    });
+});
+
+// edge cases
+test("handles empty pbject", () => {
+    expect(invert({})).toEqual({});
+});
+
+test("handles duplicate values (last key wins)", () => {
+    expect(invert({ a:1, b: 1 })).toEqual({ "1": "b" });
+});
+
+// error handling
+test("throws TypeError for invalid input", () => {
+    expect(() => invert(null)).toThrow(TypeError);
+    expect(() => invert([1, 2])).toThrow(TypeError);
+    expect(() => invert("string")).toThrow(TypeError);
+});