Week 5: JavaScript Array Methods

Week	Topic	Learning Objectives	Key Resources
5	JavaScript Array Methods	Students will be able to employ array methods including map(), filter(), and reduce() to transform array data; employ callbacks and higher order functions with existing arrays.	Week 5 Slides

• Array methods like map(), filter(), and reduce() allow powerful data transformation
• map()
  • Transforms each element in an array
  • Returns a new array with the transformed elements
• filter()
  • Filters array to elements that pass a conditional test
  • Returns subset of array
• reduce()
  • Reduces array to a single value
  • Callback takes accumulator and current value
  • Initial value can be specified
• Benefits
  • More concise, readable code
  • Avoid bugs associated with manual iteration

Guidance for Mentors

• Start with simple examples and sample data that demonstrate each method visually. Leverage analogies like Project Odin's sandwich analogy to explain the methods in an intuitive way first.
• Let students experiment with running different types of functions as callbacks.
• Have students describe in their own words what each method does before coding. Check for gaps.
• Contrast benefits of methods vs manual iteration. When might manual work make sense still?

You can mark the student's assignment as complete if they:

• Fork the Replit file and submit their own link.
• Use proper syntax and good JavaScript logic to answer the questions. Example outputs (Note that some student work variation is expected!):

Question 1:

let names = ["Beyonce", "Kelly", "Michelle"];

function printNames(arr) {
  arr.forEach(name => console.log(name));
}

console.log("Q1: ");
printNames(names);

Question 2:

let trees = [
  { type: "Dogwood", height: 10 },
  { type: "Maple", height: 15 },
  { type: "Oak", height: 20 }
];

function logTreeType(arr) {
  arr.forEach(tree => console.log(tree.type));
}

console.log("Q2: ");
logTreeType(trees);

Question 3:

let myNumbers = [1, 2, 3, 4, 5];

function totalPoints(arr) {
  let sum = 0;
  arr.forEach(num => sum += num);
  return sum;
}

console.log("Q3: ", totalPoints(myNumbers));

Question 4:

let myWords = ["Hello", "world", "JavaScript"];

function buildSentence(arr) {
  let sentence = "";
  arr.forEach(word => sentence += word + " ");
  return sentence.trim();
}

console.log("Q4: ", buildSentence(myWords));

Question 5:

let decimals = [0.75, 0.91, 0.2, 1.34];

const logPercentagesV1 = (arr) =>
  arr.map((el) => el.toLocaleString(undefined, { style: "percent" }));

console.log("Q5: ", logPercentagesV1(decimals));

Question 6:

let startingNums = [1, 2, 3, 4, 5];

function addThreeToAll(arr) {
  return arr.map(num => num + 3);
}

console.log("Q6: ", addThreeToAll(startingNums));

Question 7:

let baseNums = [1, 2, 3, 4, 5];

function squareAll(arr) {
  return arr.map(num => num ** 2);
}

console.log("Q7: ", squareAll(baseNums));

Question 8:

function allGreetings(names) {
  return names.map(name => `Hello, ${name}, nice to meet you!`); 
}

console.log("Q8: ", allGreetings(names));

Question 9:

let users = [
  {
    "username": "juan.marcos",
    "isAdmin": false,
  },
  {
    "username": "aleksandra.ivanov",
    "isAdmin": false
  },
  {
    "username": "zhang.wei",
    "isAdmin": false
  },
  {
    "username": "bernice.king",
    "isAdmin": true
  }
   ];

function getUsernames(arr) {
  return arr.map(user => user.username);
}

console.log("Q9: ", getUsernames(users));

Question 10:

function pluck(arr, key) {
  return arr.map(obj => obj[key]);
}

console.log("Q10: ", pluck(users, 'username'));

Question 11:

function evenNumbers(arr) {
  return arr.filter(num => num % 2 === 0);
}

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

console.log("Q11: ", evenNumbers(numbers));

Question 12:

function greaterThan100(arr) {
  return arr.filter(num => num > 100);
}

let numbers2 = [50, 75, 100, 125, 150, 175];

console.log("Q12: ", greaterThan100(numbers2));

Question 13:

function nonAdminUsers(arr) {
  return arr.filter(user => !user.isAdmin);
}

console.log("Q13: ", nonAdminUsers(users));

Question 14:

function countAdminUsers(arr) {
  return arr.filter(user => user.isAdmin).length;
}

console.log("Q14: ", countAdminUsers(users));

Question 15:

function shorterThanX(arr, length) {
  return arr.filter(str => str.length < length);
}

let strings = ["apple", "banana", "kiwi", "orange", "grape"];
let maxLength = 6;

console.log("Q15: ", maxLength, ":", shorterThanX(strings, maxLength));

Question 16:

function onlyStrings(arr) {
  return arr.filter(element => typeof element === "string");
}

let mixedArray = [1, "apple", true, "banana", 42, false, "kiwi"];

console.log("Q16: ", onlyStrings(mixedArray));

Question 17:

function firstOdd(arr) {
  return arr.find(number => number % 2 === 1);
}

let numbers3 = [2, 4, 6, 8, 9, 10, 11, 12];

console.log("Q17: ", firstOdd(numbers3));

Question 18:

function getAdministrator(users) {
  return users.find(user => user.isAdmin === true);
}
console.log("Q18: ", getAdministrator(users));

Question 19:

function divisibleByTen(arr) {
  return arr.find(number => number % 10 === 0);
}

let numArray = [15, 27, 30, 42, 55, 60, 75];

console.log("Q19: ", divisibleByTen(numArray));

Question 20:

function divisibleByX(arr, num) {
  return arr.find(number => number % num === 0);
}

let divideArray = [15, 27, 30, 42, 55, 60, 75];
let divisor = 5;

console.log("Q20: ", divisibleByX(divideArray, divisor));

Question 21:

With the stretch goal:

function startsWithLetter(strings, letter) {
  return strings.find(string => string.charAt(0) === letter);
}

let words = ["apple", "banana", "cherry", "date"];
let letter = "b";

console.log("Q21: ", startsWithLetter(words, letter));

Without the stretch goal:

function startsWithLetter(strings, letter) {
  if (typeof letter !== "string" || letter.length !== 1) {
    throw new Error("Letter must be a string of length 1");
  }
  return strings.find(string => string.charAt(0) === letter);
}

let words = ["apple", "banana", "cherry", "date"];
let letter = "b";

try {
  console.log("Q21: ", startsWithLetter(words, letter));
} catch (error) {
  console.log("Q21 - Error:", error.message);
}