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javascript

JavaScript Coding Challenges for Beginners

1. Multiples of 3 or 5

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Finish the solution so that it returns the sum of all the multiples of 3 or 5 below the number passed in.

Note: If the number is a multiple of both 3 and 5, only count it once. Also, if a number is negative, return 0.

const solution = number => {
  // Your solution
};

console.log(solution(0)); // 0
console.log(solution(-15)); // 0
console.log(solution(10)); // 23
console.log(solution(20)); // 78
console.log(solution(200)); // 9168
Solution 
const solution = number => {
  let sum = 0;
  for (let i = 3; i < number; i++) {
    if (i % 3 === 0 || i % 5 === 0) {
      sum += i;
    }
  }
  return sum;
};

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2. Even or Odd

Create a function that takes an integer as an argument and returns "Even" for even numbers or "Odd" for odd numbers.

const even_or_odd = number => {
  // Your solution
};

console.log(even_or_odd(0)); // 'Even'
console.log(even_or_odd(2)); // 'Even'
console.log(even_or_odd(3)); // 'Odd'
console.log(even_or_odd(-3)); // 'Odd'
Solution 
const even_or_odd = number => {
  // Let's use a ternary operator
  return number % 2 === 0 ? 'Even' : 'Odd';
};

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3. Clock

The clock shows h hours (0 <= h <= 23), m minutes (0 <= m <= 59) and s seconds (0 <= s <= 59) after midnight. Your task is to write a function which returns the time since midnight in milliseconds. There are 1,000 milliseconds in a second.

const past = (h, m, s) => {
  // Your solution
};

console.log(past(0, 0, 0)); // 0
console.log(past(0, 1, 1)); // 61000
console.log(past(1, 0, 0)); // 3600000
console.log(past(1, 0, 1)); // 3601000
console.log(past(1, 1, 1)); // 3661000
Solution 
const past = (h, m, s) => {
  return (h * 60 * 60 + m * 60 + s) * 1000;
};

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4. Returning Strings

Write a function that given the input string name, returns the greeting statement Hello, <name> how are you doing today?

const greet = name => {
  //Your solution
};

console.log(greet('Ryan')); // "Hello, Ryan how are you doing today?"
console.log(greet('Sara')); // "Hello, Sara how are you doing today?"
Solution 
const greet = name => {
  // Let's use a template literal
  return `Hello, ${name} how are you doing today?`;
};

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5. Century From Year

The first century spans from the year 1 up to and including the year 100, The second - from the year 101 up to and including the year 200, etc. Given a year, return the century it is in.

const century = year => {
  // Your solution
};

console.log(century(1705)); // 18
console.log(century(1900)); // 19
console.log(century(1601)); // 17
console.log(century(2000)); // 20
console.log(century(89)); // 1
Solution 
const century = year => {
  return Math.ceil(year / 100);
};

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6. Keep Hydrated!

Nathan loves cycling. Because Nathan knows it is important to stay hydrated, he drinks 0.5 litres of water per hour of cycling. Given the time in hours, you need to return the number of litres of water that Nathan will drink, rounded to the smallest value.

const litres = time => {
  // Your solution
};

console.log(litres(0)); // 0
console.log(litres(2)); // 1
console.log(litres(1.4)); // 0
console.log(litres(12.3)); // 6
console.log(litres(0.82)); // 0
console.log(litres(11.8)); // 5
console.log(litres(1787)); // 893
Solution 
const litres = time => {
  return Math.floor(time * 0.5);
};

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7. Is n Divisible by x and y?

Create a function that checks if a number n is divisible by two numbers x AND y. All inputs are positive, non-zero digits.

const isDivisible = (n, x, y) => {
  // Your solution
};

console.log(isDivisible(3, 3, 4)); // false
console.log(isDivisible(12, 3, 4)); // true
console.log(isDivisible(8, 3, 4)); // false
console.log(isDivisible(48, 3, 4)); // true
Solution 
const isDivisible = (n, x, y) => {
  return n % x === 0 && n % y === 0;
};

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8. Vowel Count

Return the number (count) of vowels (a, e, i, o, u) in the given string. The input string will only consist of lower case letters and/or spaces.

const getCount = str => {
  // Your solution
};

console.log(getCount('my pyx')); // 0
console.log(getCount('pear tree')); // 4
console.log(getCount('abracadabra')); // 5
console.log(getCount('o a kak ushakov lil vo kashu kakao')); // 13
Solution 
const getCount = str => {
  let vowelsCount = 0;
  for (let char of str) {
    if ('aeiou'.includes(char)) vowelsCount++;
  }
  return vowelsCount;
};

// An alternative solution could be to convert the string to an array (using the spread syntax)
// and filtering that array by vowels
const getCount = str => {
  const arr = [...str];
  return arr.filter(ele => ['a', 'e', 'i', 'o', 'u'].includes(ele)).length;
};

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9. Disemvowel Trolls

Trolls are attacking your comment section! A common way to deal with this situation is to remove all of the vowels from the trolls' comments, neutralizing the threat. Your task is to write a function that takes a string and returns a new string with all vowels (a, e, i, o, u) removed.

const disemvowel = str => {
  // Your solution
};

console.log(disemvowel('This website is for losers LOL!')); // 'Ths wbst s fr lsrs LL!'
Solution 
const disemvowel = str => {
  // Let's use regular expressions (regex)
  return str.replace(/[aeiou]/gi, '');
};

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10. Find the Odd Int

Given an array of integers, find the one that appears an odd number of times. There will always be only one integer that appears an odd number of times.

const findOdd = arr => {
  // Your solution
};

console.log(findOdd([20, 1, -1, 2, -2, 3, 3, 
10000
5, 5, 1, 2, 4, 20, 4, -1, -2, 5])); // 5
console.log(findOdd([20, 1, 1, 2, 2, 3, 3, 5, 5, 4, 20, 4, 5])); // 5
console.log(findOdd([1, 1, 2, -2, 5, 2, 4, 4, -1, -2, 5])); // -1
console.log(findOdd([5, 4, 3, 2, 1, 5, 4, 3, 2, 10, 10])); // 1
console.log(findOdd([1, 1, 1, 1, 1, 1, 10, 1, 1, 1, 1])); // 10
console.log(findOdd([10])); // 10
Solution 
const findOdd = arr => {
  return arr.reduce((a, b) => a ^ b);
};

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11. Get the Middle Character

Given a word, your job is to return the middle character(s) of the word. If the word's length is odd, return the middle character. If the word's length is even, return the middle 2 characters.

const getMiddle = str => {
  // Your solution
};

console.log(getMiddle('test')); // 'es'
console.log(getMiddle('testing')); // 't'
console.log(getMiddle('middle')); // 'dd'
console.log(getMiddle('A')); // 'A'
Solution 
const getMiddle = str => {
  const len = str.length;
  const mid = len / 2;
  // For an odd length, len % 2 equals 1 which is truthy
  return len % 2 ? str[Math.floor(mid)] : str[mid - 1] + str[mid];
};

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12. Who likes it?

You probably know the "like" system from Facebook and other social media. People can "like" posts, photos or other items. We want to create the text that should be displayed next to such an item.

Implement a function that takes an input array, containing the names of people who like an item and returns an output string formatted nicely as shown below.

const likes = names => {
  // Your solution
};

console.log(likes([])); // 'no one likes this'
console.log(likes(['Peter'])); // 'Peter likes this'
console.log(likes(['Jacob', 'Alex'])); // 'Jacob and Alex like this'
console.log(likes(['Max', 'John', 'Mark'])); // 'Max, John and Mark like this'
console.log(likes(['Alex', 'Jacob', 'Mark', 'Max'])); // 'Alex, Jacob and 2 others like this'
Solution 
const likes = names => {
  const len = names.length;
  let output;
  if (len === 0) {
    output = 'no one likes this';
  } else if (len === 1) {
    output = `${names[0]} likes this`;
  } else if (len === 2) {
    output = `${names[0]} and ${names[1]} like this`;
  } else if (len === 3) {
    output = `${names[0]}, ${names[1]} and ${names[2]} like this`;
  } else {
    output = `${names[0]}, ${names[1]} and ${len - 2} others like this`;
  }

  return output;
};

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13. Create Phone Number

Write a function that accepts an array of 10 integers (between 0 and 9), and returns a string of those numbers in the form of a phone number.

const createPhoneNumber = numbers => {
  // Your solution
};

console.log(createPhoneNumber([1, 2, 3, 4, 5, 6, 7, 8, 9, 0])); // '(123) 456-7890'
console.log(createPhoneNumber([1, 1, 1, 1, 1, 1, 1, 1, 1, 1])); // '(111) 111-1111'
console.log(createPhoneNumber([1, 2, 3, 4, 5, 6, 7, 8, 9, 0])); // '(123) 456-7890'
Solution 
const createPhoneNumber = numbers => {
  // Using substrings
  const str = numbers.join('');
  return `(${str.substring(0, 3)}) ${str.substring(3, 6)}-${str.substring(6)}`;

  // Alternative solution using RegEx
  // return numbers.join('').replace(/(\d{3})(\d{3})(\d+)/, '($1) $2-$3');

  // Alternative solution using reduce()
  // return numbers.reduce((acc, cur) => acc.replace('x', cur), '(xxx) xxx-xxxx');
};

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14. Square Every Digit

Given an integer, your task is to square every digit of it and concatenate them to produce a new integer.

const squareDigits = num => {
  // Your solution
};

console.log(squareDigits(2112)); // 4114
console.log(squareDigits(3212)); // 9414
console.log(squareDigits(9159)); // 8112581
Solution 
const squareDigits = num => {
  return Number(
    num
      .toString()
      .split('')
      .map(ele => ele * ele)
      .join('')
  );
};

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15. You're a Square!

Write a function that given an integer, checks to see if it is a square number. A square number or perfect square is an integer that is the square of an integer; in other words, it is the product of some integer with itself.

const isSquare = n => {
  // Your solution
};

console.log(isSquare(0)); // true
console.log(isSquare(4)); // true
console.log(isSquare(25)); // true
console.log(isSquare(3)); // false
console.log(isSquare(93)); // false
console.log(isSquare(-1)); // false
Solution 
const isSquare = n => {
  return Math.sqrt(n) % 1 === 0;
};

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16. Highest and Lowest

Given a string of space-separated numbers, write a function that returns the highest and lowest numbers. There will always be at least one number in the input string.

const highAndLow = numbers => {
  // Your solution
};

console.log(highAndLow('1 2 3 4 5')); // '5 1'
console.log(highAndLow('1 2 -3 4 5')); // '5 -3'
console.log(highAndLow('1 9 3 4 -5')); // '9 -5'
console.log(highAndLow('0 -214 542')); // '542 -214'
Solution 
const highAndLow = numbers => {
  const arr = numbers.split(' ');
  return `${Math.max(...arr)} ${Math.min(...arr)}`;
};

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17. Descending Order

Write a function that takes any non-negative integer as an argument and returns it with its digits in descending order. Essentially, rearrange the digits to create the highest possible number.

const descendingOrder = n => {
  // Your solution
};

console.log(descendingOrder(0)); // 0
console.log(descendingOrder(1)); // 1
console.log(descendingOrder(1021)); // 2110
console.log(descendingOrder(42145)); // 54421
console.log(descendingOrder(145263)); // 654321
console.log(descendingOrder(123456789)); // 987654321
Solution 
const descendingOrder = n => {
  return parseInt(
    n
      .toString()
      .split('')
      .sort((a, b) => b - a)
      .join('')
  );
};

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18. Mumbling

Given a string which includes only letters, write a function that produces the outputs below.

const accum = str => {
  // Your solution
};

console.log(accum('abcd')); // 'A-Bb-Ccc-Dddd'
console.log(accum('cwAt')); // 'C-Ww-Aaa-Tttt'
console.log(accum('RqaEzty')); // 'R-Qq-Aaa-Eeee-Zzzzz-Tttttt-Yyyyyyy'
Solution 
const accum = str => {
  return str
    .split('')
    .map((ele, index) => ele.toUpperCase() + ele.toLowerCase().repeat(index))
    .join('-');
};

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19. Stop gninnipS My sdroW!

Write a function that takes in a string of one or more words, and returns the same string, but with all five or more letter words reversed. Strings passed in will consist of only letters and spaces.

const spinWords = str => {
  // Your solution
};

console.log(spinWords('This is a test')); // 'This is a test'
console.log(spinWords('Hey fellow warriors')); // 'Hey wollef sroirraw'
console.log(spinWords('This is another test')); // 'This is rehtona test'
Solution 
const spinWords = str => {
  return str
    .split(' ')
    .map(word => (word.length < 5 ? word : word.split('').reverse().join('')))
    .join(' ');
};

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20. Shortest Word

Given a non-empty string of words, return the length of the shortest word(s).

const findShort = str => {
  // Your solution
};

console.log(findShort('Test where final word shortest see')); // 3
console.log(findShort('Lets all go on holiday somewhere very cold')); // 2
console.log(findShort('i want to travel the world writing code one day')); // 1
Solution 
const findShort = str => {
  return Math.min(...str.split(' ').map(word => word.length));
};

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21. Bit Counting

Write a function that takes an integer as input, and returns the number of bits that are equal to 1 in the binary representation of that number. You can guarantee that input is non-negative. For example the binary representation of 1234 is 10011010010, so the function should return 5 in this case.

const countBits = n => {
  // Your solution
};

console.log(countBits(0)); // 0
console.log(countBits(4)); // 1
console.log(countBits(7)); // 3
console.log(countBits(9)); // 2
Solution 
const countBits = n => {
  return n.toString(2).split('0').join('').length;
};

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22. Exes and Ohs

Check to see if a string has the same amount of 'x's and 'o's. The method must return a boolean and be case insensitive. The input string can contain any character.

const XO = str => {
  // Your solution
};

console.log(XO('xo')); // true
console.log(XO('Oo')); // false
console.log(XO('xxOo')); // true
console.log(XO('xxxm')); // false
console.log(XO('ooom')); // false
console.log(XO('ty')); // true (when no 'x' and 'o' is present should return true)
Solution 
const XO = str => {
  const lowerStr = str.toLowerCase();
  let result = 0;
  for (const letter of lowerStr) {
    if (letter === 'x') {
      result++;
    } else if (letter === 'o') {
      result--;
    }
  }

  return !result;
};

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23. Sum of Positives

Given an array of numbers, write a function that returns the sum of all of the positives ones. If the array is empty, the sum should be 0.

const positiveSum = arr => {
  // Your solution
};

console.log(positiveSum([1, 2, 3, 4, 5])); // 15
console.log(positiveSum([1, -2, 3, 4, 5])); // 13
console.log(positiveSum([-1, 2, 3, 4, -5])); // 9
console.log(positiveSum([-1, -2, -3, -4, -5])); // 0
console.log(positiveSum([])); // 0
Solution 
const positiveSum = arr => {
  return arr.reduce((acc, cur) => (cur > 0 ? acc + cur : acc), 0);
};

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24. Find The Parity Outlier

You are given an array of at least length 3 containing integers. The array is either entirely comprised of odd integers or entirely comprised of even integers except for a single integer N. Write a function that takes the array as an argument and returns this "outlier" N.

const findOutlier = arr => {
  // Your solution
};

console.log(findOutlier([0, 1, 2])); // 1
console.log(findOutlier([1, 2, 3])); // 2
console.log(findOutlier([1, 1, 0, 1, 1])); // 0
console.log(findOutlier([0, 0, 3, 0, 0])); // 3
console.log(findOutlier([160, 3, 1719, 19, 13, -21])); // 160
console.log(findOutlier([4, 0, 100, 4, 11, 2602, 36])); // 11
Solution 
const findOutlier = arr => {
  const evenArray = arr.filter(ele => ele % 2 === 0);
  const oddArray = arr.filter(ele => ele % 2 !== 0);
  return evenArray.length === 1 ? evenArray[0] : oddArray[0];
};

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25. Array.diff

Write a function that subtracts one list from another and returns the result. It should remove all values from array a, which are present in array b.

const arrayDiff = (a, b) => {
  // Your solution
};

console.log(arrayDiff([1, 8, 2], [])); // [1, 8, 2]
console.log(arrayDiff([1, 2, 3], [1, 2])); // [3]
console.log(arrayDiff([3, 4], [3])); // [4]
console.log(arrayDiff([], [4, 5])); // []
Solution 
const arrayDiff = (a, b) => {
  return a.filter(ele => !b.includes(ele));
};

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26. Capitalize Words

Write a function that capitalizes each word in a given input string.

String.prototype.capitalize = function () {
  // Your solution
};

var str = "How can mirrors be real if our eyes aren't real";
console.log(str.capitalize()); // 'How Can Mirrors Be Real If Our Eyes Aren't Real'
Solution 
String.prototype.capitalize = function () {
  return this.split(' ')
    .map(ele => ele[0].toUpperCase() + ele.slice(1))
    .join(' ');
};

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27. Complementary DNA

DNA is a chemical found in the nucleus of cells and carries the "instructions" for the development and functioning of living organisms. In DNA strings, symbols "A" and "T" are complements of each other, as are "C" and "G". Given one side of the DNA, write a function that returns the other complementary side. The DNA strand is never empty.

const DNAStrand = dna => {
  // Your solution
};

console.log(DNAStrand('AAAA')); // 'TTTT'
console.log(DNAStrand('ATTGC')); // 'TAACG'
console.log(DNAStrand('GTAT')); // 'CATA'
Solution 
const DNAStrand = dna => {
  const dnaMap = {
    A: 'T',
    T: 'A',
    G: 'C',
    C: 'G',
  };
  return [...dna].map(ele => dnaMap[ele]).join('');
};

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28. Isograms

An isogram is a word that has no repeating letters, consecutive or non-consecutive. Implement a function that determines whether a string that contains only letters is an isogram. Assume the empty string is an isogram. Ignore letter case.

const isIsogram = str => {
  // Your solution
};

console.log(isIsogram('Dermatoglyphics')); // true
console.log(isIsogram('isIsogram')); // false
console.log(isIsogram('isogram')); // true
console.log(isIsogram('moOse')); // false
console.log(isIsogram('aba')); // false
console.log(isIsogram('')); // true
Solution 
const isIsogram = str => {
  const lowerStr = str.toLowerCase();

  // Using Set
  return str.length === new Set(lowerStr).size;

  // Alternative solution using every()
  // return [...lowerStr].every((ele, index) => lowerStr.indexOf(ele) === index);
};

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29. FizzBuzz

Write a program that prints the numbers from 1 to 100. But for multiples of 3 prints "Fizz" instead of the number and for the multiples of 5 prints "Buzz". For numbers which are multiples of both 3 and 5 prints "FizzBuzz".

const fizzBuzz = () => {
  // Your solution
};

fizzBuzz(); // 1, 2, 'Fizz', 4, 'Buzz', 'Fizz', 7, ...
Solution 
const fizzBuzz = () => {
  let output;
  for (let num = 1; num <= 100; num++) {
    output = '';
    if (num % 3 === 0) output += 'Fizz';
    if (num % 5 === 0) output += 'Buzz';
    console.log(output || num);
  }
};

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30. Counting Duplicates

Write a function that will return the count of distinct case-insensitive alphanumeric characters that occur more than once in the input string.

const duplicateCount = text => {
  // Your solution
};

console.log(duplicateCount('')); // 0
console.log(duplicateCount('abcde')); // 0
console.log(duplicateCount('abA11')); // 2
console.log(duplicateCount('aabbcde')); // 2
console.log(duplicateCount('aabBcde')); // 2
console.log(duplicateCount('Indivisibility')); // 1
console.log(duplicateCount('Indivisibilities')); // 2
Solution 
const duplicateCount = text => {
  const lowercaseText = text.toLowerCase();
  let frequency = {};
  let count = 0;

  for (const letter of lowercaseText) {
    frequency[letter] = (frequency[letter] || 0) + 1;
    if (frequency[letter] === 2) count++;
  }
  return count;
};

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31. Duplicate Encoder

Write a function that converts a string to a new string where each character in the new string is ( if that character appears only once in the original string, or ) if that character appears more than once in the original string. Ignore capitalization when determining if a character is a duplicate.

const duplicateEncode = word => {
  // Your solution
};

console.log(duplicateEncode('din')); // '((('
console.log(duplicateEncode('(( @')); // '))(('
console.log(duplicateEncode('recede')); // '()()()'
console.log(duplicateEncode('Success')); // ')())())'
Solution 
const duplicateEncode = word => {
  const lowerWord = word.toLowerCase();
  let result = '';
  for (const char of lowerWord) {
    lowerWord.indexOf(char) !== lowerWord.lastIndexOf(char)
      ? (result += ')')
      : (result += '(');
  }
  return result;
};

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32. Reversed Strings

Write a function that reverses the string that is passed to it. For this challenge, you may NOT use the JavaScript built-in reverse() method.

const reverseString = str => {
  // Your solution
};

console.log(reverseString('hello')); // 'olleh'
console.log(reverseString('world')); // 'dlrow'
console.log(reverseString('')); // ''
console.log(reverseString('h')); // 'h'
Solution 
const reverseString = str => {
  let result = '';
  for (let char of str) {
    result = char + result;
  }
  return result;
};

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33. Persistent Bugger

Write a function that takes a positive number num and returns its multiplicative persistence, which is the number of steps it takes to multiply all the digits of num by each other, and repeating with the product until a single digit is obtained.

const persistence = num => {
  // Your solution
};

console.log(persistence(999)); // 4
// because 9*9*9=729, 7*2*9=126, 1*2*6=12, and finally 1*2=2

console.log(persistence(93)); // 3
// because 9*3=27, 2*7=14, 1*4=4 and 4 has only one digit

console.log(persistence(5)); // 0
// because 5 is already a single-digit number
Solution 
const persistence = num => {
  if (num < 10) return 0;

  let product = 1;
  while (num >= 10) {
    product *= num % 10;
    num = Math.floor(num / 10);
  }

  // Using recursion
  return 1 + persistence(product * num);
};

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34. Fibonacci Number

Fibonacci number (Fibonacci sequence), named after mathematician Fibonacci, is a sequence of numbers that looks like this: 0, 1, 1, 2, 3, 5, 8, 13,.... You get first two starting numbers, 0 and 1, and the next number in the sequence is always the sum of the previous two numbers.

Write a function fib() that takes one parameter steps, and returns a number from the Fibonacci sequence, based on the parameter steps, which determines the position in Fibonacci number. For example fib(0) returns 0, fib(4) returns 3, and fib(15) returns 610.

const fib = steps => {
  // Your solution
};

console.log(fib(0)); // 0
console.log(fib(4)); // 3
console.log(fib(17)); // 1597
console.log(fib(20)); // 6765
Solution 
// Recursive solution
const fib = steps => {
  if (steps < 2) return steps;
  return fib(steps - 2) + fib(steps - 1);
};

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35. Replace with Alphabet Position

Given a string, write a function that replaces every letter with its position in the alphabet: 'a' = 1, 'b' = 2, .... If anything in the input isn't a letter, ignore it and don't return it.

const alphabetPosition = text => {
  // Your solution
};

console.log(alphabetPosition('The narwhal bacons at midnight.'));
// '20 8 5 14 1 18 23 8 1 12 2 1 3 15 14 19 1 20 13 9 4 14 9 7 8 20'

console.log(alphabetPosition("The sunset sets at twelve o' clock."));
// '20 8 5 19 21 14 19 5 20 19 5 20 19 1 20 20 23 5 12 22 5 15 3 12 15 3 11'
Solution 
const alphabetPosition = text => {
  const startingIndex = 'a'.charCodeAt() - 1;
  return text
    .toLowerCase()
    .match(/[a-z]/g)
    .map(letter => letter.charCodeAt() - startingIndex)
    .join(' ');
};

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36. Two Sum

Given an array of integers nums and an integer target, return indices of the two numbers in the array such that they add up to target. You may assume that each input would have exactly one solution, and you may not use the same element twice.

const twoSum = (nums, target) => {
  // Your solution
};

console.log(twoSum([2, 7, 11, 15], 9)); // [0, 1]
console.log(twoSum([3, 2, 4], 6)); // [1, 2]
Solution 
const twoSum = (nums, target) => {
  const len = nums.length;
  for (let i = 0; i < len; i++) {
    const j = nums.lastIndexOf(target - nums[i]);
    if (j > i) {
      return [i, j];
    }
  }

  // Alternative solution using Map
  // const map = new Map();

  // for (let i = 0; i < nums.length; i++) {
  //   const otherIndex = map.get(target - nums[i]);
  //   if (typeof otherIndex !== 'undefined') {
  //     return [otherIndex, i];
  //   }
  //   map.set(nums[i], i);
  // }
};

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37. Unique In Order

Implement a function that takes an iterable argument (a string or an array) as input and returns a list of items without any elements with the same value next to each other and preserving the original order of elements.

const uniqueInOrder = iterable => {
  // Your solution
};

console.log(uniqueInOrder([1, 2, 2, 3, 3])); // [1, 2, 3]
console.log(uniqueInOrder('ABBCcAD')); // ['A', 'B', 'C', 'c', 'A', 'D']
console.log(uniqueInOrder('AAAABBBCCDAABBB')); // ['A', 'B', 'C', 'D', 'A', 'B']
Solution 
const uniqueInOrder = iterable => {
  const arr = [...iterable];
  return arr.filter((ele, index) => ele !== arr[index - 1]);
};

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38. Best Time to Buy and Sell Stock

You are given an array prices where prices[i] is the price of a given stock on the ith day. You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock. Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

Example 1:

Input: prices = [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5.
Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.

Example 2:

Input: prices = [7,6,4,3,1]
Output: 0
Explanation: In this case, no transactions are done and the max profit = 0.
const maxProfit = prices => {
  // Your solution
};

console.log(maxProfit([7, 1, 5, 3, 6, 4])); // 5
console.log(maxProfit([7, 6, 4, 3, 1])); // 0
Solution 
const maxProfit = prices => {
  let min = Number.MAX_SAFE_INTEGER;
  let profit = 0;

  for (let i = 0; i < prices.length; i++) {
    min = Math.min(prices[i], min);
    if (prices[i] - min > profit) {
      profit = prices[i] - min;
    }
  }
  return profit;
};

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39. Dubstep

Mike works as a DJ in the best London nightclub, and he often uses dubstep music in his performance. Recently, he has decided to take a couple of old songs and make dubstep remixes from them. Let's assume that a song consists of some number of words (that don't contain WUB). To make the dubstep remix of this song, Mike inserts a certain number of words WUB before the first word of the song (the number may be zero), after the last word (the number may be zero), and between words (at least one between any pair of neighboring words), and then the boy glues together all the words, including WUB, in one string and plays the song at the club.

For example, a song with words "I AM X" can transform into a dubstep remix as WUBWUBIWUBAMWUBWUBX and cannot transform into WUBWUBIAMWUBX.

Recently, Johnny has heard Mike's new dubstep track, but since he isn't into modern music, he decided to find out what was the initial song that Mike remixed. Help Johnny restore the original song.

Input: The input consists of a single non-empty string, consisting only of uppercase English letters, the string's length doesn't exceed 200 characters

Output: Return the words of the initial song that Mike used to make a dubsteb remix. Separate the words with a space.

const songDecoder = song => {
  // Your solution
};

console.log(songDecoder('AWUBBWUBC'));
// 'A B C' (WUB should be replaced by 1 space)
console.log(songDecoder('AWUBWUBWUBBWUBWUBWUBC'));
// 'A B C' (Multiples WUBs should be replaced by only 1 space)
console.log(songDecoder('WUBAWUBBWUBCWUB'));
// 'A B C' (Any starting or trailing WUBs should be removed)
console.log(songDecoder('WUBWEWUBAREWUBWUBTHEWUBCHAMPIONSWUBMYWUBFRIENDWUB'));
// 'WE ARE THE CHAMPIONS MY FRIEND'
Solution 
const songDecoder = song => {
  return song.replace(/(WUB)+/g, ' ').trim();

  // Alternative solution
  // return song.split('WUB').filter(Boolean).join(' ');
};

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40. Valid Parentheses

Given a non-empty string s containing just the characters (, ), {, }, [, ], determine if the input string is valid. An input string is valid if open brackets are closed by the same type of brackets, and open brackets are closed in the correct order.

const isValid = s => {
  // Your solution
};

console.log(isValid('[')); //false
console.log(isValid('()')); //true
console.log(isValid('(]')); //false
console.log(isValid('{[]}')); //true
console.log(isValid('([)]')); //false
console.log(isValid('()[]{}')); //true
Solution 
const isValid = s => {
  const stack = [];
  const pairs = {
    '(': ')',
    '[': ']',
    '{': '}',
  };

  for (const char of s) {
    if (pairs[char]) {
      stack.push(char);
    } else if (pairs[stack.pop()] !== char) {
      return false;
    }
  }
  return !stack.length;
};

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41. Reverse Integer

Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 - 1], then return 0.

const reverse = x => {
  // Your solution
};

console.log(reverse(0)); // 0
console.log(reverse(120)); // 21
console.log(reverse(123)); // 321
console.log(reverse(-123)); // -321
console.log(reverse(1534236469)); // 0
Solution 
const reverse = x => {
  const MAX = Math.pow(2, 31) - 1;
  const MIN = -1 * Math.pow(2, 31);
  const arr = Math.abs(x).toString().split('');
  const reversed = Math.sign(x) * Number(arr.reverse().join(''));
  return reversed < MIN || reversed > MAX ? 0 : reversed;
};

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42. Remove Duplicates from Sorted Array

Given a sorted array nums, write a function that removes the duplicates in-place such that each element appears only once and returns the new length. Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.

const removeDuplicates = nums => {
  // Your solution
};

console.log(removeDuplicates([1, 1, 2])); // 2 (because [1, 2] has length 2)
console.log(removeDuplicates([0, 1, 1, 1, 2, 2, 3, 3, 4])); // 5
console.log(removeDuplicates([])); // 0
Solution 
const removeDuplicates = nums => {
  if (nums.length <= 1) return nums.length;
  let currentIndex = 0;

  for (let i = 1; i < nums.length; i++) {
    if (nums[currentIndex] !== nums[i]) {
      currentIndex++;
      nums[currentIndex] = nums[i];
    }
  }

  nums.length = currentIndex + 1;
  return nums.length;
};

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43. Kids With the Greatest Number of Candies

Given an array candies where candies[i] represents the number of candies that the ith kid has, and an integer extraCandies, write a function that for each kid checks if he/she would have the greatest number of candies in the group if they were given extraCandies. Note that multiple kids can have the greatest number of candies. For example,

Input: candies = [2,3,5,1,3], extraCandies = 3
Output: [true,true,true,false,true]

Explanation:
- Kid 1 has 2 candies and if he or she receives all extra candies (3) will have 5 candies --- the greatest number of candies among the kids.
- Kid 2 has 3 candies and if he or she receives at least 2 extra candies will have the greatest number of candies among the kids.
- Kid 3 has 5 candies and this is already the greatest number of candies among the kids.
- Kid 4 has 1 candy and even if he or she receives all extra candies will only have 4 candies.
- Kid 5 has 3 candies and if he or she receives at least 2 extra candies will have the greatest number of candies among the kids.
const kidsWithCandies = (candies, extraCandies) => {
  // Your solution
};

console.log(kidsWithCandies([12, 1, 12], 10)); // [true, false, true]
console.log(kidsWithCandies([4, 2, 1, 1, 2], 1)); // [true, false, false, false, false
10000
]
Solution 
const kidsWithCandies = (candies, extraCandies) => {
  const MAX = Math.max(...candies);
  return candies.map(candy => candy + extraCandies >= MAX);
};

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44. Rot13

ROT13 is a simple letter substitution cipher that replaces a letter with the letter 13 letters after it in the alphabet. ROT13 is an example of the Caesar cipher. Create a function that takes a string and returns the string ciphered with Rot13. If there are numbers or special characters included in the string, they should be returned as they are. Only letters from the latin/english alphabet should be shifted.

const rot13 = str => {
  // Your solution
};

console.log(rot13('az AZ')); // nm NM
console.log(rot13('10+2 is twelve.')); // 10+2 vf gjryir.
console.log(rot13('abcdefghijklmnopqrstuvwxyz')); // nopqrstuvwxyzabcdefghijklm
Solution 
const rot13 = str => {
  return str.replace(/[a-z]/gi, letter =>
    String.fromCharCode(
      letter.charCodeAt() + (letter.toLowerCase() <= 'm' ? 13 : -13)
    )
  );
};

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45. Richest Customer Wealth

You are given an m x n integer grid accounts, where accounts[i][j] is the amount of money the i​​​​​​​​​​​th​​​​ customer has in the j​​​​​​​​​​​th​​​​ bank. Return the wealth that the richest customer has. A customer's wealth is the amount of money they have in all their bank accounts. The richest customer is the customer that has the maximum wealth. For example:

Input: accounts = [[1,5],[7,3],[3,5]]
Output: 10
Explanation:
1st customer has wealth = 1 + 5 = 6
2nd customer has wealth = 7 + 3 = 10
3rd customer has wealth = 3 + 5 = 8
The 2nd customer is the richest with a wealth of 10.
const maximumWealth = accounts => {
  // Your solution
};

console.log(
  maximumWealth([
    [2, 8, 7],
    [7, 1, 3],
    [1, 9, 5],
  ])
); // 17
console.log(
  maximumWealth([
    [1, 5],
    [7, 3],
    [3, 5],
  ])
); // 10
console.log(
  maximumWealth([
    [1, 2, 3],
    [3, 2, 1],
  ])
); // 6
Solution 
const maximumWealth = accounts => {
  return Math.max(
    ...accounts.map(customer => customer.reduce((a, b) => a + b))
  );
};

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46. The Hashtag Generator

The marketing team is spending way too much time typing in hashtags. Let's help them with our own Hashtag Generator! Here's the deal:

  • It must start with a hashtag #.
  • Ignore spaces in the input.
  • All words must have their first letter capitalized.
  • If the final result is longer than 140 chars it must return false.
  • If the input or the result is an empty string it must return false.
const generateHashtag = str => {
  // Your solution
};

console.log(generateHashtag('JavaScript')); // #JavaScript
console.log(generateHashtag('Do we have a Hashtag')); // #DoWeHaveAHashtag
console.log(generateHashtag('    Hello     World   ')); // #HelloWorld
console.log(generateHashtag('coding' + ' '.repeat(140) + 'for life')); // #CodingForLife
console.log(generateHashtag('')); // false
console.log(generateHashtag(' ')); // false
console.log(generateHashtag('a'.repeat(140))); // false
console.log(generateHashtag(' '.repeat(200))); // false
Solution 
const generateHashtag = str => {
  const hashtag = str
    .split(' ')
    .reduce(
      (tag, word) => tag + word.charAt(0).toUpperCase() + word.slice(1),
      '#'
    );

  // Alternative solution
  // const hashtag =
  //   '#' +
  //   str
  //     .split(' ')
  //     .map(ele => ele.charAt(0).toUpperCase() + ele.substring(1))
  //     .join('');

  return hashtag.length === 1 || hashtag.length > 140 ? false : hashtag;
};

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47. Pete, the Baker

Pete likes to bake some cakes. He has some recipes and ingredients. Unfortunately he is not good in maths. Can you help him to find out, how many cakes he could bake considering his recipes?

Write a function cakes(), which takes the recipe object and the available ingredients object and returns the maximum number of cakes Pete can bake. Ingredients that are not present in the objects, can be considered as 0.

const cakes = (recipe, available) => {
  // Your solution
};

let recipe = { flour: 500, sugar: 200, eggs: 1 };
let available = { flour: 1200, sugar: 1200, eggs: 5, milk: 200 };
console.log(cakes(recipe, available)); // 2

recipe = { apples: 3, flour: 300, sugar: 150, milk: 100, oil: 100 };
available = { sugar: 500, flour: 2000, milk: 2000 };
console.log(cakes(recipe, available)); // 0
Solution 
const cakes = (recipe, available) => {
  let num_cakes = Infinity;
  for (let ingredient in recipe) {
    if (!available[ingredient] || recipe[ingredient] > available[ingredient])
      return 0;
    num_cakes = Math.min(
      num_cakes,
      Math.floor(available[ingredient] / recipe[ingredient])
    );
  }
  return num_cakes;
};

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48. Count Characters in Your String

Write a function that counts the frequency of all the characters in a given string.

const count = string => {
  // Your solution
};

console.log(count('')); // {}
console.log(count('aba')); // { a: 2, b: 1 }
Solution 
const count = string => {
  const frequency = {};
  for (const char of string) {
    frequency[char] = (frequency[char] || 0) + 1;
  }
  return frequency;
};

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49. Break camelCase

Complete the solution so that the function will break up camel casing, using a space between words.

const solution = str => {
  // Your solution
};

console.log(solution('camelCasingHere')); // camel Casing Here
console.log(solution('No Camels here')); // No Camels here
console.log(solution('ABC')); // ABC
console.log(solution('')); // ''
Solution 
const solution = str => {
  return str.replace(/([a-z])([A-Z])/g, '$1 $2');
};

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50. Check if Word Equals Summation of Two Words

Let's assume that the numeric value of a letter is its position in the alphabet starting from 0 (i.e. a -> 0, b -> 1, c -> 2, etc.). Similarly, the numerical value of a string str consisting of some lowercase English letters is the concatenation (not sum!) of the numeric values of each letter in str, which is then converted into an integer. For example, if str = 'acb', we concatenate each letter's numeric value, resulting in 021 which is then converted to integer 21.

You are given three strings firstWord, secondWord, and targetWord, each consisting of lowercase English letters a through j inclusive. Write a function that returns true if the sum of the numerical values of firstWord and secondWord equals the numerical value of targetWord.

const isSumEqual = (firstWord, secondWord, targetWord) => {
  // Your solution
};

console.log(isSumEqual('acb', 'cba', 'cdb')); // true
// The numerical value of firstWord 'acb' is '021' -> 21
// The numerical value of secondWord 'cba' is '210' -> 210
// The numerical value of targetWord 'cdb' is '231' -> 231
// So we return true because 21 + 210 == 231

console.log(isSumEqual('aaa', 'a', 'aab')); // false
// The numerical value of firstWord 'aaa' is '000' -> 0
// The numerical value of secondWord 'a' is '0' -> 0
// The numerical value of targetWord 'aab' is '001' -> 1
// So we return false because 0 + 0 != 1

console.log(isSumEqual('aaa', 'a', 'aaaa')); // true
// The numerical value of firstWord 'aaa' is '000' -> 0
// The numerical value of secondWord 'a' is '0' -> 0
// The numerical value of targetWord 'aaaa' is '0000' -> 0
// So we return true because 0 + 0 == 0
Solution 
const getNumericValue = str => {
  const offset = 'a'.charCodeAt();
  const arr = [];

  for (const char of str) {
    arr.push(char.charCodeAt() - offset);
  }

  return parseInt(arr.join(''));
};

const isSumEqual = (firstWord, secondWord, targetWord) => {
  return (
    getNumericValue(firstWord) + getNumericValue(secondWord) ===
    getNumericValue(targetWord)
  );
};

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51. Extract the Domain Name From a URL

Write a function that given an input URL, returns its domain name.

const domainName = url => {
  // Your solution
};

console.log(domainName('www.google.ca')); // google
console.log(domainName('http://google.com')); // google
console.log(domainName('https://google.com')); // google
console.log(domainName('http://google.co.jp')); // google
console.log(domainName('https://www.google.com')); // google
Solution 
const domainName = url => {
  return url.replace(/(www\.|.*\/\/|\..+)/g, '');
};

// Alternative solution with no regex
// const domainName = url => url.replace('http://', '').replace('https://', '').replace('www.', '').split('.')[0];

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52. First Non-repeating Character

Write a function that takes an input string and returns the first character that is not repeated anywhere in the string. Upper- and lowercase letters are considered the same character, but the function should return the correct case for the initial letter.

const firstNonRepeatingLetter = str => {
  // Your solution
};

console.log(firstNonRepeatingLetter('a')); // 'a'
console.log(firstNonRepeatingLetter('stress')); // 't'
console.log(firstNonRepeatingLetter('sTreSS')); // 'T'
console.log(firstNonRepeatingLetter('abba')); // ''
console.log(firstNonRepeatingLetter("Go hang a salami, I'm a lasagna hog!")); // ','
Solution 
const firstNonRepeatingLetter = str => {
  const strToLower = str.toLowerCase();
  for (let char of str) {
    if (
      strToLower.indexOf(char.toLowerCase()) ===
      strToLower.lastIndexOf(char.toLowerCase())
    ) {
      return char;
    }
  }
  return '';
};

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53. Roman Numerals Encoder

Create a function that takes a positive integer less than 4,000 as its input and returns a string containing the Roman numeral representation of that integer. Modern Roman numerals are written by expressing each digit separately starting with the leftmost digit and skipping any digit with a value of zero. There can't be more than 3 identical symbols in a row. More about Roman numerals: http://en.wikipedia.org/wiki/Roman_numerals

Table of individual decimal places for your reference:

Thousands Hundreds Tens Units
1 M C X I
2 MM CC XX II
3 MMM CCC XXX III
4 CD XL IV
5 D L V
6 DC LX VI
7 DCC LXX VII
8 DCCC LXXX VIII
9 CM XC IX 
const convertToRoman = number => {
  // Your solution
};

console.log(convertToRoman(4)); // IV
console.log(convertToRoman(9)); // IX
console.log(convertToRoman(11)); // XI
console.log(convertToRoman(19)); // XIX
console.log(convertToRoman(22)); // XXII
console.log(convertToRoman(15)); // XV
console.log(convertToRoman(39)); // XXX + IX = XXXIX
console.log(convertToRoman(160)); // C + LX = CLX
console.log(convertToRoman(207)); // CC + VII = CCVII
console.log(convertToRoman(246)); // CC + XL + VI = CCXLVI
console.log(convertToRoman(789)); // DCC + LXXX + IX = DCCLXXXIX
console.log(convertToRoman(1009)); // M + IX = MIX
console.log(convertToRoman(1066)); // M + LX + VI = MLXVI
console.log(convertToRoman(1776)); // M + DCC + LXX + VI = MDCCLXXVI
console.log(convertToRoman(1918)); // M + CM + X + VIII = MCMXVIII
console.log(convertToRoman(1954)); // M + CM + L + IV = MCMLIV
console.log(convertToRoman(2014)); // MM + X + IV = MMXIV
console.log(convertToRoman(2421)); // MM + CD + XX + I = MMCDXXI
console.log(convertToRoman(3999)); // MMM + CM + XC + IX = MMMCMXCIX
Solution 
const convertToRoman = number => {
  const decimals = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
  // prettier-ignore
  const romans = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];
  let result = '';

  decimals.map((value, index) => {
    while (number >= value) {
      result += romans[index];
      number -= value;
    }
  });

  return result;
};

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54. Scramble

Write a function that accepts two strings and returns true if some or all of the characters in the first string can be rearranged to match the second string.

const scramble = (str1, str2) => {
  // Your solution
};

console.log(scramble('scriptjava', 'javascript')); // true
console.log(scramble('scriptingjava', 'javascript')); // true
console.log(scramble('scriptsjava', 'javascripts')); // true
console.log(scramble('jscripts', 'javascript')); // false
console.log(scramble('javscripts', 'javascript')); // false
Solution 
const scramble = (str1, str2) => {
  return [...str2].every(
    letter => str2.split(letter).length <= str1.split(letter).length
  );

  // Alternative solution
  // const freq = {};
  // for (const letter of str1) {
  //   freq[letter] = (freq[letter] ?? 0) + 1;
  // }

  // for (const letter of str2) {
  //   if (!freq[letter] || freq[letter] < 0) return false;
  //   freq[letter]--;
  // }
  // return true;
};

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55. Wave, wAve, waVe, wavE

Write a function that turns a given string into a wave! You will be passed a string and you must return that string in an array where each letter takes turns to become uppercase. The input string will always be lowercase but may be empty. If you encounter a whitespace then pass over it.

const wave = str => {
  // Your solution
};

console.log(wave('hello')); // ['Hello', 'hEllo', 'heLlo', 'helLo', 'hellO'];
console.log(wave(' gap ')); // [' Gap ', ' gAp ', ' gaP '];
console.log(wave('Two words')); // ['Two words', 'tWo words', 'twO words', 'two Words', 'two wOrds', 'two woRds', 'two worDs', 'two wordS'];
Solution 
const wave = str => {
  const result = [];
  const len = str.length;
  for (let i = 0; i < len; i++) {
    if (str[i] !== ' ') {
      const word =
        str.substring(0, i).toLowerCase() +
        str[i].toUpperCase() +
        str.substring(i + 1).toLowerCase();
      result.push(word);
    }
  }
  return result;
};

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56. Concatenation of Array

Write a function that given an integer array nums of length n, returns an array of length 2n where nums is concatenated to itself.

const getConcatenation = nums => {
  // Your solution
};

console.log(getConcatenation([1, 2, 3])); // [1, 2, 3, 1, 2, 3]
console.log(getConcatenation([4, 3, 2, 1])); // [4, 3, 2, 1, 4, 3, 2, 1]
Solution 
const getConcatenation = nums => {
  return [...nums, ...nums];
};

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57. Get Names

Write a function that given an array of users, returns an array of their names. Can you achieve this in one line of code?

const getNames = users => {
  // Your solution
};

const users = [
  {
    id: 001,
    name: 'Alice',
    startDate: '2021-01-03',
  },
  {
    id: 002,
    name: 'Bob',
    startDate: '2020-02-01',
  },
  {
    id: 003,
    name: 'Claire',
    startDate: '2019-03-01',
  },
];

console.log(getNames(users)); // ['Alice', 'Bob', 'Claire']
Solution 
const getNames = users => users.map(user => user.name);

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58. Object Keys from snake_case to camelCase

Write a function that converts all the keys in an object from snake case to camel case.

const toCamel = obj => {
  // Your solution
};

console.log(
  toCamel({
    first_name: 'John',
    last_name: 'Rambo',
    favorite_movie: 'First Blood',
  })
); // {'firstName': 'John', 'lastName': 'Rambo', 'favoriteMovie': 'First Blood'}
Solution 
const toCamel = obj => {
  const result = {};
  for (const [key, value] of Object.entries(obj)) {
    // Let's use regex capture groups
    const camelKey = key.replace(/(_[a-z])/gi, $1 =>
      $1.replace('_', '').toUpperCase()
    );
    result[camelKey] = value;
  }
  return result;
};

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59. Valid Palindrome

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers. Given a string s, return true if it is a palindrome, or false otherwise.

const isPalindrome = s => {
  // Your solution
};

console.log(isPalindrome('A man, a plan, a canal: Panama')); // true
// Explanation: "amanaplanacanalpanama" is a palindrome.
console.log(isPalindrome('race a car')); // false
// Explanation: "raceacar" is not a palindrome.
console.log(isPalindrome('ab_a')); // true
// Explanation: "aba" is a palindrome.
console.log(isPalindrome(' ')); // true
// Explanation: `s` is an empty string "" after removing non-alphanumeric characters.
// Since an empty string reads the same forward and backward, it is a palindrome.
Solution 
const isPalindrome = s => {
  const str = s.toLowerCase().replace(/[^a-z0-9]/g, '');
  let start = 0;
  let end = str.length - 1;

  while (start < end) {
    if (str[start] !== str[end]) return false;
    start++;
    end--;
  }
  return true;

  // Alternative solution using built-in reverse() method
  // const str = s.toLowerCase().replace(/[^a-z0-9]/g, '');
  // return str === [...str].reverse().join('');
};

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60. Move Zeroes

Given an integer array nums, move all 0's in the array to the end of it while maintaining the relative order of the non-zero elements. Achieve this without copying the array or creating a new array.

const moveZeroes = nums => {
  // Your solution
};

console.log(moveZeroes([0, 1, 0, 3, 12])); // [1, 3, 12, 0, 0]
console.log(moveZeroes([0, 0, 1])); // [1, 0, 0]
console.log(moveZeroes([0])); // [0]
Solution 
const moveZeroes = nums => {
  const length = nums.length;
  let index = 0;

  for (let i = 0; i < length; i++) {
    if (nums[i] !== 0) {
      nums[index] = nums[i];
      if (index !== i) {
        nums[i] = 0;
      }
      index++;
    }
  }

  return nums;
};

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JavaScript Coding Challenges for Beginners

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