1. Palindrome Check
Write a function to check if a string is a palindrome.
function isPalindrome(str) {
let reversed = str.split('').reverse().join('');
return str === reversed;
}
console.log(isPalindrome('racecar')); // true
console.log(isPalindrome('hello')); // false
2. Anagram Check
Write a function that checks if two strings are anagrams of each other.
function isAnagram(str1, str2) {
let normalize = str => str.toLowerCase().replace(/[\W_]+/g, '').split('').sort().join('');
return normalize(str1) === normalize(str2);
}
console.log(isAnagram('listen', 'silent')); // true
console.log(isAnagram('hello', 'world')); // false
3. FizzBuzz
Write a function that prints the numbers from 1 to 100, but for multiples of 3, print "Fizz", for multiples of 5 print "Buzz", and for multiples of both 3 and 5 print "FizzBuzz".
function fizzBuzz() {
for (let i = 1; i <= 100; i++) {
if (i % 3 === 0 && i % 5 === 0) {
console.log('FizzBuzz');
} else if (i % 3 === 0) {
console.log('Fizz');
} else if (i % 5 === 0) {
console.log('Buzz');
} else {
console.log(i);
}
}
}
fizzBuzz();
4. Sum of Array
Write a function to return the sum of all elements in an array.
function sumArray(arr) {
return arr.reduce((sum, num) => sum + num, 0);
}
console.log(sumArray([1, 2, 3, 4])); // 10
5. Find Maximum
Write a function that returns the maximum number in an array.
function findMax(arr) {
return Math.max(...arr);
}
console.log(findMax([3, 5, 7, 2, 8])); // 8
6. Remove Duplicates
Write a function that removes duplicates from an array.
function removeDuplicates(arr) {
return [...new Set(arr)];
}
console.log(removeDuplicates([1, 2, 2, 3, 4, 4, 5])); // [1, 2, 3, 4, 5]
7. Flatten an Array
Write a function to flatten an array of arrays into a single array.
function flattenArray(arr) {
return arr.flat();
}
console.log(flattenArray([[1, 2], [3, 4], [5]])); // [1, 2, 3, 4, 5]
8. Find Missing Number
Write a function that finds the missing number in an array of consecutive numbers.
function findMissingNumber(arr) {
let n = arr.length + 1;
let totalSum = (n * (n + 1)) / 2;
let arrSum = arr.reduce((sum, num) => sum + num, 0);
return totalSum - arrSum;
}
console.log(findMissingNumber([1, 2, 4, 5, 6])); // 3
9. Factorial
Write a function to calculate the factorial of a number.
function factorial(n) {
if (n === 0) return 1;
return n * factorial(n - 1);
}
console.log(factorial(5)); // 120
10. Debounce Function
Write a debounce function in JavaScript.
function debounce(func, delay) {
let timeoutId;
return function (...args) {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => {
func.apply(this, args);
}, delay);
};
}
const logMessage = debounce(() => console.log('Hello'), 1000);
logMessage(); // Will log "Hello" after 1 second if not called again within that time.
11. Reverse a String
Write a function to reverse a given string without using the built-in reverse method.
function reverseString(str) {
let reversed = '';
for (let i = str.length - 1; i >= 0; i--) {
reversed += str[i];
}
return reversed;
}
console.log(reverseString('hello')); // 'olleh'
12. Capitalize First Letter of Each Word
Write a function to capitalize the first letter of each word in a sentence.
function capitalizeWords(str) {
return str.split(' ').map(word => word[0].toUpperCase() + word.slice(1)).join(' ');
}
console.log(capitalizeWords('hello world')); // 'Hello World'
13. Find Second Largest Element
Write a function to find the second largest number in an array.
function secondLargest(arr) {
let uniqueArr = [...new Set(arr)];
uniqueArr.sort((a, b) => b - a);
return uniqueArr[1];
}
console.log(secondLargest([1, 5, 3, 9, 7])); // 7
14. Sum of Digits
Write a function to compute the sum of the digits of a number.
function sumOfDigits(num) {
return num.toString().split('').reduce((sum, digit) => sum + parseInt(digit), 0);
}
console.log(sumOfDigits(1234)); // 10
15. Count Vowels and Consonants
Write a function to count the number of vowels and consonants in a string.
function countVowelsAndConsonants(str) {
let vowels = 'aeiouAEIOU';
let vowelCount = 0;
let consonantCount = 0;
for (let char of str) {
if (vowels.includes(char)) {
vowelCount++;
} else if (/[a-zA-Z]/.test(char)) {
consonantCount++;
}
}
return { vowels: vowelCount, consonants: consonantCount };
}
console.log(countVowelsAndConsonants('hello world')); // { vowels: 3, consonants: 7 }
16. Sum of Even Numbers
Write a function that returns the sum of all even numbers in an array.
function sumEvenNumbers(arr) {
return arr.filter(num => num % 2 === 0).reduce((sum, num) => sum + num, 0);
}
console.log(sumEvenNumbers([1, 2, 3, 4, 5, 6])); // 12
17. Check for Duplicates
Write a function that checks if an array contains any duplicate elements.
function hasDuplicates(arr) {
return new Set(arr).size !== arr.length;
}
console.log(hasDuplicates([1, 2, 3, 4, 5])); // false
console.log(hasDuplicates([1, 2, 3, 4, 4])); // true
18. Merge and Sort Two Arrays
Write a function to merge two arrays and return a sorted array.
function mergeAndSort(arr1, arr2) {
return [...arr1, ...arr2].sort((a, b) => a - b);
}
console.log(mergeAndSort([3, 1, 4], [6, 5, 2])); // [1, 2, 3, 4, 5, 6]
19. Generate Fibonacci Sequence
Write a function to generate the Fibonacci sequence up to n numbers.
function fibonacci(n) {
let fib = [0, 1];
for (let i = 2; i < n; i++) {
fib[i] = fib[i - 1] + fib[i - 2];
}
return fib;
}
console.log(fibonacci(6)); // [0, 1, 1, 2, 3, 5]
20. Find Intersection of Two Arrays
Write a function that finds the intersection (common elements) of two arrays.
function arrayIntersection(arr1, arr2) {
return arr1.filter(item => arr2.includes(item));
}
console.log(arrayIntersection([1, 2, 3], [2, 3, 4])); // [2, 3]
21. Group Elements by Frequency
Write a function that groups elements of an array by their frequency.
function groupByFrequency(arr) {
let frequencyMap = {};
arr.forEach(item => frequencyMap[item] = (frequencyMap[item] || 0) + 1);
return frequencyMap;
}
console.log(groupByFrequency([1, 2, 2, 3, 3, 3])); // {1: 1, 2: 2, 3: 3}
22. Remove Null and Undefined Values
Write a function to remove null and undefined values from an array.
function removeNullUndefined(arr) {
return arr.filter(item => item !== null && item !== undefined);
}
console.log(removeNullUndefined([1, null, 2, undefined, 3])); // [1, 2, 3]
23. Find Longest Word
Write a function to find the longest word in a sentence.
function findLongestWord(sentence) {
let words = sentence.split(' ');
let longestWord = words.reduce((longest, currentWord) => currentWord.length > longest.length ? currentWord : longest, '');
return longestWord;
}
console.log(findLongestWord('The quick brown fox jumped over the lazy dog')); // 'jumped'
24. Binary Search Algorithm
Write a function to implement binary search for a sorted array.
function binarySearch(arr, target) {
let left = 0, right = arr.length - 1;
while (left <= right) {
let mid = Math.floor((left + right) / 2);
if (arr[mid] === target) return mid;
else if (arr[mid] < target) left = mid + 1;
else right = mid - 1;
}
return -1;
}
console.log(binarySearch([1, 2, 3, 4, 5, 6, 7], 4)); // 3
console.log(binarySearch([1, 2, 3, 4, 5, 6, 7], 10)); // -1
25. Throttle Function
Write a throttle function that limits the execution of a function to once every n milliseconds.
function throttle(func, delay) {
let lastCall = 0;
return function (...args) {
const now = new Date().getTime();
if (now - lastCall >= delay) {
lastCall = now;
return func(...args);
}
};
}
const throttledLog = throttle(() => console.log('Throttled Function Call'), 2000);
setInterval(throttledLog, 500); // Will log once every 2 seconds
26. Count Occurrences of Each Character
Write a function that counts the occurrences of each character in a string.
function countOccurrences(str) {
let charCount = {};
for (let char of str) {
charCount[char] = (charCount[char] || 0) + 1;
}
return charCount;
}
console.log(countOccurrences('hello')); // {h: 1, e: 1, l: 2, o: 1}
27. Flatten Nested Arrays
Write a function that flattens a deeply nested array (any level of depth).
function flattenDeep(arr) {
return arr.reduce((flat, toFlatten) => flat.concat(Array.isArray(toFlatten) ? flattenDeep(toFlatten) : toFlatten), []);
}
console.log(flattenDeep([1, [2, [3, [4]], 5]])); // [1, 2, 3, 4, 5]
28. Prime Number Check
Write a function that checks if a number is prime.
function isPrime(num) {
if (num <= 1) return false;
for (let i = 2; i <= Math.sqrt(num); i++) {
if (num % i === 0) return false;
}
return true;
}
console.log(isPrime(7)); // true
console.log(isPrime(10)); // false
29. Sum of Nested Arrays
Write a function that computes the sum of all numbers in a deeply nested array.
function sumNested(arr) {
return arr.reduce((sum, item) => sum + (Array.isArray(item) ? sumNested(item) : item), 0);
}
console.log(sumNested([1, [2, 3], [4, [5]]])); // 15
30. Convert Array to Object
Write a function that converts an array of key-value pairs into an object.
function arrayToObject(arr) {
return arr.reduce((obj, [key, value]) => {
obj[key] = value;
return obj;
}, {});
}
console.log(arrayToObject([['name', 'John'], ['age', 30]])); // { name: 'John', age: 30 }
31. Find Unique Elements
Write a function that returns only unique elements from an array (without using Set).
function uniqueElements(arr) {
let uniqueArr = [];
arr.forEach(item => {
if (!uniqueArr.includes(item)) uniqueArr.push(item);
});
return uniqueArr;
}
console.log(uniqueElements([1, 2, 2, 3, 4, 4, 5])); // [1, 2, 3, 4, 5]
32. Deep Clone Object
Write a function to perform a deep clone of an object.
function deepClone(obj) {
return JSON.parse(JSON.stringify(obj));
}
const obj = { a: 1, b: { c: 2 } };
const clone = deepClone(obj);
console.log(clone); // { a: 1, b: { c: 2 } }
33. Check if Arrays are Equal
Write a function to check if two arrays are equal (order and elements).
function arraysEqual(arr1, arr2) {
if (arr1.length !== arr2.length) return false;
return arr1.every((val, index) => val === arr2[index]);
}
console.log(arraysEqual([1, 2, 3], [1, 2, 3])); // true
console.log(arraysEqual([1, 2, 3], [1, 2, 4])); // false
34. Find Duplicates in Array
Write a function that finds all duplicates in an array.
function findDuplicates(arr) {
let seen = new Set();
let duplicates = new Set();
arr.forEach(num => {
if (seen.has(num)) {
duplicates.add(num);
} else {
seen.add(num);
}
});
return [...duplicates];
}
console.log(findDuplicates([1, 2, 3, 4, 4, 5, 6, 2])); // [2, 4]
35. Check Balanced Parentheses
Write a function that checks if a string has balanced parentheses.
function isBalanced(str) {
let stack = [];
for (let char of str) {
if (char === '(') {
stack.push('(');
} else if (char === ')') {
if (stack.length === 0) return false;
stack.pop();
}
}
return stack.length === 0;
}
console.log(isBalanced('(())')); // true
console.log(isBalanced('(()')); // false
36. Rotate Array
Write a function that rotates an array n times.
function rotateArray(arr, n) {
n = n % arr.length;
return arr.slice(n).concat(arr.slice(0, n));
}
console.log(rotateArray([1, 2, 3, 4, 5], 2)); // [3, 4, 5, 1, 2]
37. Memoized Fibonacci
Write a function that generates the Fibonacci sequence using memoization.
function memoizedFibonacci() {
let cache = {};
return function fib(n) {
if (n in cache) return cache[n];
if (n <= 1) return n;
cache[n] = fib(n - 1) + fib(n - 2);
return cache[n];
};
}
const fib = memoizedFibonacci();
console.log(fib(10)); // 55
38. Throttle Function Using Timestamps
Write a throttle function that uses timestamps to limit the function calls.
function throttle(func, limit) {
let lastFunc;
let lastRan;
return function () {
const context = this;
const args = arguments;
if (!lastRan) {
func.apply(context, args);
lastRan = Date.now();
} else {
clearTimeout(lastFunc);
lastFunc = setTimeout(() => {
if (Date.now() - lastRan >= limit) {
func.apply(context, args);
lastRan = Date.now();
}
}, limit - (Date.now() - lastRan));
}
};
}
const throttledLog = throttle(() => console.log('Throttled'), 2000);
setInterval(throttledLog, 500); // Logs "Throttled" once every 2 seconds
39. Count Words in a String
Write a function to count the number of words in a string.
function countWords(str) {
return str.split(/\s+/).filter(word => word.length > 0).length;
}
console.log(countWords('Hello world! How are you?')); // 5
40. Event Delegation
Write a simple example that demonstrates event delegation in JavaScript.
document.getElementById('parent').addEventListener('click', function (event) {
if (event.target && event.target.nodeName === 'BUTTON') {
console.log('Button clicked:', event.target.innerText);
}
});
HTML:
<div id="parent">
<button>Button 1</button>
<button>Button 2</button>
<button>Button 3</button>
</div>
41. Find Missing Number in Sorted Array
Write a function to find the missing number in a sorted array.
function findMissing(arr) {
let n = arr.length;
let totalSum = ((n + 1) * (n + 2)) / 2;
let arrSum = arr.reduce((sum, num) => sum + num, 0);
return totalSum - arrSum;
}
console.log(findMissing([1, 2, 3, 5])); // 4
42. Debounce Function with Immediate Call
Write a debounce function that has an option to run the function immediately.
function debounce(func, delay, immediate = false) {
let timeout;
return function () {
const context = this;
const args = arguments;
const callNow = immediate && !timeout;
clearTimeout(timeout);
timeout = setTimeout(() => {
timeout = null;
if (!immediate) func.apply(context, args);
}, delay);
if (callNow) func.apply(context, args);
};
}
const debouncedLog = debounce(() => console.log('Debounced'), 2000, true);
debouncedLog(); // Executes immediately, subsequent calls are debounced
43. Implement a Curry Function
Write a function that curries another function.
function curry(func) {
return function curried(...args) {
if (args.length >= func.length) {
return func.apply(this, args);
} else {
return function (...nextArgs) {
return curried.apply(this, args.concat(nextArgs));
};
}
};
}
function add(a, b, c) {
return a + b + c;
}
const curriedAdd = curry(add);
console.log(curriedAdd(1)(2)(3)); // 6
44. Shuffle an Array
Write a function to randomly shuffle the elements of an array.
function shuffleArray(arr) {
for (let i = arr.length - 1; i > 0; i--) {
const j = Math.floor(Math.random() * (i + 1));
[arr[i], arr[j]] = [arr[j], arr[i]];
}
return arr;
}
console.log(shuffleArray([1, 2, 3, 4, 5])); // Randomly shuffled array
45. Implement Object.create() Polyfill
Write a polyfill for Object.create() in JavaScript.
function myObjectCreate(proto) {
function F() {}
F.prototype = proto;
return new F();
}
const obj = { name: 'John' };
const newObj = myObjectCreate(obj);
console.log(newObj.name); // 'John'
46. Implement Promise.all() Polyfill
Write a polyfill for Promise.all() in JavaScript.
function myPromiseAll(promises) {
return new Promise((resolve, reject) => {
let results = [];
let completed = 0;
promises.forEach((promise, index) => {
Promise.resolve(promise)
.then(result => {
results[index] = result;
completed++;
if (completed === promises.length) {
resolve(results);
}
})
.catch(error => reject(error));
});
});
}
myPromiseAll([Promise.resolve(1), Promise.resolve(2), Promise.resolve(3)])
.then(console.log); // [1, 2, 3]
47. Find Longest Palindrome in a String
Write a function that finds the longest palindromic substring in a string.
function longestPalindrome(str) {
let result = '';
for (let i = 0; i < str.length; i++) {
for (let j = i; j < str.length; j++) {
let substr = str.slice(i, j + 1);
if (substr === substr.split('').reverse().join('') && substr.length > result.length) {
result = substr;
}
}
}
return result;
}
console.log(longestPalindrome('babad')); // 'bab' or 'aba'
48. Get All Subsets of an Array
Write a function that returns all possible subsets of a given array.
function getSubsets(arr) {
return arr.reduce((subsets, value) => {
return subsets.concat(subsets.map(set => [value, ...set]));
}, [[]]);
}
console.log(getSubsets([1, 2, 3])); // [[], [1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]
49. Implement Debounce with Trailing and Leading Edge
Write a debounce function that allows execution on both the leading and trailing edge.
function debounce(func, delay, leading = false) {
let timeout;
return function (...args) {
const context = this;
const callNow = leading && !timeout;
clearTimeout(timeout);
timeout = setTimeout(() => {
timeout = null;
if (!leading) func.apply(context, args);
}, delay);
if (callNow) func.apply(context, args);
};
}
const debouncedFunc = debounce(() => console.log('Debounced!'), 2000, true);
debouncedFunc();
50. Find the Deepest Node in a Tree
Write a function to find the deepest node in a binary tree.
function findDeepestNode(root) {
let maxDepth = -1;
let deepestNode = null;
function traverse(node, depth) {
if (node === null) return;
if (depth > maxDepth) {
maxDepth = depth;
deepestNode = node;
}
traverse(node.left, depth + 1);
traverse(node.right, depth + 1);
}
traverse(root, 0);
return deepestNode;
}
// Example tree: { value: 1, left: {...}, right: {...} }
51. Sum of Ranges in Array
Write a function that takes an array and returns the sum of all ranges in it (e.g., consecutive numbers).
function sumOfRanges(arr) {
let sum = 0;
for (let i = 1; i < arr.length; i++) {
sum += Math.abs(arr[i] - arr[i - 1]);
}
return sum;
}
console.log(sumOfRanges([1, 4, 6, 8])); // 7
52. Find Intersection of Two Arrays
Write a function that returns the intersection of two arrays.
function arrayIntersection(arr1, arr2) {
return arr1.filter(value => arr2.includes(value));
}
console.log(arrayIntersection([1, 2, 3], [2, 3, 4])); // [2, 3]
53. Sort Array of Objects by Property
Write a function that sorts an array of objects by a given property.
function sortByProperty(arr, prop) {
return arr.sort((a, b) => (a[prop] > b[prop] ? 1 : -1));
}
const people = [{ name: 'John', age: 25 }, { name: 'Jane', age: 22 }];
console.log(sortByProperty(people, 'age')); // [{ name: 'Jane', age: 22 }, { name: 'John', age: 25 }]
54. Binary Search in a Sorted Array
Write a function to implement binary search in a sorted array.
function binarySearch(arr, target) {
let low = 0;
let high = arr.length - 1;
while (low <= high) {
let mid = Math.floor((low + high) / 2);
if (arr[mid] === target) return mid;
if (arr[mid] < target) low = mid + 1;
else high = mid - 1;
}
return -1;
}
console.log(binarySearch([1, 2, 3, 4, 5], 3)); // 2
55. Check if Two Strings Are Anagrams
Write a function to check if two strings are anagrams.
function areAnagrams(str1, str2) {
const formatStr = str => str.replace(/[^\w]/g, '').toLowerCase().split('').sort().join('');
return formatStr(str1) === formatStr(str2);
}
console.log(areAnagrams('listen', 'silent')); // true
console.log(areAnagrams('hello', 'world')); // false
56. Count Vowels in a String
Write a function to count the number of vowels in a string.
function countVowels(str) {
return str.match(/[aeiou]/gi)?.length || 0;
}
console.log(countVowels('hello world')); // 3
57. Generate Pascal’s Triangle
Write a function to generate Pascal’s triangle up to a given number of rows.
function generatePascalsTriangle(rows) {
let triangle = [[1]];
for (let i = 1; i < rows; i++) {
let prevRow = triangle[i - 1];
let newRow = [1];
for (let j = 1; j < prevRow.length; j++) {
newRow.push(prevRow[j - 1] + prevRow[j]);
}
newRow.push(1);
triangle.push(newRow);
}
return triangle;
}
console.log(generatePascalsTriangle(5));
// [
// [1],
// [1, 1],
// [1, 2, 1],
// [1, 3, 3, 1],
// [1, 4, 6, 4, 1]
// ]
58. Check if a String is a Substring
Write a function to check if one string is a substring of another.
function isSubstring(str, sub) {
return str.includes(sub);
}
console.log(isSubstring('hello world', 'world')); // true
59. Find the First Non-Repeating Character in a String
Write a function to find the first non-repeating character in a string.
function firstNonRepeatingChar(str) {
for (let i = 0; i < str.length; i++) {
if (str.indexOf(str[i]) === str.lastIndexOf(str[i])) {
return str[i];
}
}
return null;
}
console.log(firstNonRepeatingChar('swiss')); // 'w'
60. Find Maximum Profit from Stock Prices
Write a function that finds the maximum profit from an array of stock prices.
function maxProfit(prices) {
let minPrice = Infinity;
let maxProfit = 0;
for (let price of prices) {
minPrice = Math.min(minPrice, price);
maxProfit = Math.max(maxProfit, price - minPrice);
}
return maxProfit;
}
console.log(maxProfit([7, 1, 5, 3, 6, 4])); // 5
Feel free to continue practicing with these or adapt them for your specific use cases!