20 Essential JavaScript Coding Questions – Numbers, Strings & Arrays

Convert number to string, reverse it, and convert back to number, preserving the sign.

function reverseNumber(n) {
  return parseInt(n.toString().split('').reverse().join('')) * Math.sign(n);
}
console.log(reverseNumber(123)); // 321

Loop from 2 to √n and check if any number divides n evenly.

function isPrime(n) {
  if (n <= 1) return false;
  for (let i = 2; i <= Math.sqrt(n); i++) {
    if (n % i === 0) return false;
  }
  return true;
}
console.log(isPrime(7)); // true

Use regular expression /^\d+$/ to test for only numeric characters.

console.log(/^\d+$/.test("12345")); // true

Use \b\w regex to target first letter of each word and capitalize it.

const str = "hello world";
const capitalized = str.replace(/\b\w/g, c => c.toUpperCase());
console.log(capitalized); // "Hello World"

Compare each element with the previous one to ensure non-decreasing order.

function isSorted(arr) {
  return arr.every((v, i, a) => i === 0 || a[i - 1] <= v);
}
console.log(isSorted([1, 2, 3])); // true

Match all vowels (case-insensitive) and count them using regex.

const str = "Hello World";
const vowelCount = str.match(/[aeiou]/gi)?.length || 0;
console.log(vowelCount); // 3

Convert number to string, split digits, convert back and sum them using reduce().

const num = 123;
const sum = num.toString().split('').reduce((a, b) => a + Number(b), 0);
console.log(sum); // 6

Use Object.keys() to check if an object has any properties.

const obj = {};
console.log(Object.keys(obj).length === 0); // true

Use array destructuring to swap variables without a temporary one.

let a = 1, b = 2;
[a, b] = [b, a];
console.log(a, b); // 2 1

Use spread operator to merge arrays, and Set to remove duplicates.

const arr1 = [1, 2];
const arr2 = [2, 3];
const merged = [...new Set([...arr1, ...arr2])];
console.log(merged); // [1, 2, 3]

Use recursion to multiply the number by the factorial of the number minus one.

function factorial(n) {
        return n <= 1 ? 1 : n * factorial(n - 1);
      }
console.log(factorial(5)); // 120

Use modulo operator (%) to determine if number is divisible by 2.

const n = 7; 
console.log(n % 2 === 0 ? 'Even' : 'Odd'); // "Odd"

Trim and split the string on one or more spaces to count words.

const str = "Hello world from JS";
const wordCount = str.trim().split(/\s+/).length;
console.log(wordCount); // 4

Use Math.max with spread operator to find the largest number.

const arr = [3, 6, 2, 9];
      console.log(Math.max(...arr)); // 9

Split sentence into words, reverse each word, then join them back.

const str = "Hello World";
const result = str.split(' ').map(w => w.split('').reverse().join('')).join(' ');
console.log(result); // "olleH dlroW"

Replace dashes or underscores followed by characters with the uppercase version of that character.

const str = "hello_world-test";
const camel = str.replace(/[-_](\w)/g, (_, c) => c.toUpperCase());
console.log(camel); // "helloWorldTest"

Compare the string to its uppercase version.

const str = "HELLO";
console.log(str === str.toUpperCase()); // true

Compare size of Set (unique elements) to original array length.

const arr = [1, 2, 3, 3];
console.log(new Set(arr).size !== arr.length); // true

Use a compare function (a - b) to sort numerically.

const arr = [10, 2, 5];
arr.sort((a, b) => a - b);
console.log(arr); // [2, 5, 10]

Convert Set back to array using Array.from to remove duplicates.

const arr = [1, 2, 2, 3];
const unique = Array.from(new Set(arr));
console.log(unique); // [1, 2, 3]