Top Essential Coding Interview Problems in JavaScript

1. Reverse a String function reverseString(str) { let result = ""; for (let i = str.length - 1; i >= 0; i--) { result += str[i]; } return result; } console.log(reverseString("hello")); // "olleh" 2. Find Duplicates function findDuplicates(arr) { let flatArr = arr.flat(Infinity); let seen = new Set(); let duplicates = []; for (let item of flatArr) { if (seen.has(item)) { duplicates.push(item); } else { seen.add(item); } } return [...new Set(duplicates)]; } console.log(findDuplicates([1, 2, [3, 4, 2], 5, 1])); // [2, 1] 3. First Non-Repeating Character function firstNonRepeatingChar(str) { let freq = {}; for (let char of str) { freq[char] = (freq[char] || 0) + 1; } for (let char of str) { if (freq[char] === 1) return char; } return null; } console.log(firstNonRepeatingChar("swiss")); // "w" 4. Flatten Array (recursive approach) function flattenArray(arr) { let result = []; function helper(subArr) { for (let item of subArr) { if (Array.isArray(item)) { helper(item); } else { result.push(i