Top 10 Front-End Interview Algorithm Questions and Solutions

Although front‑end developers rarely work with algorithms, interviewers still test algorithmic thinking; learning data structures helps engineers analyze and optimize complex problems.

Q1: Determine if a word is a palindrome

A palindrome reads the same forward and backward (e.g., "mamam" or "redivider"). The solution often involves reversing the string—using built‑in functions or converting the string to an array for flexible manipulation.

Q2: Remove duplicate values from an integer array

Example: input [1,13,24,11,11,14,1,2] → output [1,13,24,11,14,2]. The typical approach uses an object’s keys to filter unique elements.

Q3: Find the most frequent letter in a string

Given a continuous English string, count occurrences of each character and return the one with the highest count. Example: input "afjghdfraaaasdenas" → output "a".

Q4: Basic sorting algorithms

Bubble sort is a fundamental algorithm that repeatedly swaps adjacent out‑of‑order elements. Knowing it is essential, though other sorts like insertion, quick, and shell sort each have distinct characteristics.

Q5: Swap two integers without a temporary variable

Using arithmetic: a = a + (b - a) results in a taking b 's original value, then assign b = a - (b - a) to complete the swap.

Q6: Draw a limited Fibonacci curve with Canvas

Generate the first nine Fibonacci numbers (0,1,1,2,3,5,8,13,21) using the recurrence fibo[i] = fibo[i-1] + fibo[i-2], then plot them with canvas.arc to create a smooth curve.

Q7: Find the maximum difference in a positive array

Example: input [10,5,11,7,8,9] → output 6. The maximum difference equals the difference between the array’s maximum and minimum values.

Q8: Generate a random string of a given length

For a specified length (e.g., 8), produce a random alphanumeric string such as "4ldkfg9j" by selecting characters from a predefined set.

Q9: Implement a custom getElementsByClassName

Write a function that traverses a DOM subtree and collects all elements whose className matches a target, without using native getElementsByClassName or querySelectorAll.

Q10: Implement a Binary Search Tree (BST) in JavaScript

A BST is a binary tree where each node’s left subtree contains smaller values and the right subtree contains larger values; duplicate keys are not allowed. It offers O(log n) search and insertion complexity and serves as a foundation for sets, maps, etc.

Left subtree values < left node value

Right subtree values > right node value

Both sub‑trees are themselves BSTs

No duplicate keys

Full source code is available at GitHub .