How to Crack JavaScript Coding Rounds – Real Examples & Deep Dives 💻🔥

If you are getting ready for Frontend or Full-Stack interviews, it is likely that you will face significant testing in JavaScript coding rounds.

These challenges go beyond mere syntax; they evaluate your logical reasoning, problem-solving abilities, and capacity to produce clean code while under pressure. In this article, I will guide you on how to crack javascript coding rounds, the typical types of questions you may encounter, and the methods to effectively resolve them—complete with real-world examples, code walkthroughs, and detailed explanations.

📌 What Interviewers Actually Look For

Before we explore examples, let's first clarify what interviewers look for in JavaScript coding rounds:

• Ability to break down problems logically.
• Understanding the principles of JavaScript (arrays, objects, functions, scope, closures, async, etc.).
• Clean, understandable, and optimized code
• Confident in using vanilla JS without libraries.

🚀 Strategy to Crack JavaScript Coding Rounds -

1. Master the Fundamentals – Understand your data types, loops, conditions, and scope rules.
2. Practice with Limitations – No Array.prototype methods allowed? Unable to use sort()? Prepare yourself!
3. Break It Down – Always draft a plan in comments prior to diving into the code.
4. Perform a Dry Run of Your Code – Manually follow the inputs and outputs.
5. Edge Cases – Consider negative inputs, empty arrays, large values, and so on.

💡 Example 1: Reverse a String Without Built-in Methods

Question:
Reverse the string "frontend" without using .reverse().

Approach:

• Manually convert a string into an array
• Use a loop to reverse
• Join the characters

function reverseString(str) {
  let reversed = '';
  for (let i = str.length - 1; i >= 0; i--) {
    reversed += str[i];
  }
  return reversed;
}

console.log(reverseString("frontend")); // "dnetnorf"

✅ Why It’s Good

• No built-in methods are used
• Helps in understanding loops better
• Efficient for small & medium strings

💡 Example 2: Remove Duplicates from Array (No Set)

Question:
Remove duplicates from [1, 2, 2, 3, 4, 4, 5].

Approach:

• You can use a result array
• Loop through the input, add only if not present

function removeDuplicates(arr) {
  let unique = [];
  for (let i = 0; i < arr.length; i++) {
    if (!unique.includes(arr[i])) {
      unique.push(arr[i]);
    }
  }
  return unique;
}

console.log(removeDuplicates([1, 2, 2, 3, 4, 4, 5])); // [1,2,3,4,5]

✅ Interview Tips:

• If interviewer allows you to use Set, mention Set as a one-liner: Array.from(new Set(arr))
• But also explain the interviewer that you can do it manually, which proves that you have good logical skills.

💡 Example 3: Find First Non-Repeating Character

Question:
Return the first non-repeating character in a string like "aabbcddee" → output: "c"

Approach:

• Create a frequency map
• Loop to find the first char with count 1

function firstUniqueChar(str) {
  let freq = {};

  // Count frequency
  for (let char of str) {
    freq[char] = (freq[char] || 0) + 1;
  }

  // Find first unique
  for (let char of str) {
    if (freq[char] === 1) return char;
  }

  return null;
}

console.log(firstUniqueChar("aabbcddee")); // "c"

🧠 Key Concepts Tested:

• Object as a HashMap
• Looping through string
• Frequency counting pattern

💡 Example 4: Flatten a Nested Array

Question:
Flatten [[1, 2], [3, [4, 5]], 6] into [1, 2, 3, 4, 5, 6] without .flat().

Approach:

• Use recursion method to dive into nested arrays

function flattenArray(arr) {
  let result = [];

  for (let item of arr) {
    if (Array.isArray(item)) {
      result = result.concat(flattenArray(item));
    } else {
      result.push(item);
    }
  }

  return result;
}

console.log(flattenArray([[1, 2], [3, [4, 5]], 6]));
// [1, 2, 3, 4, 5, 6]

✅ Good For:

• Showing how Recursion works
• How Arrays can be handled without built-in methods

💡 Example 5: Implement Debounce Function (Advanced)

Question:
Implement a basic debounce(fn, delay) to control function execution.

Approach:

• Return a function
• Use setTimeout and clearTimeout inside closure

function debounce(func, delay) {
  let timer;
  return function(...args) {
    clearTimeout(timer);
    timer = setTimeout(() => {
      func.apply(this, args);
    }, delay);
  };
}

// Example usage:
const handleScroll = debounce(() => {
  console.log("Scrolled!");
}, 500);

window.addEventListener("scroll", handleScroll);

🔍 What It Tests:

• How Closures works
• setTimeout & clearTimeout functions
• Function context and performance understanding

✅ Bonus Tips for Real Interviews

1. Talk while coding – Explain your thoughts while writing the code.
2. Write tests – Add basic test cases after coding to show you are confident.
3. Optimize if time allows – Once the logic works, discuss time complexity.
4. Don’t panic – If you are stuck between the code, describe what you would do and move forward.

🧠 Final Thoughts

Successfully navigating JavaScript coding interviews requires clear reasoning, organized problem-solving skills, and proficiency in coding. The majority of questions are grounded in practical logic or traditional data structures and algorithms (DSA) patterns, thus regular practice yields significant benefits.
If you are getting ready for interviews, aim to tackle 1–2 problems daily from platforms such as:

• LeetCode
• CodeWars
• Frontend Mentor You do not have to tackle 1000 problems—focus intently on 50 to 100, truly comprehend them, and you will gain confidence.

Have you encountered a challenging problem lately? Please share it in the comments section. Together, we can find a solution! 👇
Happy coding!