Top 5 Mistakes JavaScript Beginners Make in Interviews (And How to Avoid Them)

JavaScript is one of the most in-demand programming languages today, especially for front-end and full-stack development roles. But even with hours of practice and tutorials, many beginners stumble during job interviews.

In this article, we will break down the top 5 mistakes JavaScript beginners make in interviews, and how you can avoid them. If you are preparing for a JavaScript job or coding round, this guide is your cheat sheet to avoid common pitfalls.

🔥 1. Relying Too Much on var Instead of let and const

Many beginners still use var everywhere, even though ES6 introduced let and const in 2015.

Why it’s a mistake:
var has function scope and strange hoisting behaviours unexpected outcomes. Interviewers want to see modern JS practices, and let/const provide block scope which is more predictable and safe.

Better approach:
Use const by default, and let only if the variable needs to change.

// Bad
var count = 0;

// Good
let score = 10;
const MAX_LIMIT = 100;

Interview Tip:
Be ready to explain the difference between var, let, and const.

🔄 2. Not Understanding this Keyword

Ask any JavaScript interviewer – questions on this are almost guaranteed. Yet, beginners often fail to explain how this behaves in different contexts.

Why it’s a mistake:
Misunderstanding this leads to incorrect code, especially in object methods, callbacks, or when using arrow functions.

What you should know:

• this depends on how a function is called, not where it's written.
• Arrow functions don't have their own this.

const obj = {
  name: 'Siddhesh',
  greet() {
    console.log(`Hello, I am ${this.name}`);
  }
};
obj.greet(); // Correct usage

Interview Tip:
Expect a follow-up question involving nested functions or event handlers.

🧠 3. Memorizing Without Understanding Core Concepts

You can’t fake your way through a JavaScript interview. Interviewers can easily tell when a candidate memorized code snippets but doesn’t understand the why behind them.

Common examples:

• Using map() instead of forEach() without knowing the difference.
• Copy-pasting a debounce function without knowing what it does.

How to fix it:
Focus on understanding:

• Execution context & call stack
• Closures
• Event loop
• Promises and async/await

Interview Tip:
Explain concepts in your own words. If you’re asked about closures, say what a closure is, not just the definition.

🪝 4. Ignoring Asynchronous JavaScript: Callbacks, Promises, and Async/Await

Async programming is a must-know in real-world JS development. Still, many beginners fumble when asked to explain or use promises or async/await.

Why it matters:
APIs, timers, and I/O operations all use asynchronous patterns. If you don’t know how to handle them, you’ll struggle in any practical project.

What to learn:

• What is the difference between setTimeout, Promise, and async/await?
• How does the event loop work?
• How to catch errors in async code?

async function fetchData() {
  try {
    const res = await fetch('https://api.example.com/data');
    const data = await res.json();
    console.log(data);
  } catch (err) {
    console.error('Error:', err);
  }
}

Interview Tip:
Expect tasks like fetching mock API data or writing chained promises.

🧪 5. Not Practicing Enough Real Coding Problems

Watching tutorials isn't enough. If you can’t solve basic coding challenges during a live interview, it’s a red flag.

Typical beginner errors:

• Poor logic structuring
• Using inbuilt methods without knowing how they work
• Failing to explain time complexity

What to practice:

• Array and string manipulation (e.g., reverse, sort, filter)
• Object operations
• DOM manipulation (for front-end roles)
• Writing simple algorithms without using Array.prototype.* methods

Example Question:
Write a function to find the second largest number in an array without using .sort().

function secondLargest(arr) {
  let first = -Infinity, second = -Infinity;
  for (let num of arr) {
    if (num > first) {
      second = first;
      first = num;
    } else if (num > second && num !== first) {
      second = num;
    }
  }
  return second;
}

✅ Final Thoughts

JavaScript interviews are not just about syntax – they test your understanding, problem-solving ability, and modern JS practices.

✅ Recap: Avoid These Mistakes

1. Using var instead of let/const
2. Not understanding this
3. Memorizing code without real understanding
4. Ignoring async concepts like promises and async/await
5. Skipping hands-on coding practice