The Closure Trap: A JavaScript Bug That Shows Why Fundamentals Still Matter in the AI Era

You know that feeling when your code should work, but doesn't? Welcome to one of JavaScript's classic gotchas—a closure problem that trips up developers at every level.

Let me show you a bug I encountered recently that perfectly illustrates why understanding fundamentals is more important than ever, even (especially) in this age of AI coding assistants.

The Bug That Shouldn't Exist

Here's the scenario: Imagine you need to create a collection of worker functions, each remembering its own ID number. Simple enough, right?

function createWorkers() {
  let workers = [];

  let count = 0;
  while (count < 10) {
    let worker = function() {
      alert(count); // Should show 0, 1, 2, 3...
    };
    workers.push(worker);
    count++;
  }

  return workers;
}

let team = createWorkers();

team[0](); // Expected: 0, Got: 10 😱
team[1](); // Expected: 1, Got: 10
team[5](); // Expected: 5, Got: 10

Wait, what? Every single worker function outputs 10 instead of their individual numbers. But we clearly incremented count and created separate functions... What's going on?

Understanding the Problem: It's All About References

Here's the thing: all the worker functions share the same lexical environment. They don't store the value of count when they're created—they store a reference to the count variable itself.

By the time we actually call any of these functions, the loop has long finished, and count has been incremented to 10. So every function, when executed, looks up count and finds... you guessed it, 10.

Think of it like this: imagine you're giving directions to 10 people, but instead of telling them specific addresses, you just say "go to wherever this GPS pin is pointing." If you move the pin after giving the directions, everyone ends up at the same final location.

Solution #1: Create a New Scope for Each Iteration

The fix is to capture the current value in a separate scope for each iteration:

function createWorkers() {
  let workers = [];

  let count = 0;
  while (count < 10) {
    let worker = function(n) {
      return function() {
        alert(n);
      };
    }(count); // Immediately pass current count value

    workers.push(worker);
    count++;
  }

  return workers;
}

Here, we're using an IIFE (Immediately Invoked Function Expression) to create a new scope for each iteration. The parameter n captures the current value of count, and the inner function closes over n instead of count.

Solution #2: The Modern Way with for Loop

Actually, there's a much cleaner solution. Modern JavaScript's for loop with let creates a new binding for each iteration:

function createWorkers() {
  let workers = [];

  for (let count = 0; count < 10; count++) {
    let worker = function() {
      alert(count); // Now each function has its own count!
    };
    workers.push(worker);
  }

  return workers;
}

let team = createWorkers();
team[0](); // 0 ✅
team[1](); // 1 ✅
team[5](); // 5 ✅

This works because let in a for loop creates a new lexical environment for each iteration. Each count is a separate variable, not the same variable being updated.

Pro tip: This is one of the key reasons why we moved from var to let. With var, even the for loop wouldn't save you from this bug!

Solution #3: The Array Method Approach

For bonus points, here's a functional programming approach:

function createWorkers() {
  return Array.from({ length: 10 }, (_, index) => {
    return function() {
      alert(index);
    };
  });
}

Clean, modern, and each function naturally captures its own index value.

Why This Matters More Than Ever in the AI Era

Now, here's the real talk: we're living in an age where ChatGPT, GitHub Copilot, and other AI tools can generate code in seconds. You might think, "Why do I need to understand closures? AI can write this for me!"

Here's why that's dangerous thinking:

1. AI Doesn't Debug—You Do

When AI-generated code doesn't work (and trust me, it often doesn't on the first try), you need to understand what's wrong. If you don't understand closures, you can't fix closure bugs. You'll be stuck in a loop of asking AI to "fix it," getting slightly different broken code each time.

2. Code Reviews and Maintenance

You'll be reading and maintaining code written by others (or by AI) for years. If you don't understand the fundamentals, you can't spot bugs, can't suggest improvements, and can't maintain the codebase effectively.

3. Knowing When AI is Wrong

AI confidently generates incorrect code all the time. Without solid fundamentals, you won't know when to trust it and when to question it. You become dependent on a tool that's wrong 20-30% of the time.

4. Problem-Solving > Code Generation

The hardest part of programming isn't writing code—it's understanding what code to write. AI can help with syntax, but only you can solve the actual problem. And that requires deep understanding of how your language works.

5. Standing Out in the Job Market

As AI makes basic coding more accessible, what separates good developers from great ones? Deep understanding. The ability to debug complex issues. The knowledge to architect solutions. These fundamentals are becoming more valuable, not less.

The Bottom Line

This closure bug is a perfect example. An AI might generate the buggy code. It might even generate a fix if you describe the problem perfectly. But if you don't understand why it was broken and why the fix works, you're just copying and pasting without learning.

Fundamentals aren't becoming obsolete—they're becoming your competitive advantage.

In a world where everyone has access to AI code generators, the developers who thrive will be those who:

• Understand what's happening under the hood
• Can debug when things go wrong
• Know which solutions are elegant vs. hacky
• Can architect systems, not just write functions

So yes, use AI tools. They're fantastic for productivity. But don't let them become a crutch that prevents you from understanding the fundamentals.

The irony? The better you understand fundamentals, the better you can use AI tools. You'll write better prompts, spot errors faster, and know when to override the AI's suggestions.

Your Turn

Have you encountered this closure bug before? How do you balance using AI tools with learning fundamentals? Drop your thoughts in the comments—I'd love to hear your experiences!

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Key Takeaways:

• Closures reference variables, not values
• Use for loops with let to create new bindings per iteration
• Fundamentals matter more in the AI era, not less
• AI is a tool, not a replacement for understanding

Keep coding, keep learning! 🚀

JavaScript #WebDevelopment #Closures #Programming #AI #SoftwareDevelopment #LearnToCode