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calendar_month : August 8, 2025

10 JavaScript Mistakes Every Developer Makes

Table of Contents

  1. Introduction: Why JavaScript Mistakes Happen

  2. JavaScript Mistake #1: Using var Instead of let and const

  3. JavaScript Mistake #2: Not Understanding Asynchronous Code

  4. JavaScript Mistake #3: Ignoring Type Coercion

  5. JavaScript Mistake #4: Forgetting to Use Strict Equality (===)

  6. JavaScript Mistake #5: Misusing this Keyword

  7. JavaScript Mistake #6: Not Handling Errors Properly

  8. JavaScript Mistake #7: Overusing Global Variables

  9. JavaScript Mistake #8: Not Understanding Scope and Closures

  10. JavaScript Mistake #9: Inefficient DOM Manipulation

  11. JavaScript Mistake #10: Forgetting About null and undefined

  12. Final Tips for Writing Clean JavaScript

  13. Resources & Further Reading

1. Introduction: Why JavaScript Mistake Happen

JavaScript powers the modern web. Whether you’re building a simple to-do app or a full-fledged SaaS platform, chances are JavaScript is part of your stack.

However, JavaScript’s flexibility can be a double-edged sword. Its loose typing, asynchronous nature, and unique quirks make it easy to write code that’s functional but inefficient—or worse, buggy.

In this guide, we’ll cover the 10 most common JavaScript Mistake developers make, explain why they happen, and provide clear ES6+ solutions so you can avoid them.

2. JavaScript Mistake #1: Using var Instead of let and const

In older JavaScript, var was the only way to declare variables. But ES6 introduced let and const, which have block scope and prevent many logical bugs.

 Bad Example:

javascript
var count = 5;
for (var i = 0; i < count; i++) {
console.log(i);
}
console.log(i); // 5 — i is still accessible here (bug-prone)

 Good Example:

javascript
for (let i = 0; i < 5; i++) {
console.log(i);
}
// console.log(i); // ReferenceError: i is not defined

Why this matters:
Using let or const avoids accidental variable leakage and makes your code more predictable.
Best practice: Use const by default and let only when reassigning.

3. JavaScript Mistake #2: Not Understanding Asynchronous Code

JavaScript is single-threaded but uses asynchronous callbacks for non-blocking operations. Beginners often write code expecting synchronous behavior.

 Bad Example:

javascript
console.log("Start");
setTimeout(() => console.log("Async task done"), 1000);
console.log("End");
// Output: Start → End → Async task done

 Good Example (Using async/await):

javascript
async function fetchData() {
console.log("Start");
await new Promise(resolve => setTimeout(resolve, 1000));
console.log("Async task done");
console.log("End");
}
fetchData();

Why this matters:
Misunderstanding async behavior leads to race conditions and unexpected outputs.

4. JavaScript Mistake #3: Ignoring Type Coercion

JavaScript will automatically convert types in certain situations, sometimes producing unexpected results.

 Bad Example:

javascript
console.log(1 + '1'); // "11"
console.log('5' - 2); // 3

 Good Example:

javascript
console.log(Number('1') + 1); // 2
console.log(parseInt('5', 10) - 2); // 3

Best practice: Always explicitly convert types to avoid unintended coercion.

5. JavaScript Mistake #4: Forgetting to Use Strict Equality (===)

 Bad Example:

javascript
console.log(0 == false); // true
console.log('' == 0); // true

 Good Example:

javascript
console.log(0 === false); // false
console.log('' === 0); // false

Why this matters:
== performs type coercion, which can cause subtle bugs. Always use ===.

6. JavaScript Mistake #5: Misusing this Keyword

The value of this depends on how a function is called, not where it’s defined.

 Bad Example:

javascript
const obj = {
name: "JS",
greet: function() {
setTimeout(function() {
console.log(this.name);
}, 1000);
}
};
obj.greet(); // undefined

 Good Example (Arrow Function):

javascript
const obj = {
name: "JS",
greet: function() {
setTimeout(() => {
console.log(this.name);
}, 1000);
}
};
obj.greet(); // "JS"

7. Mistake #6: Not Handling Errors Properly

 Bad Example:

javascript
fetch('invalid-url')
.then(response => response.json())
.then(data => console.log(data));

 Good Example:

javascript
async function getData() {
try {
const res = await fetch('invalid-url');
const data = await res.json();
console.log(data);
} catch (error) {
console.error('Error fetching data:', error);
}
}
getData();

8. Mistake #7: Overusing Global Variables

Too many global variables increase the risk of name collisions.

 Best Practice: Encapsulate logic inside functions, modules, or classes.

9. Mistake #8: Not Understanding Scope and Closures

Closures are powerful but can cause memory leaks if misunderstood.

Example:

javascript
function makeCounter() {
let count = 0;
return function() {
count++;
return count;
};
}
const counter = makeCounter();
console.log(counter()); // 1

10. Mistake #9: Inefficient DOM Manipulation

Repeated DOM changes are slow. Use document fragments or frameworks like React for efficiency.

11. Mistake #10: Forgetting About null and undefined

Always check values before accessing properties.

 Example:

javascript
const user = null;
console.log(user?.name); // undefined

12. Final Tips for Writing Clean JavaScript

  • Use ESLint to catch mistakes early.

  • Follow a style guide like Airbnb’s JavaScript Style Guide.

  • Write unit tests.

13. Resources & Further Reading

 

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