Continuously JavaScript is in high demand by all types of programmers. Its high demand has encouraged many programmers and students to learn it. Most of them are not adequately trained by any training institute. Rather they learned themselves without any proper guidance and evaluation. It has increased errors in JavaScript code.
In this blog, we’ll discuss about the most common JavaScript mistakes done by developers. It will further help new developers to avoid and solve such errors. Let’s make sure that your JavaScript training path is smoother and your code is cleaner.
Latest JavaScript Mistakes made by Developers
Mistake #1 – Using var too much instead of let and const
One of the most common mistakes is using “var” to declare variables. While “var” can lead to unexpected behavior due to its function scope. This can cause the bugs that are hard to track, especially in larger projects.
For example, when a variable is declared with “var” inside a loop, it gets hoisted (i.e., moved to the top of its function scope), which can create confusion about its value.
Here’s what can go wrong with “var”:
Code:
for (var i = 0; i < 3; i++) {<br>
setTimeout(function () <br>{<br>
console.log(i);<br>
document.writeln(i);<br> }, 1000);<br>}Output
In this example, “var” causes the loop variable ‘i’ to be shared across all iterations, resulting in ‘i’ begin ‘3’ when the “setTimeout()” callback executes.
The Solution:
Instead of “var”, use “let” or “const”:
• “let” has block scope and is perfect for variables whose value might change.
• “const” also has block scope but is used when the variable’s value shouldn’t change.
Now let’s fix this with “let”:
Code:
for(let i = 0; i < 3; i++){<br>
setTimeout(function(){<br>
console.log(i);<br>
document.writeln(i);<br>
}, 1000);<br>}Output
By using “let,” each iteration will get its own scoped variable, and the output would just be as expected.
Mistake #2 – Not Using Asynchronous Approach in JavaScript
JavaScript has asynchronous features like setTimeout(), promises, and API calls. Many beginners are either not aware or not comfortable with managing asynchronous code. It leads to callback hell or unhandled promise rejections. Callback hell is a situation when there are many unnecessary function callbacks in a code.
Example: Callback Hell
Code:
setTimeout(() => {<br>
document.writeln("Task 1<br>");<br>
setTimeout(() => {<br>
document.writeln("Task 2<br>");<br>
setTimeout(() => {<br>
document.writeln("Task 3<br>");<br>
setTimeout(() => {<br>
document.writeln("Task 4<br>");<br>
setTimeout(() => {<br>
document.writeln("Task 5<br>");<br>
}, 1000);<br>
}, 1000);<br>
}, 1000);<br>
}, 1000);<br>},
1000);Output:
It works for sure but once you add a few more tasks, the code turns into a tangled mess.
The Solution
Use promises or async/await to handle asynchronous code in a cleaner and more readable way. This helps ensure that your code runs in the correct order and that errors are handled properly.
Example: A Cleaner Way – Using Promises with async/await
Code:
function delayLog(message, time) {<br>
return new Promise(resolve => {<br>
setTimeout(() => {<br>
document.writeln(message);<br>
resolve();<br>
}, time);<br>
});<br>}<br><br>async function runTasks() {<br>
await delayLog("Task 1<br>", 1000);<br>
await delayLog("Task 2<br>", 1000);<br>
await delayLog("Task 3<br>", 1000);<br>
await delayLog("Task 4<br>", 1000);<br>
await delayLog("Task 5<br>", 1000);<br>}<br><br>runTasks();Output:
With this structure, the same logic becomes much easier to read, manage and modify.
Mistake #3 – Confusing “==” with “===”
A common trap many beginners fall into is using the wrong equality operator. In JavaScript, there are two ways to check if values are equal: == and ===. At first, they look similar but they behave difference. “==” (equality) tries to be smart by changing the types of the values to match before comparing.
- “===” (strict equality) doesn’t change anything, it checks both the value and the type exactly as they are
- This small difference can cause bugs that are really hard to track down later
Example:
console.log(5 == '5'); // true - because '5' is converted to number<br>
console.log(5 === '5'); // false - both are different type: number vs stringWhy it matters:
- If you’re not careful using “==” might make your code behave in ways you didn’t intend.
- For example, Code:
if ('0' == false) {<br>
document.write("This runs!");<br>}Output:
- This prints “This runs!”, which is often not what you want because ‘0’ is converted to false.
The Solution:
To stay safe and avoid bugs, always use “===” and “!==” unless you have a very specific reason not to. It’s a good habit that keeps your comparisons predictable and your code more reliable.
Mistake #4 – Misunderstanding Variable Scope
Many beginners don’t realize that where you declare a variable matters a lot in JavaScript. If you use “var”, the variable function-scoped not block-scoped. That means it doesn’t behave the way you might expect inside blocks like if statements or loops. This often leads to bugs that are super confusing to track down.
Example code:
if (true) {<br>
var name = "John";<br>}<br><br>
document.writeln(name); // Output: JohnOutput:
Even though name variable was declared inside the if block, it still exists outside of it!
The Solution:
- Prefer using let and const, instead of var.
- “let” allows to reassign values but keeps the scope inside the block {}.
- “const” keeps the value constant and also block-scoped.
Example with let, Code:
if (true) {<br>
let city = "New Delhi";<br>}<br><br>
console.log(city); // Reference errorOutput:
Now, the variable “city” is safely locked inside the if block, exactly what you expect.
Mistake #5 – Not Understanding How “this” Works in JavaScript
In JavaScript, the value of “this” can change depending on how a function is called, not where it’s written. Many beginners expect “this” to always point to the object that contains the function. But that’s not always true, especially inside regular functions, event handlers, or when working with “setTimeout”. This leads to strange, unexpected bugs.
Example, Code:
const user = {<br>
username: "Alice",<br>
greet: function(){<br>
document.writeln("Hello, " + this.username);<br>
}<br>};<br><br>setTimeout(user.greet, 1000);Output:
Wait, why undefined? Because when “setTimeout” calls “user.greet”, it loses its original object reference, so “this” becomes “window” (or “undefined” in strict mode).
The Solution:
Use an arrow function “()=>{}” or explicitly bind “this” to the correct object.
Example 1 (Arrow function fix),
Code:
const user = {<br>
username: "Alice",<br>
greet: function(){<br>
document.writeln("Hello, " + this.username);<br>
}<br>};<br><br>setTimeout(() => user.greet(), 1000);Output:
Example 1 (blind fix),
Code:
const user = {<br>
username: "Alice",<br>
greet: function(){<br>
document.writeln("Hello, " + this.username);<br>
}<br>};<br><br>setTimeout(user.greet.bind(user), 1000);Output:
Now, “this” stays linked to user, you’ll get “Hello, Alice” as expected.
Mistake #6 – Modifying Objects or Arrays Directly Without Cloning
In JavaScript, data structures like Arrays and Objects are reference types. That means, when you assign an object or array to a new variable, you’re not creating a new copy, you’re just pointing to the same thing in the memory. Beginners often change the new variable thinking it’s a separate copy, but they’re changing the original data too, sometimes causing huge, hidden bugs in applications.
Example for Arrays,
Code:
const originalArray = [1, 2, 3];<br>
const newArray = originalArray;<br>
newArray.push(4);<br><br>
console.log(originalArray);<br>
console.log(newArray);Output:
Example for objects,
Code:
const user = { name: "Alice", age: 25 };<br>
const copiedUser = user;<br>
copiedUser.age = 30;<br><br>
console.log(user.age);<br>
console.log(copiedUser.age);Output:
Notice how “originalArray” also changed, even though we only pushed (4) to “newArray”? That’s because both variables are pointing to the same array.
The Solution:
Always clone the array or object if you want a real copy before modifying it.
Example for Arrays,
Code:
const originalArray = [1, 2, 3];<br>
const newArray = [...originalArray];<br>
newArray.push(4);<br><br>
console.log(originalArray);<br>
console.log(newArray);Output:
Example for Object,
Code:
const user = { name: "Alice", age: 25 };<br>
const copiedUser = { ...user };<br>
copiedUser.age = 30;<br><br>
console.log(user.age);<br>
console.log(copiedUser.age);Output:
Using the spread operator (…) is a super clean way to make shallow copies.
Helps prevent data mutation bugs that very hard to debug later.
Conclusion
These 6 mistakes of JavaScript are game changer if a JavaScript developer like you follow them. You will be able to write bug free code which offers great performance. Above given points don’t just make you better at JavaScript—they give you a unique way to solve such problems. This way, you can enhance your career in web design easily.
Further you can explore and master more advanced options of JS with JavaScript 3 months master plus and 4 months JS certificate course.
You will get a wonderful growth as a programmer with continuous practice with proper approach. Keep reading similar blogs and apply these suggestions in your code. You will surely see a difference as a developer in you.
