8 Common Mistakes of JavaScript Developers and How to Avoid Them

mistakes of javascript developers

JavaScript is a high-level programming language that enjoys widespread adoption among developers globally. It has a global usage share of over 98.2% among web pages. Although renowned for its ease of use as an untyped language, novice programmers may encounter difficulties when implementing real-world projects. 

If a company seeks to hire Javascript developers, they typically evaluate the candidates’ proficiency by posing these problems during the interview.

In this post, we shall delve into 8 common errors encountered by JavaScript programmers and ways to circumvent them.

8 Common Mistakes JavaScript Developers Should Avoid

JavaScript is a versatile language that allows developers to build interactive and dynamic web pages that can respond to user input in real time. However, even experienced JavaScript developers can make mistakes that cause bugs, slow the application, or compromise security.

list of mistakes javascript developers

1. Memory leaks

A program may encounter memory leaks if it fails to deallocate memory that is no longer required, leading to a gradual increase in memory usage until the program crashes. This is a frequent error committed by JavaScript programmers who need to gain knowledge of JavaScript’s memory management and garbage collection principles.

Consider the following example code:

					function createArray() {
  var arr = new Array(1000000);
  return arr;

var myArray = createArray();
In this code, we have created an array of one million elements and then returned the array from a function. However, we never release the memory allocated for the array. If we call this function many times, it will cause a memory leak. To avoid memory leaks, a programmer must release unnecessary memory allocations. This can be done by using the delete operator or setting a variable to null.
					function createArray() {
  var arr = new Array(1000000);
  return arr;

var myArray = createArray();

// Release memory
myArray = null;

2. Type

Javascript is not a strongly-typed language, which means that variables can hold different types of data at different times. However, this flexibility can lead to problems if you are a former C/C++ developer and not familiar with the type concept of Javascript.

Consider the following example:

					var x = "10";
var y = 5;
var z = x + y;
In this code, x is a string and y is an integer. However, when we add them, Javascript will automatically convert y to a string and concatenate the two values, resulting in “105”. To avoid type-related mistakes, always make sure to explicitly convert values to the correct type before performing operations on them.
					var x = "10";
var y = 5;
var z = parseInt(x) + y;

3. Scope

The term “scope” refers to the portion of a program where a variable is reachable. In Javascript, variables declared inside a function have a function-level scope and are not accessible outside of it.

Consider the following example:

					function myFunction() {
  var x = 1;

In this code, x is declared inside the myFunction() function and is not accessible outside of it. If we try to access x outside of the function, we will get an error. To avoid scope-related mistakes, always make sure to declare variables in the appropriate scope.
					var x;

function myFunction() {
  x = 1;


4. Misunderstanding Mutability Concepts

In JavaScript, some data types such as strings and numbers are immutable, meaning that their value cannot be changed once they are created. On the other hand, objects and arrays are mutable, which means that their contents can be modified.

Consider the following example:

					var x = 5;
var y = x;
y = 10;
console.log(x); // Output: 5
In this code, we create a variable x with a value of 5. We then create a new variable y and assign it the value of x. When we change the value of y to 10, it does not affect the value of x. However, with mutable objects, changes made to one variable can affect others that reference the same object.
					var x = [1, 2, 3];
var y = x;
console.log(x); // Output: [1, 2, 3, 4]
To avoid misunderstandings related to mutability concepts, always make sure to understand the data types you are working with and whether they are immutable or mutable. Again, in JavaScript, the const the keyword is often misunderstood to mean that a variable declared with it is immutable. While it is true that you cannot reassign a new value to a const variable, the variable itself is not actually immutable. For example, if you declare a const object, you can still modify its properties:
					const myObj = { foo: 'bar' }; = 'baz';
console.log(myObj); // Output: { foo: 'baz' }

Similarly, if you declare a const array, you can still modify its elements:

					const myArr = [1, 2, 3];
console.log(myArr); // Output: [1, 2, 3, 4]
This means that while const provides some level of immutability by preventing you from assigning a new value to a variable, it does not provide complete immutability in the sense that the contents of an object or array can still be modified. It’s important to keep this in mind when using const in your code, as it can lead to unexpected behavior if you assume that a const variable is completely immutable.

5. Equality

Javascript has two equality operators: == and ===. The == the operator performs type coercion, which means that it will try to convert values to the same type before comparing them. The === operator, on the other hand, is strict and will only return true if the values being compared have the same type and value. Consider the following example:
					console.log(5 == "5"); // Output: true
In this code, 5 and “5” are different types (integer and string), but the == operator coerces them to the same type before comparing them, resulting in true. To avoid equality-related mistakes, always use the strict equality operator === unless you specifically need type coercion.

6. Misunderstanding ‘this’ keyword

In JavaScript, the this keyword refers to the object that the function is a method of. However, this can be confusing for developers who are not familiar with Javascript’s object-oriented programming concepts. Consider the following example:
					var obj = {
  x: 10,
  getX: function() {
    return this.x;

console.log(obj.getX()); // Output: 10
var getX = obj.getX;
console.log(getX()); // Output: undefined
In this code, we create an object obj with a property x and a method getX that returns the value of x. When we call obj.getX(), it correctly returns 10. However, when we assign obj.getX to a new variable getX and call it without the context of the obj object, it returns undefined. To avoid misunderstandings related to the this keyword, always make sure to understand the context in which a function is called and how the this keyword will be resolved.

7. Promises

Asynchronous programming is a fundamental component of JavaScript, and Promises are a popular method to manage asynchronous operations. A Promise represents a value that is currently unavailable but will become available in the future, either with a resolved value or a rejection reason.

There are three promise states:

  • Pending: The initial state; neither the Promise is fulfilled nor is it rejected.
  • Fulfillment: The process was completed, and the Promise’s value was resolved.
  • Rejection: Because of the operation’s failure, the Promise is being rejected.

Here’s an example of a Promise that simulates a network request:

					const getData = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("Data received successfully!");
    }, 2000);

  .then((data) => console.log(data))
  .catch((error) => console.log(error));
In this example, a function named getData is defined which returns a new Promise. We utilize the setTimeout function within this Promise to mimic a network request that has a completion time of two seconds. If the request is successful, invoke the resolve function with the message “Data received successfully!”. To execute this Promise, we invoke the function getData and utilize the then function to manage the resolved value. We utilize the catch method for rejection handling.

8. Callback Hell

The term “callback hell” refers to a scenario in which there are numerous nested callbacks that render the code challenging to comprehend and sustain. This is a frequent error committed by Javascript programmers, particularly those who are inexperienced with asynchronous programming.

Here’s an example of what callback hell might look like-

					getUser(userId, (user) => {
  getProfile(user.profileId, (profile) => {
    getPosts(profile.posts, (posts) => {
      posts.forEach((post) => {
        getComments(post.comments, (comments) => {

In this example, we have four levels of nested callbacks, resulting in code that is challenging to comprehend.

To reduce callback hell, one may utilize Promises, async/await, or libraries such as Async.js or Bluebird to achieve code that is more legible and sustainable.

Way out of these mistakes

If you aspire to become a Javascript developer, it is highly unlikely that you will not encounter any of these challenges during your early career as a developer. By carefully studying the Javascript documentation, such mistakes can be significantly reduced. 

Fortunately, Javascript’s documentation is very thorough and well-crafted. By thoroughly studying and understanding it, you can circumvent numerous prevalent errors. Moreover, there exist plenty of online resources, tutorials, and JavaScript bootcamp that can furnish supplementary guidance and assistance as you acquire knowledge and advance as a Javascript programmer.

To minimize errors, it is recommended to practice good coding practices, such as producing tidy, structured, and sustainable code. Implementing early error detection can facilitate prompt issue identification and resolution.

Working with more experienced developers or participating in online communities and forums can also be valuable for getting feedback, advice, and support. Don’t be afraid to ask questions and seek out help when needed, as this can be an effective way to learn and avoid mistakes in the future.


If you are an employer who is looking for Javascript developers who hardly fall for such naive mistakes, our suggestion would be Vivasoft. Vivasoft has a team of seasoned Javascript developers for hire at your disposal for team augmentation.

To hire a dedicated developers team, contact us.

Final Thoughts

Dealing with Javascript errors can be quite a hassle and can take up a lot of your time. However, it’s important to note that these mistakes can actually serve as valuable learning opportunities that can aid in your growth and development as a skilled developer. 

As a developer, it’s crucial to acknowledge that mistakes are inevitable. However, taking the time to learn from them and adopting good coding habits can make all the difference in building robust, reliable, and effective Javascript applications. By doing so, you can avoid many common issues that may arise during the development process.

Hire a Talented Developer from Vivasoft

By Vivasoft Team
By Vivasoft Team
Hire Exceptional Developers Quickly

Find Your Talent

Get Updated with us Regularly
Share this blog on

Hire a Talented Developer from Vivasoft

Lets discuss about your offshore project. We can Help you by our skillful Global team. You can take a free counciling by taking a schedule

Related Post