Debugging JavaScript: Tips and Techniques

Debugging JavaScript: Tips and Techniques

"Mastering the Art of Debugging: Expert Tips and Techniques for JavaScript Development


18 min read

Debugging is an essential part of the software development process. No matter how good you are at coding, you're bound to encounter errors or unexpected behavior in your code. Debugging is the process of finding and fixing those errors, and it's an essential skill for every developer.

In this article, we'll explore some tips and techniques for debugging JavaScript code, along with an example to demonstrate the process.

Tip #1: Use console.log

One of the most straightforward and useful ways to debug JavaScript code is to use console.log. Console.log is a built-in method that allows you to output messages to the console. You can use console.log to print out variable values, function outputs, and other data types to help you understand what's happening in your code.


let num1 = 10;
let num2 = 20;
let sum = num1 + num2;
console.log(sum); // Output: 30

In the example above, we're using console.log to print out the value of the sum variable, which is the sum of num1 and num2. This can be helpful when you're trying to figure out why your code is not working as expected.

Tip #2: Use breakpoints

Another powerful debugging technique is to use breakpoints. Breakpoints allow you to pause the execution of your code at a particular line and examine the state of your code. You can inspect variable values, function calls, and the call stack to get a better understanding of what's happening in your code.

To set a breakpoint in your code, you can use the debugger keyword. When the debugger keyword is encountered in your code, the execution of your code will pause, and you can use the browser's developer tools to inspect your code.


function multiply(num1, num2) {
  return num1 * num2;

let result = multiply(5, 10);

In the example above, we're using the debugger keyword to set a breakpoint inside the multiply function. When the function is called, the execution of the code will pause at the debugger keyword, and you can use the browser's developer tools to inspect the values of the num1 and num2 variables.

Tip #3: Check your syntax

Syntax errors are one of the most common errors that developers encounter. A syntax error occurs when the code violates the rules of the JavaScript syntax. These errors can be easy to overlook, but they can cause your code to break.

To check your syntax, you can use an IDE or text editor that has built-in syntax highlighting and error checking. Alternatively, you can use an online JavaScript validator, such as JSHint or ESLint, to check your code for syntax errors.


function add(num1, num2) {
  return num1 + num2;

let result = add(5, 10;

In the example above, we're missing a closing parenthesis on the second line, which will result in a syntax error when the code is executed. By using an IDE or online validator, we can quickly identify and fix syntax errors.

Tip #4: Use try-catch blocks

Another powerful technique for handling errors in JavaScript is to use try-catch blocks. A try-catch block allows you to handle errors gracefully and prevent your code from crashing.

In a try-catch block, you place the code that you want to try inside the try block. If an error occurs, the catch block is executed, and you can handle the error accordingly.


function divide(num1, num2) {
  try {
    if (num2 === 0) {
      throw new Error("Cannot divide by zero.");
    return num1 / num2;
  } catch (error) {

console.log(divide(10, 2)); // Output: 5
console.log(divide(10, 0)); // Output: Cannot divide by zero.

In this example, we define a function divide that takes in two numbers as parameters. Inside the function, we have a try-catch block that attempts to divide num1 by num2. If num2 is zero, we throw a new Error object with the message "Cannot divide by zero.".

If an error occurs in the try block (such as when num2 is zero), the catch block is executed. In this case, we simply log the error message to the console using console.error. If no error occurs, the result of the division is returned.

In summary, try-catch blocks are a powerful tool for handling errors in JavaScript code. By placing code that might throw an error inside a try block and handling potential errors in a catch block, you can ensure that your code runs smoothly even when unexpected errors occur.

Tip #5: Understand closures and scope

Closures and scope are fundamental concepts in JavaScript that can cause confusion for developers. A closure is a function that has access to variables in its outer (enclosing) function, even after the outer function has returned. Understanding closures and scope is essential for debugging complex JavaScript code.

function outerFunction() {
  let outerVariable = 'outer';

  function innerFunction() {
    let innerVariable = 'inner';
    console.log(`${innerVariable} ${outerVariable}`);

  return innerFunction;

let innerFunc = outerFunction();
innerFunc(); // Output: "inner outer"

In this example, outerFunction creates a local variable outerVariable and a nested function innerFunction. innerFunction also creates a local variable innerVariable. When innerFunction is called, it has access to both innerVariable and outerVariable, even though outerFunction has already returned. This is because innerFunction forms a closure that retains access to the variables in its outer function.

It's important to understand how closures and scope work in JavaScript to avoid bugs related to variable scoping and function scope.

Tip #6: Use a debugger

Debuggers are powerful tools that can help you find and fix bugs in your JavaScript code. Most modern browsers have built-in debuggers that you can use to step through your code line by line, set breakpoints, and inspect variables. Learning how to use a debugger is an essential skill for any JavaScript developer.

The best example to learn a debugger

Tip #7: Avoid global variables

Global variables can make it difficult to understand and debug your code, especially if you're working on a large project. Instead, it's recommended to use local variables and pass data between functions using parameters and return values. This will make your code easier to read, understand, and debug.

Here's an example:

// Bad Example
let x = 10;

function addNumbers(y) {
  return x + y;

console.log(addNumbers(5)); // Output: 15

// Good Example
function addNumbers(x, y) {
  return x + y;

console.log(addNumbers(10, 5)); // Output: 15

In the bad example, the variable x is a global variable that can be accessed from anywhere in the code, which can cause issues if other parts of the code accidentally modify its value. In the good example, x is a local variable that is passed into the addNumbers function as a parameter. This makes the code easier to reason about and debug, as there are no unexpected side effects caused by global variables.

Tip #8: Use strict mode

Strict mode is a feature in JavaScript that allows you to opt into a stricter mode of operation. When you use strict mode, the JavaScript engine will enforce stricter rules and generate more errors for certain code behaviors that could cause problems. This can help you catch errors earlier in development and make your code more reliable.

To use strict mode in your JavaScript code, simply add the following line at the beginning of your script or function:

'use strict';

For example:

'use strict';

function myFunction() {
  // strict mode code goes here

In strict mode, you cannot use undeclared variables, which can help catch typos and other errors. It also prohibits certain unsafe actions, such as assigning values to read-only properties, which can help prevent bugs.

Overall, using strict mode is a simple way to make your JavaScript code more robust and reliable.

Tip #9: Use descriptive variable and function names

One of the most important aspects of writing maintainable and understandable code is using clear and descriptive variable and function names. Choosing descriptive names can make your code more readable and help you and other developers understand what your code is doing.

For example, instead of using generic variable names like "a", "b", or "temp", use names that describe the data or purpose of the variable. If you have a variable that stores a person's name, use a name like "personName" or "fullName" instead of "name".

Similarly, when naming functions, use names that describe what the function does or what it returns. A function that calculates the area of a rectangle could be named "calculateRectangleArea", for example.

Using descriptive names can make your code more self-documenting and can help prevent confusion or errors caused by using the wrong variable or function name.


// Not descriptive variable names
let a = 5;
let b = 10;
let temp = a;
a = b;
b = temp;

// Descriptive variable names
let firstNumber = 5;
let secondNumber = 10;
let tempNumber = firstNumber;
firstNumber = secondNumber;
secondNumber = tempNumber;

// Not descriptive function names
function calc(x, y) {
return x * y;

// Descriptive function names
function calculateRectangleArea(width, height) {
return width * height;

Tip #10: Comment your code

Commenting your code is an essential practice that can help you and other developers understand your code better. By adding comments to your code, you can explain the purpose of a function, the expected inputs and outputs, and any potential side effects. Additionally, commenting can help you keep track of your thought process as you're writing code, making it easier to return to a project after a break.


// This function calculates the area of a rectangle given its length and width
function calculateRectangleArea(length, width) {
// Check if length and width are both numbers
if (typeof length !== 'number' || typeof width !== 'number') {
console.error('Error: length and width must be numbers');
return null;

// Calculate the area and return it
const area = length * width;
return area;

// Example usage
const rectangleArea = calculateRectangleArea(5, 10);
console.log(rectangleArea); // Output: 50

In the example above, the function is commented to explain its purpose and any potential errors that could occur. This makes it easier for other developers to understand the code and for you to return to the code at a later time.

Tip #11: Use ES6 features

ES6 (also known as ECMAScript 2015) is a significant update to the JavaScript language that introduced many new features and improvements. Using ES6 features can make your code more concise, readable, and efficient. Some popular ES6 features include:

  1. Arrow functions: Arrow functions provide a more concise syntax for defining functions, especially when working with arrays and higher-order functions.


// ES5 syntax
var numbers = [1, 2, 3, 4, 5];
var squaredNumbers = {
  return number * number;

// ES6 syntax
const numbers = [1, 2, 3, 4, 5];
const squaredNumbers = => number * number);
  1. let and const: The let and const keywords were introduced in ES6 as alternatives to var for declaring variables. They provide better scoping rules and can help prevent common bugs.


// ES5 syntax
var name = 'John';
var age = 30;
if (age > 18) {
  var message = name + ' is an adult.';
console.log(message); // 'John is an adult.'

// ES6 syntax
const name = 'John';
let age = 30;
if (age > 18) {
  const message = `${name} is an adult.`;
console.log(message); // ReferenceError: message is not defined
  1. Template literals: Template literals provide a more flexible and readable way to create strings, especially when working with variables or multiline strings.


// ES5 syntax
var name = 'John';
var message = 'Hello, my name is ' + name + '.';

// ES6 syntax
const name = 'John';
const message = `Hello, my name is ${name}.`;
  1. Destructuring: Destructuring allows you to extract values from arrays or objects and assign them to variables in a more concise and readable way.


// ES5 syntax
var person = { name: 'John', age: 30 };
var name =;
var age = person.age;

// ES6 syntax
const person = { name: 'John', age: 30 };
const { name, age } = person;

By using these and other ES6 features, you can write cleaner, more expressive, and less error-prone code in JavaScript.

The Don't Repeat Yourself (DRY) principle is an important concept in software development, including JavaScript. The principle states that every piece of knowledge or logic should have a single, unambiguous, authoritative representation within a system. In other words, don't repeat the same code or logic in multiple places in your codebase.

When you find yourself copying and pasting code, it's a good indication that you should refactor your code and extract the common logic into a reusable function or module. This makes your code more modular, maintainable, and easier to debug.

Here's an example of how you can refactor repetitive code into a reusable function:


let num1 = 10;
let num2 = 5;

console.log(num1 + num2);

num1 = 20;
num2 = 15;

console.log(num1 + num2);

num1 = 30;
num2 = 25;

console.log(num1 + num2);


function add(num1, num2) {
  console.log(num1 + num2);

add(10, 5);
add(20, 15);
add(30, 25);

In the refactored code, we created a function called add that takes two arguments, num1 and num2, and logs the sum to the console. We then called the add function three times with different arguments, instead of repeating the same code three times. This makes the code more concise and easier to maintain.

Tip #13: Test your code

Testing is an essential part of software development. It helps ensure that your code works as expected and can catch bugs early on in the development process. There are various types of tests you can perform, including unit tests, integration tests, and end-to-end tests.

Unit tests involve testing individual components of your code, such as functions or modules, in isolation. Integration tests involve testing how multiple components work together. End-to-end tests involve testing your entire application to ensure that it works as expected.

You can use testing frameworks such as Mocha, Jest, and Jasmine to write and run tests for your JavaScript code. By testing your code, you can catch bugs early on and avoid issues that could arise later in production.


// Function to test
function add(num1, num2) {
  return num1 + num2;

// Unit test using Mocha
describe('add', function() {
  it('should add two numbers', function() {
    assert.equal(add(2, 3), 5);

In the example above, we have a simple add function that takes two numbers and returns their sum. We then use the Mocha testing framework to write a unit test for this function. The describe function is used to group together related tests, and the it function is used to specify individual test cases. In this case, we're testing that the add function correctly adds two numbers together. We use the assert library to check that the expected result is equal to the actual result returned by the add function.

By writing tests for your JavaScript code, you can catch bugs early on and ensure that your code works as expected. This can save you time and effort in the long run and help ensure that your code is of high quality.

Tip #13: Use code linters

Code linters are tools that analyze your code for potential errors and style issues. They can help you catch errors early on and maintain a consistent coding style throughout your project. There are several popular code linters available for JavaScript, including ESLint, JSHint, and JSLint.

To use a linter, you first need to install it as a package in your project. Once installed, you can run the linter using a command in your terminal or integrated development environment (IDE). The linter will scan your code and report any issues it finds, such as unused variables, missing semicolons, or inconsistent spacing.

Here's an example of how to use ESLint in a Node.js project:

  1. Install ESLint as a development dependency using npm:

     npm install eslint --save-dev
  2. Create a configuration file for ESLint in the root of your project:

     npx eslint --init
  3. Configure the rules you want to enforce in your project, such as "no-console" or "semi":

     module.exports = {
       rules: {
         'no-console': 'warn',
         semi: ['error', 'always'],
  4. Run ESLint on your code using the following command:

     npx eslint yourFile.js

This will scan your code and report any issues it finds based on your configured rules. You can also use ESLint plugins and extensions to extend the functionality of the linter and tailor it to your specific needs.

Using a code linter can help you catch potential issues early on and maintain a consistent coding style throughout your project.

Tip #14: Understand asynchronous programming

Asynchronous programming is a programming paradigm in which the execution of code is non-blocking, meaning that the program can continue to execute other code while waiting for certain operations to complete. Asynchronous programming is essential in JavaScript for performing operations such as making HTTP requests, reading and writing files, and handling user input without blocking the program's execution.

In JavaScript, asynchronous programming is typically achieved using callbacks, promises, and async/await. Understanding these concepts is crucial for writing efficient and maintainable asynchronous code.

Callbacks are functions that are passed as arguments to other functions and are called when a particular operation is complete. For example, when making an HTTP request, you can pass a callback function that will be called when the response is received.

Promises provide a more structured way to handle asynchronous code. A promise represents a value that may not be available yet and provides a way to handle success and error cases. Promises also allow chaining multiple asynchronous operations, making it easier to write complex asynchronous code.

Async/await is a newer feature in JavaScript that provides a cleaner syntax for working with asynchronous code. With async/await, you can write asynchronous code that looks like synchronous code, making it easier to understand and debug.

Example using callbacks:

function makeHTTPRequest(url, callback) {
  const request = new XMLHttpRequest();
  request.onreadystatechange = function() {
    if (request.readyState === 4 && request.status === 200) {
  };"GET", url, true);

makeHTTPRequest("", function(response) {

Example using promises:

  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error(error));

Example using async/await:

async function makeHTTPRequest() {
  try {
    const response = await fetch("");
    const data = await response.json();
  } catch (error) {


Tip #15: Optimize your code

Optimizing your code can improve its performance and reduce load times, making for a better user experience. There are several ways to optimize your code, including:

  • Minifying your code: Minifying your code removes unnecessary characters like spaces and comments, reducing its size and improving load times.

  • Using caching: Caching can help reduce server load and improve page load times by storing frequently used data in the browser or on a server.

  • Optimizing images: Optimizing images can help reduce their size and improve load times. Use image compression tools to reduce file sizes without sacrificing quality.

  • Avoiding unnecessary DOM manipulation: Manipulating the DOM can be slow, so avoid doing it unnecessarily. Use techniques like batch DOM update to minimize the number of updates.

  • Using web workers: Web workers allow you to run scripts in the background, freeing up the main thread for other tasks and improving performance.

  • Avoiding global variables and functions: As mentioned earlier, global variables and functions can make your code harder to read and debug. They can also slow down your code, so avoid them whenever possible.

  • Optimizing your code takes time and effort, but the benefits are worth it. Your users will appreciate the faster load times and improved performance, and you'll have a more efficient codebase to work with.

here's an example of how you can minify your code using a popular tool called UglifyJS:

// Original JavaScript code
function addNumbers(a, b) {
  return a + b;

// Minified JavaScript code
function addNumbers(n,d){return n+d}

In this example, the original JavaScript function addNumbers has been minified using UglifyJS. As you can see, the code has been compressed by removing unnecessary whitespace and shortening variable names, resulting in a smaller file size and faster load times.

Here's another example of how you can optimize your code by avoiding unnecessary DOM manipulation:

// Inefficient DOM manipulation
for (let i = 0; i < 1000; i++) {
  document.getElementById('my-element').innerHTML += '<li>' + i + '</li>';

// Efficient DOM manipulation
const list = document.getElementById('my-element');
const fragment = document.createDocumentFragment();
for (let i = 0; i < 1000; i++) {
  const listItem = document.createElement('li');
  listItem.textContent = i;

In this example, we're adding 1000 list items to an element with the ID 'my-element'. The first code block is inefficient because it's updating the element's innerHTML property 1000 times, which can be slow. The second code block is more efficient because it creates a document fragment and appends all the list items to it before appending the fragment to the element. This way, the DOM is only updated once, which can be much faster.

here's an example of inefficient code and how it can be optimized:

// Inefficient code
const arr = [1, 2, 3, 4, 5];
const newArr = [];
for(let i=0; i<arr.length; i++) {
newArr.push(arr[i] * 2);

// Efficient code
const arr = [1, 2, 3, 4, 5];
const newArr = => num * 2);

In the inefficient code, we loop through the array using a for loop and push the multiplied values into a new array. This requires multiple lines of code and can be slower for large arrays.

In the efficient code, we use the map method to create a new array with the multiplied values in one line of code. This is faster and more concise than the for loop.

By optimizing our code in this way, we can improve its performance and make it easier to read and maintain.

Tip #16: Keep learning and practicing

JavaScript is a constantly evolving language, with new features and best practices being introduced all the time. To be a successful JavaScript developer, it's important to keep learning and practicing.

Stay up-to-date with the latest developments in the language by reading blogs, attending conferences, and participating in online communities. Take online courses or workshops to deepen your knowledge and skills. And most importantly, practice writing code as often as you can.

By continuing to learn and practice, you'll not only become a better JavaScript developer, but you'll also be able to keep up with the constantly changing demands of the industry.

Thank you for taking the time to read this article on JavaScript tips and techniques for debugging and optimizing your code. We hope that these tips will help you write more efficient, readable, and bug-free code. Remember to keep learning and practicing, as the world of JavaScript is constantly evolving. ๐Ÿ˜Š Happy coding!