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Nikolas Barwicki - Javascript Blog Nikolas's Blog

Decoding JavaScript error types: handling techniques

/ 8 min read

In the intricate world of web development, JavaScript stands out as a versatile and ubiquitous language. However, with great power comes great responsibility, particularly in handling the inevitable errors that arise during development. Proper error handling is not just about preventing applications from crashing; it’s about creating a seamless and user-friendly experience and ensuring that developers can quickly address and resolve issues. This comprehensive guide dives deep into the realm of error handling in JavaScript, offering detailed explanations, best practices, and practical examples to enhance your coding skills.

The Importance of Error Handling

Neglecting error handling is like sailing a ship without a compass; when storms come, you’re likely unprepared. In the development process, errors are not just nuisances but opportunities to fortify your application against unexpected behavior. Proper error handling improves the user experience by providing informative feedback instead of cryptic messages or blank screens. For developers, a robust error handling strategy means fewer emergency fixes and a more stable, maintainable codebase.

Why Errors Occur

Errors can arise from various sources, such as syntax mistakes, type mismatches, failed external resources, or logical errors in your code. Understanding the nature of these errors is the first step in handling them effectively.

Understanding JavaScript Errors

JavaScript provides several types of built-in error objects, each corresponding to a specific kind of error. By understanding these types, you can create more precise and informative error handling mechanisms.

Syntax Error

Syntax errors are the most fundamental type of error. They occur when the code doesn’t follow the language’s syntax rules, such as missing brackets, typos, or incorrect use of language constructs.

For example:

if (userId // Missing closing parenthesis and curly brace
    const user = userService.find(userId);

This snippet will immediately throw a syntax error because the if statement is not correctly formed. Understanding syntax errors is usually straightforward, as modern development environments and linters can detect them even before the code is executed.


TypeErrors occur when an operation is performed on a value of the wrong type, such as attempting to call a method that doesn’t exist on a particular data type.

For example:

const number = 1;
number.toUpperCase(); // TypeError: number.toUpperCase is not a function

In this case, toUpperCase is a method on string objects, not numbers, leading to a TypeError. These errors are common in JavaScript due to its dynamic typing system, where variable types are determined at runtime. Tools like TypeScript add a layer of static typing to help prevent these errors.


ReferenceErrors happen when you attempt to use a variable that hasn’t been declared. They are common mistakes during development.

For example:

console.log(undeclared); // ReferenceError: undeclared is not defined

This error indicates that the script is trying to access a variable named ‘undeclared,’ which hasn’t been defined anywhere in the code.

Other Common Errors

  • RangeError: Occurs when a value is outside the allowed range.
  • InternalError: Represents errors that occur within the JavaScript engine (rare for most developers).
  • EvalError: Related to the use of the global eval() function.
  • URIError: Thrown due to incorrect use of URI handling functions.
  • Error: The generic base object for all errors. You can use this to create custom error types.

The Error Object

At the heart of JavaScript error handling is the Error object. This built-in object captures critical information about an error, including its name (type) and a message describing the issue. Here’s how the Error object is typically structured:

class Error {
    constructor(message) {
        this.message = message; // Descriptive message about the error = 'Error'; // The name/type of the error
        this.stack = '...'; // Non-standard, but provides the call stack for debugging

When an error occurs, JavaScript generates an Error object with these properties. You can also create your own Error objects to throw custom errors.

Throwing Errors

JavaScript allows you to manually signal errors or exceptional situations in your code using the throw statement. Throwing an error stops the execution of the current function and starts the process of looking for an exception handling mechanism, such as a try...catch block.

throw new Error('Oops!'); // Throws a general error with a custom message

// Throwing a string - not recommended, but possible
throw 'Oops!';

While you can throw any value in JavaScript, it’s best practice to throw instances of Error or its subclasses. This ensures consistency and provides more context when handling the error.

Custom Error Handling

To handle specific kinds of errors uniquely, you might want to create your own error types. This is particularly useful when you have complex applications with various possible error conditions.

class ValidationError extends Error {
  constructor(message) {
    super(message); // Calls the constructor of the base Error class = 'ValidationError'; // Custom name for this error type

By extending the Error class, you create a new type of error that can be thrown and caught specifically. This allows for more nuanced and readable error handling.

Error Handling Patterns

Effective error handling in JavaScript typically involves the try...catch statement, which allows you to attempt risky operations and handle any resulting errors gracefully.


The try...catch statement is a way to catch exceptions that occur in the try block. It’s like setting up a safety net for operations that might fail.

try {
  riskyOperation(); // Code that might throw an error
} catch (error) {
  console.error(error.message); // Do something with the error

When an error is thrown inside the try block, execution immediately stops and transfers to the catch block, allowing you to handle the error.

Promises and Async/Await

Promises and async/await are modern JavaScript features that make handling asynchronous operations and their errors much cleaner and more intuitive.


Promises represent future values and have built-in mechanisms for error handling through the .catch() method.

new Promise((resolve, reject) => {
  throw new Error('Oops.');
}).catch((error) => {
  console.log(error.message); // Handles any errors that occur in the Promise


Async/await makes working with Promises even more straightforward, allowing you to write asynchronous code that looks and behaves more like synchronous code.

async function fetchData() {
  try {
    const data = await someAsyncOperation(); // Wait for the Promise to resolve
    return data;
  } catch (error) {
    console.error(error.message); // Handle any errors that occur

With async/await, you use the familiar try...catch pattern to handle errors from asynchronous operations.

Practical Examples and Best Practices

Let’s apply what we’ve learned to some practical scenarios and discuss the best practices for implementing effective error handling in JavaScript applications.

User Not Found Scenario

Imagine you’re writing a function to fetch user data based on an ID. If the user doesn’t exist, you want to throw an error and handle it gracefully.

const getUser = async (id) => {
  try {
    const user = await userService.get(id);
    if (!user) {
      throw new ApiError(404, 'User not found'); // Custom error for user not found
    return user;
  } catch (error) {
    console.error(`Error: ${error.message}.`);
    // Additional error handling logic here

In this scenario, you’re not only catching potential errors from the userService.get call but also throwing a custom error if the user isn’t found. The catch block then handles any errors that occur.

Comprehensive Error Handling

In a complex application, you might encounter various types of errors. Differentiating between them allows you to handle each appropriately.

try {
  // Your code here
} catch (error) {
  if (error instanceof ValidationError) {
    // Handle validation errors specifically
  } else {
    // Log and rethrow other unexpected errors
    throw error;

This pattern gives you the flexibility to provide specific handling for different error types and ensures that unexpected errors aren’t silently ignored.

Logging and Monitoring with Tools Like Sentry

When an error occurs, understanding its context is crucial for effective debugging and resolution. This is where logging and monitoring tools like Sentry come into play. Sentry, for example, provides real-time error tracking and gives you a detailed report of the error, the stack trace, and the state of your application when the error occurred. It can even integrate with your version control system to pinpoint the exact commit that introduced the error.

Integrating a tool like Sentry into your development process means you’re not just reacting to errors; you’re proactively monitoring and addressing them before they affect a significant portion of your users. With features like issue tracking, performance monitoring, and release management, Sentry enhances your ability to understand and improve your application’s health and stability.

Good Error Handling for Great UX

Good error handling is a cornerstone of excellent user experience (UX). Users should never be left wondering what’s happening with the application. If something goes wrong, they should receive immediate, clear, and helpful feedback. Well-implemented error handling informs users of the state of the application and guides them on the next steps, whether it’s retrying an action, checking their input, or being aware that the issue is on the server side.

For instance, if a network request fails, instead of a generic “Something went wrong” message, provide a more specific “Unable to reach the server. Please check your internet connection and try again” message. This informs the user of the likely issue and suggests a course of action.

Moreover, good error handling contributes to a sense of trust and reliability. Users are more likely to continue using and recommend an application that handles problems gracefully, with clear communication and quick resolutions. In contrast, an application that frequently crashes or provides vague error messages feels unreliable and frustrating to use.

In conclusion, by expanding your comprehensive error handling strategy, integrating sophisticated tools like Sentry, and focusing on the user’s experience and understanding during errors, you can significantly enhance the robustness and user-friendliness of your JavaScript applications. These practices not only lead to a more stable product but also contribute to a more engaged and satisfied user base.