Throwing Custom Errors in JavaScript: A Complete Guide

JavaScript provides a built-in error handling system, but sometimes default errors just aren't enough. To make your code easier to debug and maintain, you can create and throw custom errors.

In this article, you’ll learn how to throw custom errors in JavaScript, how to use the Error object, and when it’s best to define your own custom error classes.

What Is the `throw` Keyword in JavaScript?

The throw keyword in JavaScript is used to manually trigger an exception. This interrupts the normal flow of code execution and sends control to the nearest catch block (if available).

Syntax:


throw expression;

You can throw almost anything: strings, numbers, or custom error objects. But the best practice is to throw instances of the built-in Error class or its subclasses.

Throwing a Simple Custom Error

You can create custom error messages by using the Error constructor and passing a message.

Example:

class ValidationError extends Error {
  constructor(message) {
    super(message);
    this.name = "ValidationError";
  }
}

function validateUsername(username) {
  if (username.length < 5) {
    throw new ValidationError("Username must be at least 5 characters long");
  }
  return true;
}

try {
  validateUsername("abc");
} catch (e) {
  console.error(`${e.name}: ${e.message}`);
}

Output:

ValidationError: Username must be at least 5 characters long

Why Use Custom Errors?

Custom errors provide several advantages:

Clarity: Gives better context for what went wrong.
Categorization: Helps differentiate between different types of errors (e.g., network vs validation).
Debugging: Stack traces are more meaningful.
Control Flow: Lets you handle specific errors differently.