How to Create Custom Error in JavaScript: Simple Guide

To create a custom error in JavaScript, define a new class that extends the built-in Error class and set a custom name property. This lets you throw and catch errors with specific types for better debugging and control.

Syntax

Use a class that extends Error. Inside the constructor, call super(message) to set the error message, and set this.name to your custom error name.

• class: defines the custom error type
• extends Error: inherits from JavaScript's built-in error
• constructor(message): receives the error message
• super(message): passes message to the base Error class
• this.name: sets the error's name for identification

javascript

class CustomError extends Error {
  constructor(message) {
    super(message);
    this.name = 'CustomError';
  }
}

Example

This example shows how to create and throw a custom error, then catch it and check its type.

javascript

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

function validateAge(age) {
  if (age < 0 || age > 120) {
    throw new ValidationError('Age must be between 0 and 120');
  }
  return true;
}

try {
  validateAge(-5);
} catch (error) {
  if (error instanceof ValidationError) {
    console.log('Caught a validation error:', error.message);
  } else {
    console.log('Unknown error:', error);
  }
}

Output

Caught a validation error: Age must be between 0 and 120

Common Pitfalls

Common mistakes include forgetting to call super(message) in the constructor, which breaks the error message setup, or not setting this.name, making it harder to identify the error type. Also, avoid using plain objects or strings as errors because they lack stack traces and type information.

javascript

/* Wrong way: missing super call and name */
class BadError extends Error {
  constructor(message) {
    // super(message) is missing
    super();
    this.message = message;
  }
}

/* Right way: call super and set name */
class GoodError extends Error {
  constructor(message) {
    super(message);
    this.name = 'GoodError';
  }
}

Quick Reference

Remember these key points when creating custom errors:

• Always extend the built-in Error class.
• Call super(message) in the constructor.
• Set this.name to your custom error's name.
• Use instanceof to check error types when catching.

Key Takeaways

Create custom errors by extending the built-in Error class.

Always call super(message) in your custom error constructor.

Set this.name to identify your custom error type clearly.

Use instanceof to detect your custom errors in catch blocks.

Avoid using plain objects or strings as errors for better debugging.