Custom Error Class – JavaScript Fundamentals

✅ What Are Custom Error Classes?

JavaScript provides built-in error types like Error, TypeError, RangeError, etc.

But sometimes you want to define your own specific error types to represent domain-specific problems.

👉 That’s where custom error classes come in.

🧱 Syntax

We create a custom error class by extending the built-in Error class using class syntax.

class MyCustomError extends Error {
  constructor(message) {
    super(message);         // Call parent constructor (Error)
    this.name = "MyCustomError";  // Set custom error name
  }
}

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

try {
  throw new ValidationError("Invalid input!");
} catch (err) {
  console.log(err.name);    // ValidationError
  console.log(err.message); // Invalid input!
}

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

function setAge(age) {
  if (age < 0) {
    throw new AgeError("Age cannot be negative.");
  }
  return `Age is set to ${age}`;
}

try {
  console.log(setAge(-5));
} catch (error) {
  if (error instanceof AgeError) {
    console.error("Custom Error:", error.message);
  } else {
    console.error("Other Error:", error.message);
  }
}

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

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

function login(user) {
  if (!user) throw new AuthError("User not found");
  if (!user.isAdmin) throw new PermissionError("Access denied");
  return "Welcome!";
}

try {
  login({ isAdmin: false });
} catch (err) {
  if (err instanceof AuthError) {
    console.log("Login failed:", err.message);
  } else if (err instanceof PermissionError) {
    console.log("Permission issue:", err.message);
  } else {
    console.log("Unknown error:", err.message);
  }
}

🔥 Why Use Custom Errors?

✅ Gives meaningful error types in your application

✅ Allows different error handling for different error types

✅ Helps with cleaner debugging and logging

✅ Best Practices

Practice	Why it matters
Always extend `Error`	Ensures stack trace and prototype chain are correct
Set `this.name` manually	So you can identify error types during handling
Use `instanceof`	To check custom error types when catching

📋 Summary

Term	Meaning
`extends Error`	Creates a custom error class
`super()`	Calls the base `Error` constructor
`this.name`	Sets your custom error name (recommended)