Custom Errors in JavaScript: Extending Error

Custom Errors in JavaScript: Extending Error

JavaScript allows you to create custom error classes by extending the built-in Error class. This helps categorize different types of errors in your application.

🔹 Why Use Custom Errors?

✅ Provides more meaningful error messages
✅ Helps with better error handling
✅ Allows catching specific error types

🔹 Creating a Custom Error Class

To create a custom error, extend the Error class and set a meaningful name property.

✅ Example: Custom Validation Error

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

try {
    throw new ValidationError("Invalid email format");
} catch (error) {
    console.log(`${error.name}: ${error.message}`); 
    // ✅ Output: ValidationError: Invalid email format
}

✔ super(message) calls the Error constructor to store the message.
✔ Setting this.name = "ValidationError" helps identify the error type.

🔹 Catching Specific Errors

You can use instanceof to handle different error types separately.

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

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

try {
    throw new DatabaseError("Connection failed");
} catch (error) {
    if (error instanceof ValidationError) {
        console.log("Handle validation issue:", error.message);
    } else if (error instanceof DatabaseError) {
        console.log("Handle database issue:", error.message);
    } else {
        console.log("General error:", error.message);
    }
}

✅ Output:

Handle database issue: Connection failed

✔️ Different error types are handled appropriately.

🔹 Adding Custom Properties

You can add extra properties to store additional information.

✅ Example: Custom Error with Extra Info

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

try {
    throw new ApiError("Not Found", 404);
} catch (error) {
    console.log(`${error.name} (${error.statusCode}): ${error.message}`);
    // ✅ Output: ApiError (404): Not Found
}

✔️ Useful for API responses when handling HTTP errors.

🔹 Reusing Custom Errors in Functions

Custom errors can be used in functions to enforce rules.

function checkAge(age) {
    if (age < 18) {
        throw new ValidationError("You must be 18 or older.");
    }
    return "Access granted!";
}

try {
    console.log(checkAge(16)); // ❌ This will throw an error
} catch (error) {
    console.log(`Error: ${error.message}`);
}

✅ Output:

Error: You must be 18 or older.

✔ Custom errors improve readability and debugging.

🔹 Summary

✔ Extend Error to create custom errors
✔ Use meaningful error names (this.name)
✔ Catch errors specifically using instanceof
✔ Add extra properties for more details

🚀 Need more help? Let me know! 😊