JavaScript Error Handling: try-catch, Custom Errors and Global Handlers

JavaScript Error Handling

Unhandled errors crash applications and expose sensitive information. Proper error handling — with meaningful custom errors, async error handling, and global handlers — is what separates production-grade code from demos.

// Built-in error types
new Error('generic error');
new TypeError('wrong type');
new RangeError('out of range');
new ReferenceError('undefined variable');
new SyntaxError('bad syntax');

// Custom error classes
class AppError extends Error {
    constructor(message, code) {
        super(message);
        this.name    = 'AppError';
        this.code    = code;
        this.isOperational = true;  // expected error (vs programmer bug)
    }
}

class ValidationError extends AppError {
    constructor(fields) {
        super('Validation failed');
        this.name   = 'ValidationError';
        this.fields = fields;
    }
}

class NotFoundError extends AppError {
    constructor(resource) {
        super(`${resource} not found`);
        this.name       = 'NotFoundError';
        this.statusCode = 404;
    }
}

// Throwing and catching
function getUser(id) {
    if (typeof id !== 'number') throw new TypeError('id must be a number');
    const user = db.find(id);
    if (!user) throw new NotFoundError('User');
    return user;
}

try {
    const user = getUser('abc');
} catch (err) {
    if (err instanceof ValidationError) {
        console.error('Fields:', err.fields);
    } else if (err instanceof NotFoundError) {
        console.error('404:', err.message);
    } else {
        throw err;  // rethrow unknown errors
    }
}

Async Error Handling

// With async/await — always use try/catch
async function fetchUser(id) {
    try {
        const res = await fetch(`/api/users/${id}`);
        if (!res.ok) throw new AppError(`HTTP ${res.status}`, res.status);
        return await res.json();
    } catch (err) {
        if (err instanceof AppError) throw err;  // pass it up
        throw new AppError('Network request failed', 503);
    }
}

// Global handler for unhandled Promise rejections
process.on('unhandledRejection', (reason) => {
    console.error('Unhandled Rejection:', reason);
    process.exit(1);
});

window.addEventListener('unhandledrejection', (event) => {
    console.error('Unhandled rejection:', event.reason);
});