SvelteKit Logging: Server and Client Error Guide

Svelte Logging Best Practices

SvelteKit runs on both server and client. Your logging strategy needs to cover both.

Server-Side Logging with Hooks

// src/hooks.server.ts
import type { Handle, HandleServerError } from '@sveltejs/kit';

export const handle: Handle = async ({ event, resolve }) => {
  const requestId = crypto.randomUUID();
  event.locals.requestId = requestId;
  const start = performance.now();

  const response = await resolve(event);
  const duration = performance.now() - start;

  console.log(JSON.stringify({
    type: 'request',
    requestId,
    method: event.request.method,
    path: event.url.pathname,
    status: response.status,
    duration_ms: Math.round(duration),
  }));

  return response;
};

export const handleError: HandleServerError = ({ error, event }) => {
  console.error(JSON.stringify({
    type: 'error',
    requestId: event.locals.requestId,
    message: error instanceof Error ? error.message : 'Unknown error',
    stack: error instanceof Error ? error.stack : undefined,
    path: event.url.pathname,
  }));

  return {
    message: 'Internal server error',
  };
};

Client-Side Error Logging

// src/hooks.client.ts
import type { HandleClientError } from '@sveltejs/kit';

export const handleError: HandleClientError = ({ error, message }) => {
  console.error('Client error:', error);

  // Send to logging endpoint
  fetch('/api/log', {
    method: 'POST',
    body: JSON.stringify({
      type: 'client_error',
      message: error instanceof Error ? error.message : message,
      stack: error instanceof Error ? error.stack : undefined,
      url: window.location.href,
      userAgent: navigator.userAgent,
    }),
    keepalive: true,
  }).catch(() => {}); // Don't let logging failures break the app

  return { message: 'Something went wrong' };
};

Structured Logging in Server Routes

// src/routes/api/orders/+server.ts
import type { RequestHandler } from './$types';

export const POST: RequestHandler = async ({ request, locals }) => {
  const logger = createLogger(locals.requestId);

  logger.info('Creating order', { userId: locals.user.id });

  try {
    const order = await createOrder(await request.json());
    logger.info('Order created', { orderId: order.id });
    return json(order, { status: 201 });
  } catch (error) {
    logger.error('Order creation failed', { error: error.message });
    throw error;
  }
};

What to Log

• All requests — method, path, status, duration
• Business events — order created, user signed up
• Errors — with full context and request ID
• External calls — API requests with latency

Connect Bugsly for comprehensive error tracking across both server and client contexts in your SvelteKit application.