Hooks • Docs • Svelte

‘Hooks’ are app-wide functions you declare that SvelteKit will call in response to specific events, giving you fine-grained control over the framework’s behaviour.

There are three hooks files, all optional:

src/hooks.server.js — your app’s server hooks
src/hooks.client.js — your app’s client hooks
src/hooks.js — your app’s hooks that run on both the client and server

Code in these modules will run when the application starts up, making them useful for initializing database clients and so on.

You can configure the location of these files with config.kit.files.hooks.

Server hooks

The following hooks can be added to src/hooks.server.js:

handle

This function runs every time the SvelteKit server receives a request — whether that happens while the app is running, or during prerendering — and determines the response. It receives an event object representing the request and a function called resolve, which renders the route and generates a Response. This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).

src/hooks.server

/** @type {import('@sveltejs/kit').Handle} */
export async function function handle({ event, resolve }: {
    event: any;
    resolve: any;
}): Promise<any>@type{import('@sveltejs/kit').Handle}handle({ event: anyevent, resolve: anyresolve }) {
	if (event: anyevent.url.pathname.startsWith('/custom')) {
		return new var Response: new (body?: BodyInit | null, init?: ResponseInit) => ResponseThis Fetch API interface represents the response to a request.
MDN Reference
Response('custom response');
	}

	const const response: anyresponse = await resolve: anyresolve(event: anyevent);
	return const response: anyresponse;
}

import type { type Handle = (input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}) => MaybePromise<...>The handle hook runs every time the SvelteKit server receives a request and
determines the response.
It receives an event object representing the request and a function called resolve, which renders the route and generates a Response.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
Handle } from '@sveltejs/kit';

export const const handle: Handlehandle: type Handle = (input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}) => MaybePromise<...>The handle hook runs every time the SvelteKit server receives a request and
determines the response.
It receives an event object representing the request and a function called resolve, which renders the route and generates a Response.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
Handle = async ({ event: RequestEvent<Partial<Record<string, string>>, string | null>event, resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve }) => {
	if (event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.url: URLThe requested URL.
url.URL.pathname: stringMDN Reference
pathname.String.startsWith(searchString: string, position?: number): booleanReturns true if the sequence of elements of searchString converted to a String is the
same as the corresponding elements of this object (converted to a String) starting at
position. Otherwise returns false.
startsWith('/custom')) {
		return new var Response: new (body?: BodyInit | null, init?: ResponseInit) => ResponseThis Fetch API interface represents the response to a request.
MDN Reference
Response('custom response');
	}

	const const response: Responseresponse = await resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve(event: RequestEvent<Partial<Record<string, string>>, string | null>event);
	return const response: Responseresponse;
};

Requests for static assets — which includes pages that were already prerendered — are not handled by SvelteKit.

If unimplemented, defaults to ({ event, resolve }) => resolve(event).

locals

To add custom data to the request, which is passed to handlers in +server.js and server load functions, populate the event.locals object, as shown below.

src/hooks.server

/** @type {import('@sveltejs/kit').Handle} */
export async function function handle(input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}): MaybePromise<...>@type{import('@sveltejs/kit').Handle}handle({ event: RequestEvent<Partial<Record<string, string>>, string | null>event, resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve }) {
	event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.locals: App.LocalsContains custom data that was added to the request within the server handle hook.
locals.App.Locals.user: Useruser = await const getUserInformation: (cookie: string | void) => Promise<User>getUserInformation(event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.get(name: string, opts?: CookieParseOptions): string | undefinedGets a cookie that was previously set with cookies.set, or from the request headers.
@paramname the name of the cookie
@paramopts the options, passed directly to cookie.parse. See documentation hereget('sessionid'));

	const const response: Responseresponse = await resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve(event: RequestEvent<Partial<Record<string, string>>, string | null>event);
	const response: Responseresponse.Response.headers: HeadersMDN Reference
headers.Headers.set(name: string, value: string): voidMDN Reference
set('x-custom-header', 'potato');

	return const response: Responseresponse;
}

import type { type Handle = (input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}) => MaybePromise<...>The handle hook runs every time the SvelteKit server receives a request and
determines the response.
It receives an event object representing the request and a function called resolve, which renders the route and generates a Response.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
Handle } from '@sveltejs/kit';

export const const handle: Handlehandle: type Handle = (input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}) => MaybePromise<...>The handle hook runs every time the SvelteKit server receives a request and
determines the response.
It receives an event object representing the request and a function called resolve, which renders the route and generates a Response.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
Handle = async ({ event: RequestEvent<Partial<Record<string, string>>, string | null>event, resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve }) => {
	event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.locals: App.LocalsContains custom data that was added to the request within the server handle hook.
locals.App.Locals.user: Useruser = await const getUserInformation: (cookie: string | void) => Promise<User>getUserInformation(event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.get(name: string, opts?: CookieParseOptions): string | undefinedGets a cookie that was previously set with cookies.set, or from the request headers.
@paramname the name of the cookie
@paramopts the options, passed directly to cookie.parse. See documentation hereget('sessionid'));

	const const response: Responseresponse = await resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve(event: RequestEvent<Partial<Record<string, string>>, string | null>event);
	const response: Responseresponse.Response.headers: HeadersMDN Reference
headers.Headers.set(name: string, value: string): voidMDN Reference
set('x-custom-header', 'potato');

	return const response: Responseresponse;
};

You can define multiple handle functions and execute them with the sequence helper function.

resolve also supports a second, optional parameter that gives you more control over how the response will be rendered. That parameter is an object that can have the following fields:

transformPageChunk(opts: { html: string, done: boolean }): MaybePromise<string | undefined> — applies custom transforms to HTML. If done is true, it’s the final chunk. Chunks are not guaranteed to be well-formed HTML (they could include an element’s opening tag but not its closing tag, for example) but they will always be split at sensible boundaries such as %sveltekit.head% or layout/page components.
filterSerializedResponseHeaders(name: string, value: string): boolean — determines which headers should be included in serialized responses when a load function loads a resource with fetch. By default, none will be included.
preload(input: { type: 'js' | 'css' | 'font' | 'asset', path: string }): boolean — determines what files should be added to the <head> tag to preload it. The method is called with each file that was found at build time while constructing the code chunks — so if you for example have import './styles.css in your +page.svelte, preload will be called with the resolved path to that CSS file when visiting that page. Note that in dev mode preload is not called, since it depends on analysis that happens at build time. Preloading can improve performance by downloading assets sooner, but it can also hurt if too much is downloaded unnecessarily. By default, js and css files will be preloaded. asset files are not preloaded at all currently, but we may add this later after evaluating feedback.

src/hooks.server

/** @type {import('@sveltejs/kit').Handle} */
export async function function handle({ event, resolve }: {
    event: any;
    resolve: any;
}): Promise<any>@type{import('@sveltejs/kit').Handle}handle({ event: anyevent, resolve: anyresolve }) {
	const const response: anyresponse = await resolve: anyresolve(event: anyevent, {
		transformPageChunk: ({ html }: {
    html: any;
}) => anytransformPageChunk: ({ html: anyhtml }) => html: anyhtml.replace('old', 'new'),
		filterSerializedResponseHeaders: (name: any) => anyfilterSerializedResponseHeaders: (name: anyname) => name: anyname.startsWith('x-'),
		preload: ({ type, path }: {
    type: any;
    path: any;
}) => anypreload: ({ type: anytype, path: anypath }) => type: anytype === 'js' || path: anypath.includes('/important/')
	});

	return const response: anyresponse;
}

import type { type Handle = (input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}) => MaybePromise<...>The handle hook runs every time the SvelteKit server receives a request and
determines the response.
It receives an event object representing the request and a function called resolve, which renders the route and generates a Response.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
Handle } from '@sveltejs/kit';

export const const handle: Handlehandle: type Handle = (input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}) => MaybePromise<...>The handle hook runs every time the SvelteKit server receives a request and
determines the response.
It receives an event object representing the request and a function called resolve, which renders the route and generates a Response.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
Handle = async ({ event: RequestEvent<Partial<Record<string, string>>, string | null>event, resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve }) => {
	const const response: Responseresponse = await resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve(event: RequestEvent<Partial<Record<string, string>>, string | null>event, {
		ResolveOptions.transformPageChunk?(input: {
    html: string;
    done: boolean;
}): MaybePromise<string | undefined>Applies custom transforms to HTML. If done is true, it’s the final chunk. Chunks are not guaranteed to be well-formed HTML
(they could include an element’s opening tag but not its closing tag, for example)
but they will always be split at sensible boundaries such as %sveltekit.head% or layout/page components.
@paraminput the html chunk and the info if this is the last chunktransformPageChunk: ({ html: stringhtml }) => html: stringhtml.String.replace(searchValue: string | RegExp, replaceValue: string): string (+3 overloads)Replaces text in a string, using a regular expression or search string.
@paramsearchValue A string or regular expression to search for.
@paramreplaceValue A string containing the text to replace. When the {@linkcode searchValue} is a RegExp, all matches are replaced if the g flag is set (or only those matches at the beginning, if the y flag is also present). Otherwise, only the first match of {@linkcode searchValue} is replaced.replace('old', 'new'),
		ResolveOptions.filterSerializedResponseHeaders?(name: string, value: string): booleanDetermines which headers should be included in serialized responses when a load function loads a resource with fetch.
By default, none will be included.
@paramname header name
@paramvalue header valuefilterSerializedResponseHeaders: (name: stringname) => name: stringname.String.startsWith(searchString: string, position?: number): booleanReturns true if the sequence of elements of searchString converted to a String is the
same as the corresponding elements of this object (converted to a String) starting at
position. Otherwise returns false.
startsWith('x-'),
		ResolveOptions.preload?(input: {
    type: "font" | "css" | "js" | "asset";
    path: string;
}): booleanDetermines what should be added to the &#x3C;head> tag to preload it.
By default, js and css files will be preloaded.
@paraminput the type of the file and its pathpreload: ({ type: "font" | "css" | "js" | "asset"type, path: stringpath }) => type: "font" | "css" | "js" | "asset"type === 'js' || path: stringpath.String.includes(searchString: string, position?: number): booleanReturns true if searchString appears as a substring of the result of converting this
object to a String, at one or more positions that are
greater than or equal to position; otherwise, returns false.
@paramsearchString search string
@paramposition If position is undefined, 0 is assumed, so as to search all of the String.includes('/important/')
	});

	return const response: Responseresponse;
};

Note that resolve(...) will never throw an error, it will always return a Promise<Response> with the appropriate status code. If an error is thrown elsewhere during handle, it is treated as fatal, and SvelteKit will respond with a JSON representation of the error or a fallback error page — which can be customised via src/error.html — depending on the Accept header. You can read more about error handling here.

handleFetch

This function allows you to modify (or replace) a fetch request that happens inside a load or action function that runs on the server (or during pre-rendering).

For example, your load function might make a request to a public URL like https://api.yourapp.com when the user performs a client-side navigation to the respective page, but during SSR it might make sense to hit the API directly (bypassing whatever proxies and load balancers sit between it and the public internet).

src/hooks.server

/** @type {import('@sveltejs/kit').HandleFetch} */
export async function function handleFetch({ request, fetch }: {
    request: any;
    fetch: any;
}): Promise<any>@type{import('@sveltejs/kit').HandleFetch}handleFetch({ request: anyrequest, fetch: anyfetch }) {
	if (request: anyrequest.url.startsWith('https://api.yourapp.com/')) {
		// clone the original request, but change the URL
		request: anyrequest = new var Request: new (input: RequestInfo | URL, init?: RequestInit) => RequestThis Fetch API interface represents a resource request.
MDN Reference
Request(
			request: anyrequest.url.replace('https://api.yourapp.com/', 'http://localhost:9999/'),
			request: anyrequest
		);
	}

	return fetch: anyfetch(request: anyrequest);
}

import type { type HandleFetch = (input: {
    event: RequestEvent;
    request: Request;
    fetch: typeof fetch;
}) => MaybePromise<Response>The handleFetch hook allows you to modify (or replace) a fetch request that happens inside a load function that runs on the server (or during pre-rendering)
HandleFetch } from '@sveltejs/kit';

export const const handleFetch: HandleFetchhandleFetch: type HandleFetch = (input: {
    event: RequestEvent;
    request: Request;
    fetch: typeof fetch;
}) => MaybePromise<Response>The handleFetch hook allows you to modify (or replace) a fetch request that happens inside a load function that runs on the server (or during pre-rendering)
HandleFetch = async ({ request: Requestrequest, fetch: {
    (input: RequestInfo | URL, init?: RequestInit): Promise<Response>;
    (input: string | URL | globalThis.Request, init?: RequestInit): Promise<Response>;
}fetch }) => {
	if (request: Requestrequest.Request.url: stringReturns the URL of request as a string.
MDN Reference
url.String.startsWith(searchString: string, position?: number): booleanReturns true if the sequence of elements of searchString converted to a String is the
same as the corresponding elements of this object (converted to a String) starting at
position. Otherwise returns false.
startsWith('https://api.yourapp.com/')) {
		// clone the original request, but change the URL
		request: Requestrequest = new var Request: new (input: RequestInfo | URL, init?: RequestInit) => RequestThis Fetch API interface represents a resource request.
MDN Reference
Request(
			request: Requestrequest.Request.url: stringReturns the URL of request as a string.
MDN Reference
url.String.replace(searchValue: string | RegExp, replaceValue: string): string (+3 overloads)Replaces text in a string, using a regular expression or search string.
@paramsearchValue A string or regular expression to search for.
@paramreplaceValue A string containing the text to replace. When the {@linkcode searchValue} is a RegExp, all matches are replaced if the g flag is set (or only those matches at the beginning, if the y flag is also present). Otherwise, only the first match of {@linkcode searchValue} is replaced.replace('https://api.yourapp.com/', 'http://localhost:9999/'),
			request: Requestrequest
		);
	}

	return fetch: (input: string | URL | globalThis.Request, init?: RequestInit) => Promise<Response> (+1 overload)MDN Reference
fetch(request: Requestrequest);
};

Credentials

For same-origin requests, SvelteKit’s fetch implementation will forward cookie and authorization headers unless the credentials option is set to "omit".

For cross-origin requests, cookie will be included if the request URL belongs to a subdomain of the app — for example if your app is on my-domain.com, and your API is on api.my-domain.com, cookies will be included in the request.

If your app and your API are on sibling subdomains — www.my-domain.com and api.my-domain.com for example — then a cookie belonging to a common parent domain like my-domain.com will not be included, because SvelteKit has no way to know which domain the cookie belongs to. In these cases you will need to manually include the cookie using handleFetch:

src/hooks.server

/** @type {import('@sveltejs/kit').HandleFetch} */
export async function function handleFetch({ event, request, fetch }: {
    event: any;
    request: any;
    fetch: any;
}): Promise<any>@type{import('@sveltejs/kit').HandleFetch}handleFetch({ event: anyevent, request: anyrequest, fetch: anyfetch }) {
	if (request: anyrequest.url.startsWith('https://api.my-domain.com/')) {
		request: anyrequest.headers.set('cookie', event: anyevent.request.headers.get('cookie'));
	}

	return fetch: anyfetch(request: anyrequest);
}

import type { type HandleFetch = (input: {
    event: RequestEvent;
    request: Request;
    fetch: typeof fetch;
}) => MaybePromise<Response>The handleFetch hook allows you to modify (or replace) a fetch request that happens inside a load function that runs on the server (or during pre-rendering)
HandleFetch } from '@sveltejs/kit';
export const const handleFetch: HandleFetchhandleFetch: type HandleFetch = (input: {
    event: RequestEvent;
    request: Request;
    fetch: typeof fetch;
}) => MaybePromise<Response>The handleFetch hook allows you to modify (or replace) a fetch request that happens inside a load function that runs on the server (or during pre-rendering)
HandleFetch = async ({ event: RequestEvent<Partial<Record<string, string>>, string | null>event, request: Requestrequest, fetch: {
    (input: RequestInfo | URL, init?: RequestInit): Promise<Response>;
    (input: string | URL | globalThis.Request, init?: RequestInit): Promise<Response>;
}fetch }) => {
	if (request: Requestrequest.Request.url: stringReturns the URL of request as a string.
MDN Reference
url.String.startsWith(searchString: string, position?: number): booleanReturns true if the sequence of elements of searchString converted to a String is the
same as the corresponding elements of this object (converted to a String) starting at
position. Otherwise returns false.
startsWith('https://api.my-domain.com/')) {
		request: Requestrequest.Request.headers: HeadersReturns a Headers object consisting of the headers associated with request. Note that headers added in the network layer by the user agent will not be accounted for in this object, e.g., the “Host” header.
MDN Reference
headers.Headers.set(name: string, value: string): voidMDN Reference
set('cookie', event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.request: RequestThe original request object
request.Request.headers: HeadersReturns a Headers object consisting of the headers associated with request. Note that headers added in the network layer by the user agent will not be accounted for in this object, e.g., the “Host” header.
MDN Reference
headers.Headers.get(name: string): string | nullMDN Reference
get('cookie'));
	}

	return fetch: (input: string | URL | globalThis.Request, init?: RequestInit) => Promise<Response> (+1 overload)MDN Reference
fetch(request: Requestrequest);
};

Shared hooks

The following can be added to src/hooks.server.js and src/hooks.client.js:

handleError

If an unexpected error is thrown during loading or rendering, this function will be called with the error, event, status code and message. This allows for two things:

you can log the error
you can generate a custom representation of the error that is safe to show to users, omitting sensitive details like messages and stack traces. The returned value, which defaults to { message }, becomes the value of $page.error.

For errors thrown from your code (or library code called by your code) the status will be 500 and the message will be “Internal Error”. While error.message may contain sensitive information that should not be exposed to users, message is safe (albeit meaningless to the average user).

To add more information to the $page.error object in a type-safe way, you can customize the expected shape by declaring an App.Error interface (which must include message: string, to guarantee sensible fallback behavior). This allows you to — for example — append a tracking ID for users to quote in correspondence with your technical support staff:

src/app.d

declare global {
	namespace App {
		interface interface App.ErrorDefines the common shape of expected and unexpected errors. Expected errors are thrown using the error function. Unexpected errors are handled by the handleError hooks which should return this shape.
Error {
			App.Error.message: stringmessage: string;
			App.Error.errorId: stringerrorId: string;
		}
	}
}

export {};

src/hooks.server

import * as module "@sentry/sveltekit"Sentry from '@sentry/sveltekit';

module "@sentry/sveltekit"Sentry.const init: (opts: any) => voidinit({/*...*/})

/** @type {import('@sveltejs/kit').HandleServerError} */
export async function function handleError(input: {
    error: unknown;
    event: RequestEvent;
    status: number;
    message: string;
}): MaybePromise<void | App.Error>@type{import('@sveltejs/kit').HandleServerError}handleError({ error: unknownerror, event: RequestEvent<Partial<Record<string, string>>, string | null>event, status: numberstatus, message: stringmessage }) {
	const const errorId: `${string}-${string}-${string}-${string}-${string}`errorId = var crypto: CryptoMDN Reference
crypto.Crypto.randomUUID(): `${string}-${string}-${string}-${string}-${string}`Available only in secure contexts.
MDN Reference
randomUUID();

	// example integration with https://sentry.io/
	module "@sentry/sveltekit"Sentry.const captureException: (error: any, opts: any) => voidcaptureException(error: unknownerror, {
		extra: {
    event: RequestEvent<Partial<Record<string, string>>, string | null>;
    errorId: `${string}-${string}-${string}-${string}-${string}`;
    status: number;
}extra: { event: RequestEvent<Partial<Record<string, string>>, string | null>event, errorId: `${string}-${string}-${string}-${string}-${string}`errorId, status: numberstatus }
	});

	return {
		App.Error.message: stringmessage: 'Whoops!',
		errorId: `${string}-${string}-${string}-${string}-${string}`errorId
	};
}

import * as module "@sentry/sveltekit"Sentry from '@sentry/sveltekit';
import type { type HandleServerError = (input: {
    error: unknown;
    event: RequestEvent;
    status: number;
    message: string;
}) => MaybePromise<void | App.Error>The server-side handleError hook runs when an unexpected error is thrown while responding to a request.
If an unexpected error is thrown during loading or rendering, this function will be called with the error and the event.
Make sure that this function never throws an error.
HandleServerError } from '@sveltejs/kit';

module "@sentry/sveltekit"Sentry.const init: (opts: any) => voidinit({/*...*/})

export const const handleError: HandleServerErrorhandleError: type HandleServerError = (input: {
    error: unknown;
    event: RequestEvent;
    status: number;
    message: string;
}) => MaybePromise<void | App.Error>The server-side handleError hook runs when an unexpected error is thrown while responding to a request.
If an unexpected error is thrown during loading or rendering, this function will be called with the error and the event.
Make sure that this function never throws an error.
HandleServerError = async ({ error: unknownerror, event: RequestEvent<Partial<Record<string, string>>, string | null>event, status: numberstatus, message: stringmessage }) => {
	const const errorId: `${string}-${string}-${string}-${string}-${string}`errorId = var crypto: CryptoMDN Reference
crypto.Crypto.randomUUID(): `${string}-${string}-${string}-${string}-${string}`Available only in secure contexts.
MDN Reference
randomUUID();

	// example integration with https://sentry.io/
	module "@sentry/sveltekit"Sentry.const captureException: (error: any, opts: any) => voidcaptureException(error: unknownerror, {
		extra: {
    event: RequestEvent<Partial<Record<string, string>>, string | null>;
    errorId: `${string}-${string}-${string}-${string}-${string}`;
    status: number;
}extra: { event: RequestEvent<Partial<Record<string, string>>, string | null>event, errorId: `${string}-${string}-${string}-${string}-${string}`errorId, status: numberstatus }
	});

	return {
		App.Error.message: stringmessage: 'Whoops!',
		errorId: `${string}-${string}-${string}-${string}-${string}`errorId
	};
};

src/hooks.client

import * as module "@sentry/sveltekit"Sentry from '@sentry/sveltekit';

module "@sentry/sveltekit"Sentry.const init: (opts: any) => voidinit({/*...*/})

/** @type {import('@sveltejs/kit').HandleClientError} */
export async function function handleError(input: {
    error: unknown;
    event: NavigationEvent;
    status: number;
    message: string;
}): MaybePromise<void | App.Error>@type{import('@sveltejs/kit').HandleClientError}handleError({ error: unknownerror, event: NavigationEvent<Partial<Record<string, string>>, string | null>event, status: numberstatus, message: stringmessage }) {
	const const errorId: `${string}-${string}-${string}-${string}-${string}`errorId = var crypto: CryptoMDN Reference
crypto.Crypto.randomUUID(): `${string}-${string}-${string}-${string}-${string}`Available only in secure contexts.
MDN Reference
randomUUID();

	// example integration with https://sentry.io/
	module "@sentry/sveltekit"Sentry.const captureException: (error: any, opts: any) => voidcaptureException(error: unknownerror, {
		extra: {
    event: NavigationEvent<Partial<Record<string, string>>, string | null>;
    errorId: `${string}-${string}-${string}-${string}-${string}`;
    status: number;
}extra: { event: NavigationEvent<Partial<Record<string, string>>, string | null>event, errorId: `${string}-${string}-${string}-${string}-${string}`errorId, status: numberstatus }
	});

	return {
		App.Error.message: stringmessage: 'Whoops!',
		errorId: `${string}-${string}-${string}-${string}-${string}`errorId
	};
}

import * as module "@sentry/sveltekit"Sentry from '@sentry/sveltekit';
import type { type HandleClientError = (input: {
    error: unknown;
    event: NavigationEvent;
    status: number;
    message: string;
}) => MaybePromise<void | App.Error>The client-side handleError hook runs when an unexpected error is thrown while navigating.
If an unexpected error is thrown during loading or the following render, this function will be called with the error and the event.
Make sure that this function never throws an error.
HandleClientError } from '@sveltejs/kit';

module "@sentry/sveltekit"Sentry.const init: (opts: any) => voidinit({/*...*/})

export const const handleError: HandleClientErrorhandleError: type HandleClientError = (input: {
    error: unknown;
    event: NavigationEvent;
    status: number;
    message: string;
}) => MaybePromise<void | App.Error>The client-side handleError hook runs when an unexpected error is thrown while navigating.
If an unexpected error is thrown during loading or the following render, this function will be called with the error and the event.
Make sure that this function never throws an error.
HandleClientError = async ({ error: unknownerror, event: NavigationEvent<Partial<Record<string, string>>, string | null>event, status: numberstatus, message: stringmessage }) => {
	const const errorId: `${string}-${string}-${string}-${string}-${string}`errorId = var crypto: CryptoMDN Reference
crypto.Crypto.randomUUID(): `${string}-${string}-${string}-${string}-${string}`Available only in secure contexts.
MDN Reference
randomUUID();

	// example integration with https://sentry.io/
	module "@sentry/sveltekit"Sentry.const captureException: (error: any, opts: any) => voidcaptureException(error: unknownerror, {
		extra: {
    event: NavigationEvent<Partial<Record<string, string>>, string | null>;
    errorId: `${string}-${string}-${string}-${string}-${string}`;
    status: number;
}extra: { event: NavigationEvent<Partial<Record<string, string>>, string | null>event, errorId: `${string}-${string}-${string}-${string}-${string}`errorId, status: numberstatus }
	});

	return {
		App.Error.message: stringmessage: 'Whoops!',
		errorId: `${string}-${string}-${string}-${string}-${string}`errorId
	};
};

In src/hooks.client.js, the type of handleError is HandleClientError instead of HandleServerError, and event is a NavigationEvent rather than a RequestEvent.

This function is not called for expected errors (those thrown with the error function imported from @sveltejs/kit).

During development, if an error occurs because of a syntax error in your Svelte code, the passed in error has a frame property appended highlighting the location of the error.

Make sure that handleError never throws an error

init

This function runs once, when the server is created or the app starts in the browser, and is a useful place to do asynchronous work such as initializing a database connection.

If your environment supports top-level await, the init function is really no different from writing your initialisation logic at the top level of the module, but some environments — most notably, Safari — don’t.

src/hooks.server

import * as import dbdb from '$lib/server/database';

/** @type {import('@sveltejs/kit').ServerInit} */
export async function function init(): Promise<void>@type{import('@sveltejs/kit').ServerInit}init() {
	await import dbdb.connect();
}

import * as import dbdb from '$lib/server/database';
import type { type ServerInit = () => MaybePromise<void>The init will be invoked before the server responds to its first request
ServerInit } from '@sveltejs/kit';

export const const init: ServerInitinit: type ServerInit = () => MaybePromise<void>The init will be invoked before the server responds to its first request
ServerInit = async () => {
	await import dbdb.connect();
};

In the browser, asynchronous work in init will delay hydration, so be mindful of what you put in there.

Universal hooks

The following can be added to src/hooks.js. Universal hooks run on both server and client (not to be confused with shared hooks, which are environment-specific).

reroute

This function runs before handle and allows you to change how URLs are translated into routes. The returned pathname (which defaults to url.pathname) is used to select the route and its parameters.

For example, you might have a src/routes/[[lang]]/about/+page.svelte page, which should be accessible as /en/about or /de/ueber-uns or /fr/a-propos. You could implement this with reroute:

src/hooks


/** @type {Record<string, string>} */
const const translated: {
    '/en/about': string;
    '/de/ueber-uns': string;
    '/fr/a-propos': string;
}@type{Record<string, string>}translated = {
	'/en/about': '/en/about',
	'/de/ueber-uns': '/de/about',
	'/fr/a-propos': '/fr/about',
};

/** @type {import('@sveltejs/kit').Reroute} */
export function function reroute({ url }: {
    url: any;
}): any@type{import('@sveltejs/kit').Reroute}reroute({ url: anyurl }) {
	if (url: anyurl.pathname in const translated: {
    '/en/about': string;
    '/de/ueber-uns': string;
    '/fr/a-propos': string;
}@type{Record<string, string>}translated) {
		return const translated: {
    '/en/about': string;
    '/de/ueber-uns': string;
    '/fr/a-propos': string;
}@type{Record<string, string>}translated[url: anyurl.pathname];
	}
}

import type { type Reroute = (event: {
    url: URL;
}) => void | stringThe reroute hook allows you to modify the URL before it is used to determine which route to render.
@since2.3.0Reroute } from '@sveltejs/kit';

const const translated: Record<string, string>translated: type Record<K extends keyof any, T> = { [P in K]: T; }Construct a type with a set of properties K of type T
Record<string, string> = {
	'/en/about': '/en/about',
	'/de/ueber-uns': '/de/about',
	'/fr/a-propos': '/fr/about',
};

export const const reroute: Reroutereroute: type Reroute = (event: {
    url: URL;
}) => void | stringThe reroute hook allows you to modify the URL before it is used to determine which route to render.
@since2.3.0Reroute = ({ url: URLurl }) => {
	if (url: URLurl.URL.pathname: stringMDN Reference
pathname in const translated: Record<string, string>translated) {
		return const translated: Record<string, string>translated[url: URLurl.URL.pathname: stringMDN Reference
pathname];
	}
};

The lang parameter will be correctly derived from the returned pathname.

Using reroute will not change the contents of the browser’s address bar, or the value of event.url.

Server hooks

handle

locals

handleFetch

Shared hooks

handleError

init

Universal hooks

reroute

Further reading