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SvelteKitAdvanced

Hooks

‘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) => Response
This 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: URL
The requested URL.


url.URL.pathname: string
MDN Reference


pathname.String.startsWith(searchString: string, position?: number): boolean
Returns 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) => Response
This 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).

During prerendering, SvelteKit crawls your pages for links and renders each route it finds. Rendering the route invokes the handle function (and all other route dependencies, like load). If you need to exclude some code from running during this phase, check that the app is not building beforehand.

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.Locals
Contains 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: Cookies
Get or set cookies related to the current request


cookies.Cookies.get: (name: string, opts?: CookieParseOptions) => string | undefined
Gets 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 here
get('sessionid'));

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

	// Note that modifying response headers isn't always safe.
	// Response objects can have immutable headers
	// (e.g. Response.redirect() returned from an endpoint).
	// Modifying immutable headers throws a TypeError.
	// In that case, clone the response or avoid creating a
	// response object with immutable headers.
	const response: Responseresponse.Response.headers: Headers
MDN Reference


headers.Headers.set(name: string, value: string): void
MDN 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.Locals
Contains 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: Cookies
Get or set cookies related to the current request


cookies.Cookies.get: (name: string, opts?: CookieParseOptions) => string | undefined
Gets 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 here
get('sessionid'));

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

	// Note that modifying response headers isn't always safe.
	// Response objects can have immutable headers
	// (e.g. Response.redirect() returned from an endpoint).
	// Modifying immutable headers throws a TypeError.
	// In that case, clone the response or avoid creating a
	// response object with immutable headers.
	const response: Responseresponse.Response.headers: Headers
MDN Reference


headers.Headers.set(name: string, value: string): void
MDN 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;
}) => any
transformPageChunk: ({ html: anyhtml }) => html: anyhtml.replace('old', 'new'),
		filterSerializedResponseHeaders: (name: any) => anyfilterSerializedResponseHeaders: (name: anyname) => name: anyname.startsWith('x-'),
		
preload: ({ type, path }: {
    type: any;
    path: any;
}) => any
preload: ({ 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>) | 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 chunk
transformPageChunk: ({ 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) => boolean) | undefined
Determines 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 value
filterSerializedResponseHeaders: (name: stringname) => name: stringname.String.startsWith(searchString: string, position?: number): boolean
Returns 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;
}) => boolean) | undefined
Determines 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 path
preload: ({ type: "font" | "css" | "js" | "asset"type, path: stringpath }) => type: "font" | "css" | "js" | "asset"type === 'js' || path: stringpath.String.includes(searchString: string, position?: number): boolean
Returns 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) the result of an event.fetch call that runs on the server (or during prerendering) inside an endpoint, load, action, handle, handleError or reroute.

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) => Request
This 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) the result of an event.fetch call that runs on the server (or during prerendering) inside an endpoint, load, action, handle, handleError or reroute.


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) the result of an event.fetch call that runs on the server (or during prerendering) inside an endpoint, load, action, handle, handleError or reroute.


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: string
Returns the URL of request as a string.


MDN Reference


url.String.startsWith(searchString: string, position?: number): boolean
Returns 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) => Request
This Fetch API interface represents a resource request.


MDN Reference


Request(
			request: Requestrequest.Request.url: string
Returns 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);
};

Requests made with event.fetch follow the browser’s credentials model — for same-origin requests, cookie and authorization headers are forwarded 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.

There is one caveat: 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) the result of an event.fetch call that runs on the server (or during prerendering) inside an endpoint, load, action, handle, handleError or reroute.


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) the result of an event.fetch call that runs on the server (or during prerendering) inside an endpoint, load, action, handle, handleError or reroute.


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: string
Returns the URL of request as a string.


MDN Reference


url.String.startsWith(searchString: string, position?: number): boolean
Returns 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: Headers
Returns 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): void
MDN Reference


set('cookie', event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.request: Request
The original request object.


request.Request.headers: Headers
Returns 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 | null
MDN 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, rendering, or from an endpoint, 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.Error
Defines 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: Crypto
MDN 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
	};
}
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: Crypto
MDN 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: Crypto
MDN 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
	};
}
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: Crypto
MDN 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


@since2.10.0
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


@since2.10.0
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;
    fetch: typeof fetch;
}) => MaybePromise<string | void>
The reroute hook allows you to modify the URL before it is used to determine which route to render.


@since2.3.0
Reroute } 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;
    fetch: typeof fetch;
}) => MaybePromise<string | void>
The reroute hook allows you to modify the URL before it is used to determine which route to render.


@since2.3.0
Reroute = ({ url: URLurl }) => {
	if (url: URLurl.URL.pathname: string
MDN Reference


pathname in const translated: Record<string, string>translated) {
		return const translated: Record<string, string>translated[url: URLurl.URL.pathname: string
MDN 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.

Since version 2.18, the reroute hook can be asynchronous, allowing it to (for example) fetch data from your backend to decide where to reroute to. Use this carefully and make sure it’s fast, as it will delay navigation otherwise. If you need to fetch data, use the fetch provided as an argument. It has the same benefits as the fetch provided to load functions, with the caveat that params and id are unavailable to handleFetch because the route is not yet known.

src/hooks

/** @type {import('@sveltejs/kit').Reroute} */
export async function 
function reroute({ url, fetch }: {
    url: any;
    fetch: any;
}): Promise<any>
@type{import('@sveltejs/kit').Reroute}
reroute({ url: anyurl, fetch: anyfetch }) {
	// Ask a special endpoint within your app about the destination
	if (url: anyurl.pathname === '/api/reroute') return;

	const const api: URLapi = new var URL: new (url: string | URL, base?: string | URL) => URL
The URL interface represents an object providing static methods used for creating object URLs.


MDN Reference


URL class is a global reference for import { URL } from 'node:url'
https://nodejs.org/api/url.html#the-whatwg-url-api


@sincev10.0.0
URL('/api/reroute', url: anyurl);
	const api: URLapi.URL.searchParams: URLSearchParams
MDN Reference


searchParams.URLSearchParams.set(name: string, value: string): void
Sets the value associated to a given search parameter to the given value. If there were several values, delete the others.


MDN Reference


set('pathname', url: anyurl.pathname);

	const const result: anyresult = await fetch: anyfetch(const api: URLapi).then(r: anyr => r: anyr.json());
	return const result: anyresult.pathname;
}
import type { 
type Reroute = (event: {
    url: URL;
    fetch: typeof fetch;
}) => MaybePromise<string | void>
The reroute hook allows you to modify the URL before it is used to determine which route to render.


@since2.3.0
Reroute } from '@sveltejs/kit';
export const const reroute: Reroutereroute: 
type Reroute = (event: {
    url: URL;
    fetch: typeof fetch;
}) => MaybePromise<string | void>
The reroute hook allows you to modify the URL before it is used to determine which route to render.


@since2.3.0
Reroute = async ({ url: URLurl, 
fetch: {
    (input: RequestInfo | URL, init?: RequestInit): Promise<Response>;
    (input: string | URL | globalThis.Request, init?: RequestInit): Promise<Response>;
}
fetch }) => {
	// Ask a special endpoint within your app about the destination
	if (url: URLurl.URL.pathname: string
MDN Reference


pathname === '/api/reroute') return;

	const const api: URLapi = new var URL: new (url: string | URL, base?: string | URL) => URL
The URL interface represents an object providing static methods used for creating object URLs.


MDN Reference


URL class is a global reference for import { URL } from 'node:url'
https://nodejs.org/api/url.html#the-whatwg-url-api


@sincev10.0.0
URL('/api/reroute', url: URLurl);
	const api: URLapi.URL.searchParams: URLSearchParams
MDN Reference


searchParams.URLSearchParams.set(name: string, value: string): void
Sets the value associated to a given search parameter to the given value. If there were several values, delete the others.


MDN Reference


set('pathname', url: URLurl.URL.pathname: string
MDN Reference


pathname);

	const const result: anyresult = await fetch: (input: string | URL | globalThis.Request, init?: RequestInit) => Promise<Response> (+1 overload)
MDN Reference


fetch(const api: URLapi).Promise<Response>.then<any, never>(onfulfilled?: ((value: Response) => any) | null | undefined, onrejected?: ((reason: any) => PromiseLike<never>) | null | undefined): Promise<any>
Attaches callbacks for the resolution and/or rejection of the Promise.


@paramonfulfilled The callback to execute when the Promise is resolved.
@paramonrejected The callback to execute when the Promise is rejected.
@returnsA Promise for the completion of which ever callback is executed.
then(r: Responser => r: Responser.Body.json(): Promise<any>
MDN Reference


json());
	return const result: anyresult.pathname;
};

reroute is considered a pure, idempotent function. As such, it must always return the same output for the same input and not have side effects. Under these assumptions, SvelteKit caches the result of reroute on the client so it is only called once per unique URL.

transport

This is a collection of transporters, which allow you to pass custom types — returned from load and form actions — across the server/client boundary. Each transporter contains an encode function, which encodes values on the server (or returns a falsy value for anything that isn’t an instance of the type) and a corresponding decode function:

src/hooks
import { import VectorVector } from '$lib/math';

/** @type {import('@sveltejs/kit').Transport} */
export const 
const transport: {
    Vector: {
        encode: (value: any) => false | any[];
        decode: ([x, y]: [any, any]) => any;
    };
}
@type{import('@sveltejs/kit').Transport}
transport = {
	
type Vector: {
    encode: (value: any) => false | any[];
    decode: ([x, y]: [any, any]) => any;
}
Vector: {
		encode: (value: any) => false | any[]encode: (value: anyvalue) => value: anyvalue instanceof import VectorVector && [value: anyvalue.x, value: anyvalue.y],
		decode: ([x, y]: [any, any]) => anydecode: ([x: anyx, y: anyy]) => new import VectorVector(x: anyx, y: anyy)
	}
};
import { import VectorVector } from '$lib/math';
import type { 
type Transport = {
    [x: string]: Transporter<any, any>;
}
The transport hook allows you to transport custom types across the server/client boundary.


Each transporter has a pair of encode and decode functions. On the server, encode determines whether a value is an instance of the custom type and, if so, returns a non-falsy encoding of the value which can be an object or an array (or false otherwise).


In the browser, decode turns the encoding back into an instance of the custom type.


import type { Transport } from '@sveltejs/kit';

declare class MyCustomType {
	data: any
}

// hooks.js
export const transport: Transport = {
	MyCustomType: {
		encode: (value) => value instanceof MyCustomType &#x26;&#x26; [value.data],
		decode: ([data]) => new MyCustomType(data)
	}
};
@since2.11.0
Transport } from '@sveltejs/kit';

export const const transport: Transporttransport: 
type Transport = {
    [x: string]: Transporter<any, any>;
}
The transport hook allows you to transport custom types across the server/client boundary.


Each transporter has a pair of encode and decode functions. On the server, encode determines whether a value is an instance of the custom type and, if so, returns a non-falsy encoding of the value which can be an object or an array (or false otherwise).


In the browser, decode turns the encoding back into an instance of the custom type.


import type { Transport } from '@sveltejs/kit';

declare class MyCustomType {
	data: any
}

// hooks.js
export const transport: Transport = {
	MyCustomType: {
		encode: (value) => value instanceof MyCustomType &#x26;&#x26; [value.data],
		decode: ([data]) => new MyCustomType(data)
	}
};
@since2.11.0
Transport = {
	
type Vector: {
    encode: (value: any) => false | any[];
    decode: ([x, y]: any) => any;
}
Vector: {
		Transporter<any, any>.encode: (value: any) => anyencode: (value: anyvalue) => value: anyvalue instanceof import VectorVector && [value: anyvalue.x, value: anyvalue.y],
		Transporter<any, any>.decode: (data: any) => anydecode: ([x: anyx, y: anyy]) => new import VectorVector(x: anyx, y: anyy)
	}
};

Further reading

Edit this page on GitHub llms.txt

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