Queries, Mutations & Actions

This page covers the core reactive API of bijection-svelte. Everything here works in any Svelte app — SvelteKit, Vite + Svelte, or any other setup. Make sure you’ve called setupBijection() in a root layout first — see Overview.

Two clients

Bijection Svelte talks to your backend through two clients, and the one you use decides what’s available:

	`BijectionClient` (WebSocket)	`BijectionHttpClient` (`fetch`)
Queries	`useQuery()`	`.query()`
Mutations	`useMutation()` — optimistic updates	`.mutation()`
Actions	`useAction()`	`.action()`
Live subscriptions	✓	-

BijectionClient is the live WebSocket client that setupBijection() opens — the useQuery / useMutation / useAction helpers are thin Svelte wrappers around it, and you can retrieve it directly with getBijectionClient() / useBijectionClient(). BijectionHttpClient is a stateless fetch-based client for single calls from server code or scripts — see One-time calls. Live subscriptions and optimistic updates are WebSocket-only, and optimistic updates apply to mutations only.

Queries

Use useQuery() to subscribe to a Bijection query with automatic real-time updates. When the data changes on the server, your component re-renders automatically.

<script lang="ts">
  import { useQuery } from "bijection-svelte";
  import { api } from "../bijection/_generated/api.js";

  const messages = useQuery(api.messages.list, () => ({ searchWords }), {
    keepPreviousData: true,
  });
</script>

{#if messages.isLoading}
  Loading...
{:else if messages.error != null}
  failed to load: {messages.error.toString()}
{:else}
  <ul>
    {#each messages.data as message}
      <li>
        <span>{message.author}</span>
        <span>{message.body}</span>
      </li>
    {/each}
  </ul>
{/if}

The returned object is reactive and has the following shape:

Property	Type	Description
`data`	`T \| undefined`	The query result, or `undefined` while loading
`error`	`Error \| undefined`	The error, if the query failed
`isLoading`	`boolean`	`true` until the first result or error is received
`isStale`	`boolean`	`true` when displaying cached data from previous arguments

Options

initialData — pre-loaded data for SSR/hydration, avoids the loading state (see SSR with initialData)
keepPreviousData — when true, keeps displaying the previous result while new data loads after args change

Skipping queries

You can conditionally skip a query by returning 'skip' from the arguments function. This is useful when a query depends on some condition, like authentication state or user input.

<script lang="ts">
  import { useQuery } from "bijection-svelte";
  import { api } from "../bijection/_generated/api.js";

  let auth = $state({ isAuthenticated: true });

  const user = useQuery(api.users.queries.getActiveUser, () =>
    auth.isAuthenticated ? {} : "skip",
  );
</script>

{#if user.isLoading}
  Loading user...
{:else if user.error}
  Error: {user.error}
{:else if user.data}
  Welcome, {user.data.name}!
{/if}

When a query is skipped, isLoading will be false, error will be null, and data will be undefined.

Mutations & Actions

Use useMutation() and useAction() to get callable functions for your Bijection mutations and actions. Both use the module-level singleton (getBijectionClient()) internally, so they work in .svelte components and plain .ts / .js files — anywhere after setupBijection() has been called.

<script lang="ts">
  import { useMutation } from "bijection-svelte";
  import { api } from "../bijection/_generated/api.js";

  const sendMessage = useMutation(api.messages.send);

  let toSend = $state("");
  let author = $state("me");

  function handleSubmit(event: SubmitEvent) {
    event.preventDefault();

    const data = Object.fromEntries(
      new FormData(event.target as HTMLFormElement).entries(),
    );
    sendMessage({
      author: data.author as string,
      body: data.body as string,
    });
  }
</script>

<form onsubmit={handleSubmit}>
  <input type="text" name="author" bind:value={author} />
  <input type="text" name="body" bind:value={toSend} />
  <button type="submit" disabled={!toSend}>Send</button>
</form>

Actions are similar to mutations but can have side effects like calling third-party APIs:

import { useAction } from "bijection-svelte";
import { api } from "../bijection/_generated/api.js";

const generateUploadUrl = useAction(api.files.generateUploadUrl);
const uploadUrl = await generateUploadUrl({});

Optimistic updates

Optimistic updates let you update the UI immediately when a mutation is called, without waiting for the server to respond. Pass an optimisticUpdate callback in the mutation options at the call site to update the local query cache.

<script lang="ts">
  import { useMutation } from "bijection-svelte";
  import { api } from "../bijection/_generated/api.js";

  const updateUser = useMutation(api.user.update);

  async function handleUpdate() {
    await updateUser(
      { name: "John Doe" },
      {
        optimisticUpdate: (store) => {
          store.setQuery(api.user.get, {}, { name: "John Doe" });
        },
      },
    );
  }
</script>

Inside the optimisticUpdate callback, use store.setQuery() to update the local cache for a specific query. The arguments are:

Query reference — the query to update (e.g. api.user.get)
Query arguments — must match the arguments used by the active useQuery() subscription
New value — the optimistic data to display immediately

If the mutation fails, the optimistic update is automatically rolled back and the UI reverts to the server state.

Client access

`getBijectionClient()` — universal client access

getBijectionClient() retrieves the client from a module-level singleton. It works anywhere — .svelte components, plain .ts utility files, service layers, async callbacks — as long as setupBijection() has been called first. This is the recommended way to access the client outside of the layout where setupBijection() returns it directly.

`useBijectionClient()` — Svelte context alternative

useBijectionClient() retrieves the same client from Svelte context via getContext(). It only works during component initialization — inside .svelte files or code called synchronously from a component’s <script> block. Both functions return the same BijectionClient instance.

	`getBijectionClient()`	`useBijectionClient()`
Works in	Anywhere (`.ts`, `.svelte`, hooks)	Svelte components only
Mechanism	Module singleton	Svelte `getContext()`

Using mutations in utility files

useMutation() and useAction() work in plain .ts files too, since they use the module-level singleton:

src/lib/services/tasks.ts

import { useMutation } from "bijection-svelte";
import { api } from "../bijection/_generated/api.js";

const createTaskMutation = useMutation(api.tasks.create);
const completeTaskMutation = useMutation(api.tasks.complete);

export async function createTask(text: string) {
  await createTaskMutation({ text });
}

export async function completeTask(id: string) {
  await completeTaskMutation({ id });
}

Then call these functions from any component without plumbing the client through:

<script lang="ts">
  import { createTask } from "$lib/services/tasks.js";

  let text = $state("");
</script>

<form
  onsubmit={(e) => {
    e.preventDefault();
    createTask(text);
    text = "";
  }}
>
  <input bind:value={text} />
  <button type="submit">Add</button>
</form>

The .svelte.ts file extension enables Svelte 5 runes ($state, $derived, $effect) but does not make getContext() work outside components. If you need the client in a plain .ts file, use getBijectionClient(), not useBijectionClient().

One-time calls

useQuery keeps a live subscription open. When you instead need a single result with no ongoing binding — a SvelteKit load function, a form action, an endpoint, or a one-off script — use the stateless BijectionHttpClient, which runs query / mutation / action over fetch with no WebSocket.

import { BijectionHttpClient } from "bijection/browser";
import { api } from "../bijection/_generated/api.js";

const client = new BijectionHttpClient(process.env.BIJECTION_URL!);
const count = await client.query(api.tasks.count, {});

In SvelteKit, the createBijectionHttpClient() helper builds one with per-request auth wired in. The HTTP client supports one-shot calls only — no subscriptions and no optimistic updates. Mutations and actions have no “live” form to opt out of: useMutation() and useAction() are already one-shot calls (thin wrappers over the WebSocket client), so they cover writes in both reactive and non-reactive code.

Paginated queries

For queries that return large datasets, use usePaginatedQuery() to load results incrementally. This hook manages cursor-based pagination automatically and provides a loadMore function to fetch additional pages.

<script lang="ts">
  import { usePaginatedQuery } from "bijection-svelte";
  import { api } from "../bijection/_generated/api.js";

  const paginatedMessages = usePaginatedQuery(
    api.messages.listPaginated,
    () => ({}),
    {
      initialNumItems: 10,
    },
  );
</script>

{#if paginatedMessages.isLoading}
  Loading...
{:else if paginatedMessages.error}
  Error: {paginatedMessages.error.toString()}
{:else}
  <ul>
    {#each paginatedMessages.results as message}
      <li>
        <span>{message.author}</span>
        <span>{message.body}</span>
      </li>
    {/each}
  </ul>
  {#if paginatedMessages.status === "CanLoadMore"}
    <button onclick={() => paginatedMessages.loadMore(10)}>Load more</button>
  {/if}
{/if}

Options

initialNumItems (required) — number of items to load on the first page
initialData — optional initial data for SSR/hydration
keepPreviousData — when true, keeps previous results visible while loading new data after args change

You can also skip a paginated query by returning 'skip' from the arguments function, just like with useQuery().

<script lang="ts">
  import { usePaginatedQuery } from "bijection-svelte";
  import { api } from "../bijection/_generated/api.js";

  let searchTerm = $state("");

  const searchResults = usePaginatedQuery(
    api.messages.search,
    () => (searchTerm.length > 0 ? { query: searchTerm } : "skip"),
    { initialNumItems: 20, keepPreviousData: true },
  );
</script>

API reference

Functions and types exported from bijection-svelte:

Export	Kind	Description
`setupBijection(url, options?)`	Function	Initialize the Bijection client and store it in Svelte context. Call once in a root layout. Returns `BijectionClient`.
`useBijectionClient()`	Function	Retrieve the `BijectionClient` from Svelte context. Must be called during component initialization.
`getBijectionClient()`	Function	Retrieve the `BijectionClient` module singleton. Works anywhere — no Svelte context needed.
`closeBijection()`	Function	Close the app-scoped client and clear the singleton. For explicit teardown, e.g. in tests. Returns `Promise`.
`useQuery(query, args, options?)`	Function	Subscribe to a Bijection query with reactive updates. Returns `UseQueryReturn`.
`UseQueryOptions<Query>`	Type	Options for `useQuery`: `initialData`, `keepPreviousData`.
`UseQueryReturn<Query>`	Type	Return type of `useQuery`: `data`, `error`, `isLoading`, `isStale`.
`usePaginatedQuery(query, args, options)`	Function	Subscribe to a paginated Bijection query with cursor management. Returns `UsePaginatedQueryReturn`.
`UsePaginatedQueryOptions<Query>`	Type	Options for `usePaginatedQuery`: `initialNumItems`, `initialData`, `keepPreviousData`.
`UsePaginatedQueryReturn<Query>`	Type	Return type of `usePaginatedQuery`: `results`, `status`, `isLoading`, `loadMore`, `error`.

Authentication exports (setupAuth, useAuth, and related types) are documented on the Authentication page.

Get Started

Platform

Data and Operations

Guides

Client Libraries

Tools

API Reference

Queries, Mutations & Actions

Two clients

Queries

Options

Skipping queries

Mutations & Actions

Optimistic updates

Client access

`getBijectionClient()` — universal client access

`useBijectionClient()` — Svelte context alternative

Using mutations in utility files

One-time calls

Paginated queries

Options

API reference

​Two clients

​Queries

​Options

​Skipping queries

​Mutations & Actions

​Optimistic updates

​Client access

​getBijectionClient() — universal client access

​useBijectionClient() — Svelte context alternative

​Using mutations in utility files

​One-time calls

​Paginated queries

​Options

​API reference

Two clients

Queries

Options

Skipping queries

Mutations & Actions

Optimistic updates

Client access

`getBijectionClient()` — universal client access

`useBijectionClient()` — Svelte context alternative

Using mutations in utility files

One-time calls

Paginated queries

Options

API reference