@json-render/svelte

Svelte 5 renderer that converts json-render specs into Svelte component trees.

Quick Start

<script lang="ts">
  import { Renderer, JsonUIProvider } from "@json-render/svelte";
  import type { Spec } from "@json-render/svelte";
  import Card from "./components/Card.svelte";
  import Button from "./components/Button.svelte";

  interface Props {
    spec: Spec | null;
  }

  let { spec }: Props = $props();
  const registry = { Card, Button };
</script>

<JsonUIProvider>
  <Renderer {spec} {registry} />
</JsonUIProvider>

Creating a Catalog

import { defineCatalog } from "@json-render/core";
import { schema } from "@json-render/svelte";
import { z } from "zod";

export const catalog = defineCatalog(schema, {
  components: {
    Button: {
      props: z.object({
        label: z.string(),
        variant: z.enum(["primary", "secondary"]).nullable(),
      }),
      description: "Clickable button",
    },
    Card: {
      props: z.object({ title: z.string() }),
      description: "Card container with title",
    },
  },
});

Defining Components

Components should accept BaseComponentProps<TProps>:

interface BaseComponentProps<TProps> {
  props: TProps; // Resolved props for this component
  children?: Snippet; // Child elements (use {@render children()})
  emit: (event: string) => void; // Fire a named event
  bindings?: Record<string, string>; // Map of prop names to state paths (for $bindState)
  loading?: boolean; // True while spec is streaming
}

<!-- Button.svelte -->
<script lang="ts">
  import type { BaseComponentProps } from "@json-render/svelte";

  interface Props extends BaseComponentProps<{ label: string; variant?: string }> {}
  let { props, emit }: Props = $props();
</script>

<button class={props.variant} onclick={() => emit("press")}>
  {props.label}
</button>

<!-- Card.svelte -->
<script lang="ts">
  import type { Snippet } from "svelte";
  import type { BaseComponentProps } from "@json-render/svelte";

  interface Props extends BaseComponentProps<{ title: string }> {
    children?: Snippet;
  }

  let { props, children }: Props = $props();
</script>

<div class="card">
  <h2>{props.title}</h2>
  {#if children}
    {@render children()}
  {/if}
</div>

Creating a Registry

import { defineRegistry } from "@json-render/svelte";
import { catalog } from "./catalog";
import Card from "./components/Card.svelte";
import Button from "./components/Button.svelte";

const { registry, handlers, executeAction } = defineRegistry(catalog, {
  components: {
    Card,
    Button,
  },
  actions: {
    submit: async (params, setState, state) => {
      // handle action
    },
  },
});

Spec Structure (Element Tree)

The Svelte schema uses the element tree format:

{
  "root": "card1",
  "elements": {
    "card1": {
      "type": "Card",
      "props": { "title": "Hello" },
      "children": ["btn1"]
    },
    "btn1": {
      "type": "Button",
      "props": { "label": "Click me" }
    }
  }
}

Visibility Conditions

Use visible on elements to show/hide based on state:

• { "$state": "/path" } - truthy check
• { "$state": "/path", "eq": value } - equality check
• { "$state": "/path", "not": true } - falsy check
• { "$and": [cond1, cond2] } - AND conditions
• { "$or": [cond1, cond2] } - OR conditions

Providers (via JsonUIProvider)

JsonUIProvider composes all contexts. Individual contexts:

Event System

Components use emit to fire named events. The element's on field maps events to action bindings:

<!-- Button.svelte -->
<script lang="ts">
  import type { BaseComponentProps } from "@json-render/svelte";

  interface Props extends BaseComponentProps<{ label: string }> {}

  let { props, emit }: Props = $props();
</script>

<button onclick={() => emit("press")}>{props.label}</button>

{
  "type": "Button",
  "props": { "label": "Submit" },
  "on": { "press": { "action": "submit" } }
}

Built-in Actions

The setState action is handled automatically and updates the state model:

{
  "action": "setState",
  "actionParams": { "statePath": "/activeTab", "value": "home" }
}

Other built-in actions: pushState, removeState, push, pop.

Dynamic Props and Two-Way Binding

Nested lists can set repeat.statePath to { "$item": "field" } inside an enclosing repeat.

Expression forms resolved before your component receives props:

• {"$state": "/state/key"} - read from state
• {"$bindState": "/form/email"} - read + write-back to state
• {"$bindItem": "field"} - read + write-back for repeat items
• {"$cond": <condition>, "$then": <value>, "$else": <value>} - conditional value

For writable bindings inside components, use getBoundProp:

<script lang="ts">
  import { getBoundProp } from "@json-render/svelte";
  import type { BaseComponentProps } from "@json-render/svelte";

  interface Props extends BaseComponentProps<{ value?: string }> {}
  let { props, bindings }: Props = $props();

  let value = getBoundProp<string>(
    () => props.value,
    () => bindings?.value,
  );
</script>

<input bind:value={value.current} />

Context Helpers

Preferred helpers:

• getStateValue(path) - returns { current } (read/write)
• getBoundProp(() => value, () => bindingPath) - returns { current } (read/write when bound)
• isVisible(condition) - returns { current } (boolean)
• getAction(name) - returns { current } (registered handler)

Advanced context access:

• getStateContext()
• getActionContext()
• getVisibilityContext()
• getValidationContext()
• getOptionalValidationContext()
• getFieldValidation(ctx, path, config?)

Streaming UI

Use createUIStream for spec streaming:

<script lang="ts">
  import { createUIStream, Renderer } from "@json-render/svelte";

  const stream = createUIStream({
    api: "/api/generate-ui",
    onComplete: (spec) => console.log("Done", spec),
  });

  async function generate() {
    await stream.send("Create a login form");
  }
</script>

<button onclick={generate} disabled={stream.isStreaming}>
  {stream.isStreaming ? "Generating..." : "Generate UI"}
</button>

{#if stream.spec}
  <Renderer spec={stream.spec} {registry} loading={stream.isStreaming} />
{/if}

Use createChatUI for chat + UI responses:

const chat = createChatUI({ api: "/api/chat-ui" });
await chat.send("Build a settings panel");