feat: add createContext utility for type-safe context (#16948)

* feat: add `createContext` utility for type-safe context

* regenerate

Author: Rich-Harris

.changeset/neat-melons-cheer.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: add `createContext` utility for type-safe context

documentation/docs/98-reference/.generated/shared-errors.md

@@ -60,6 +60,14 @@ Certain lifecycle methods can only be used during component initialisation. To f
 <button onclick={handleClick}>click me</button>
 ```
 
+### missing_context
+
+```
+Context was not set in a parent component
+```
+
+The [`createContext()`](svelte#createContext) utility returns a `[get, set]` pair of functions. `get` will throw an error if `set` was not used to set the context in a parent component.
+
 ### snippet_without_render_tag
 
 ```

packages/svelte/messages/shared-errors/errors.md

@@ -52,6 +52,12 @@ Certain lifecycle methods can only be used during component initialisation. To f
 <button onclick={handleClick}>click me</button>
 ```
 
+## missing_context
+
+> Context was not set in a parent component
+
+The [`createContext()`](svelte#createContext) utility returns a `[get, set]` pair of functions. `get` will throw an error if `set` was not used to set the context in a parent component.
+
 ## snippet_without_render_tag
 
 > Attempted to render a snippet without a `{@render}` block. This would cause the snippet code to be stringified instead of its content being rendered to the DOM. To fix this, change `{snippet}` to `{@render snippet()}`.

packages/svelte/src/index-client.js

@@ -242,7 +242,13 @@ function init_update_callbacks(context) {
 }
 
 export { flushSync } from './internal/client/reactivity/batch.js';
-export { getContext, getAllContexts, hasContext, setContext } from './internal/client/context.js';
+export {
+	createContext,
+	getContext,
+	getAllContexts,
+	hasContext,
+	setContext
+} from './internal/client/context.js';
 export { hydrate, mount, unmount } from './internal/client/render.js';
 export { tick, untrack, settled } from './internal/client/runtime.js';
 export { createRawSnippet } from './internal/client/dom/blocks/snippet.js';

packages/svelte/src/index-server.js

@@ -39,6 +39,12 @@ export async function settled() {}
 
 export { getAbortSignal } from './internal/server/abort-signal.js';
 
-export { getAllContexts, getContext, hasContext, setContext } from './internal/server/context.js';
+export {
+	createContext,
+	getAllContexts,
+	getContext,
+	hasContext,
+	setContext
+} from './internal/server/context.js';
 
 export { createRawSnippet } from './internal/server/blocks/snippet.js';

packages/svelte/src/internal/client/context.js

@@ -69,6 +69,26 @@ export function set_dev_current_component_function(fn) {
 	dev_current_component_function = fn;
 }
 
+/**
+ * Returns a `[get, set]` pair of functions for working with context in a type-safe way.
+ * @template T
+ * @returns {[() => T, (context: T) => T]}
+ */
+export function createContext() {
+	const key = {};
+
+	return [
+		() => {
+			if (!hasContext(key)) {
+				e.missing_context();
+			}
+
+			return getContext(key);
+		},
+		(context) => setContext(key, context)
+	];
+}
+
 /**
  * Retrieves the context that belongs to the closest parent component with the specified `key`.
  * Must be called during component initialisation.

packages/svelte/src/internal/server/context.js

@@ -10,6 +10,15 @@ export function set_ssr_context(v) {
 	ssr_context = v;
 }
 
+/**
+ * @template T
+ * @returns {[() => T, (context: T) => T]}
+ */
+export function createContext() {
+	const key = {};
+	return [() => getContext(key), (context) => setContext(key, context)];
+}
+
 /**
  * @template T
  * @param {any} key

packages/svelte/src/internal/shared/errors.js

@@ -51,6 +51,22 @@ export function lifecycle_outside_component(name) {
 	}
 }
 
+/**
+ * Context was not set in a parent component
+ * @returns {never}
+ */
+export function missing_context() {
+	if (DEV) {
+		const error = new Error(`missing_context\nContext was not set in a parent component\nhttps://svelte.dev/e/missing_context`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/missing_context`);
+	}
+}
+
 /**
  * Attempted to render a snippet without a `{@render}` block. This would cause the snippet code to be stringified instead of its content being rendered to the DOM. To fix this, change `{snippet}` to `{@render snippet()}`.
  * @returns {never}

packages/svelte/tests/runtime-runes/samples/create-context/Child.svelte

@@ -0,0 +1,7 @@
+<script>
+	import { get } from './main.svelte';
+
+	const message = get();
+</script>
+
+<h1>{message}</h1>

packages/svelte/tests/runtime-runes/samples/create-context/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<h1>hello</h1>`
+});