feat: allow use of createContext when instantiating components programmatically (sveltejs#17575)

* feat: allow use of createContext when instantiating components programmatically

* docs

Author: Rich-Harris

.changeset/orange-ants-greet.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: allow use of createContext when instantiating components programmatically

documentation/docs/06-runtime/02-context.md

@@ -97,6 +97,32 @@ import { createContext } from 'svelte';
 export const [getUserContext, setUserContext] = createContext<User>();
 ```
 
+When writing [component tests](testing#Unit-and-component-tests-with-Vitest-Component-testing), it can be useful to create a wrapper component that sets the context in order to check the behaviour of a component that uses it. As of version 5.49, you can do this sort of thing:
+
+```js
+import { mount, unmount } from 'svelte';
+import { expect, test } from 'vitest';
+import { setUserContext } from './context';
+import MyComponent from './MyComponent.svelte';
+
+test('MyComponent', () => {
+	function Wrapper(...args) {
+		setUserContext({ name: 'Bob' });
+		return MyComponent(...args);
+	}
+
+	const component = mount(Wrapper, {
+		target: document.body
+	});
+
+	expect(document.body.innerHTML).toBe('<h1>Hello Bob!</h1>');
+
+	unmount(component);
+});
+```
+
+This approach also works with [`hydrate`](imperative-component-api#hydrate) and [`render`](imperative-component-api#render).
+
 ## Replacing global state
 
 When you have state shared by many different components, you might be tempted to put it in its own module and just import it wherever it's needed:

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

@@ -208,11 +208,9 @@ function _mount(Component, { target, anchor, props = {}, events, context, intro
 				pending: () => {}
 			},
 			(anchor_node) => {
-				if (context) {
-					push({});
-					var ctx = /** @type {ComponentContext} */ (component_context);
-					ctx.c = context;
-				}
+				push({});
+				var ctx = /** @type {ComponentContext} */ (component_context);
+				if (context) ctx.c = context;
 
 				if (events) {
 					// We can't spread the object or else we'd lose the state proxy stuff, if it is one
@@ -241,9 +239,7 @@ function _mount(Component, { target, anchor, props = {}, events, context, intro
 					}
 				}
 
-				if (context) {
-					pop();
-				}
+				pop();
 			}
 		);
 

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

@@ -617,18 +617,14 @@ export class Renderer {
 
 		renderer.push(BLOCK_OPEN);
 
-		if (options.context) {
-			push();
-			/** @type {SSRContext} */ (ssr_context).c = options.context;
-			/** @type {SSRContext} */ (ssr_context).r = renderer;
-		}
+		push();
+		if (options.context) /** @type {SSRContext} */ (ssr_context).c = options.context;
+		/** @type {SSRContext} */ (ssr_context).r = renderer;
 
 		// @ts-expect-error
 		component(renderer, options.props ?? {});
 
-		if (options.context) {
-			pop();
-		}
+		pop();
 
 		renderer.push(BLOCK_CLOSE);
 

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

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	ssrHtml: `<div></div>`,
+	html: `<div><h1>hello</h1></div>`,
+
+	test() {}
+});

packages/svelte/tests/runtime-runes/samples/create-context-programmatic/main.svelte

@@ -0,0 +1,20 @@
+<script module>
+	import { createContext, mount } from 'svelte';
+	import Child from './Child.svelte';
+
+	/** @type {ReturnType<typeof createContext<string>>} */
+	const [get, set] = createContext();
+
+	export { get };
+
+	function Wrapper(Component) {
+		return (...args) => {
+			set('hello');
+			return Component(...args);
+		};
+	}
+</script>
+
+<div {@attach (target) => {
+	mount(Wrapper(Child), { target });
+}}></div>