fix!: Rerender to only update props of component without destroying it

src/__tests__/fixtures/Rerender.svelte

@@ -0,0 +1,15 @@
+<script>
+  import { getContext, onMount } from 'svelte'
+
+  export let name
+
+  const mountCounter = getContext('mountCounter')
+
+  onMount(() => {
+    mountCounter.update((i) => i + 1)
+  })
+</script>
+
+<h1 data-testid="test">Hello {name}!</h1>
+
+<div data-testid="mount-counter">{$mountCounter}</div>

src/__tests__/rerender.test.js

@@ -1,38 +1,38 @@
 /**
  * @jest-environment jsdom
  */
-import { describe, expect, test } from 'vitest'
+import { expect, test, vi } from 'vitest'
+import { writable } from 'svelte/store'
 
-import { render } from '..'
-import Comp from './fixtures/Comp.svelte'
+import { render, waitFor } from '..'
+import Comp from './fixtures/Rerender.svelte'
 
-describe('rerender', () => {
-  test('mounts new component successfully', () => {
-    const { container, rerender } = render(Comp, { props: { name: 'World 1' } })
+const mountCounter = writable(0)
 
-    expect(container.firstChild).toHaveTextContent('Hello World 1!')
-    rerender({ props: { name: 'World 2' } })
-    expect(container.firstChild).toHaveTextContent('Hello World 2!')
+test('mounts new component successfully', async () => {
+  const { getByTestId, rerender } = render(Comp, {
+    props: { name: 'World 1' },
+    context: new Map(Object.entries({ mountCounter })),
   })
 
-  test('destroys old component', () => {
-    let isDestroyed
+  const expectToRender = (content) =>
+    waitFor(() => {
+      expect(getByTestId('test')).toHaveTextContent(content)
+      expect(getByTestId('mount-counter')).toHaveTextContent('1')
+    })
 
-    const { rerender, component } = render(Comp, { props: { name: '' } })
+  await expectToRender('Hello World 1!')
 
-    component.$$.on_destroy.push(() => {
-      isDestroyed = true
-    })
-    rerender({ props: { name: '' } })
-    expect(isDestroyed).toBeTruthy()
-  })
+  console.warn = vi.fn()
 
-  test('destroys old components on multiple rerenders', () => {
-    const { rerender, queryByText } = render(Comp, { props: { name: 'Neil' } })
+  rerender({ props: { name: 'World 2' } })
+  await expectToRender('Hello World 2!')
 
-    rerender({ props: { name: 'Alex' } })
-    expect(queryByText('Hello Neil!')).not.toBeInTheDocument()
-    rerender({ props: { name: 'Geddy' } })
-    expect(queryByText('Hello Alex!')).not.toBeInTheDocument()
-  })
+  expect(console.warn).toHaveBeenCalled()
+
+  console.warn.mockClear()
+  rerender({ name: 'World 3' })
+  await expectToRender('Hello World 3!')
+
+  expect(console.warn).not.toHaveBeenCalled()
 })

src/pure.js

@@ -1,7 +1,7 @@
 import {
   fireEvent as dtlFireEvent,
   getQueriesForElement,
-  prettyDOM
+  prettyDOM,
 } from '@testing-library/dom'
 import { tick } from 'svelte'
 
@@ -14,7 +14,7 @@ const svelteComponentOptions = [
   'props',
   'hydrate',
   'intro',
-  'context'
+  'context',
 ]
 
 const render = (
@@ -56,7 +56,7 @@ const render = (
 
   let component = new ComponentConstructor({
     target,
-    ...checkProps(options)
+    ...checkProps(options),
   })
 
   containerCache.add({ container, target, component })
@@ -70,26 +70,20 @@ const render = (
     container,
     component,
     debug: (el = container) => console.log(prettyDOM(el)),
-    rerender: (options) => {
-      if (componentCache.has(component)) component.$destroy()
-
-      // eslint-disable-next-line no-new
-      component = new ComponentConstructor({
-        target,
-        ...checkProps(options)
-      })
-
-      containerCache.add({ container, target, component })
-      componentCache.add(component)
-
-      component.$$.on_destroy.push(() => {
-        componentCache.delete(component)
-      })
+    rerender: async (props) => {
+      if (props.props) {
+        console.warn(
+          'rerender({ props: {...} }) deprecated, use rerender({...}) instead'
+        )
+        props = props.props
+      }
+      component.$set(props)
+      await tick()
     },
     unmount: () => {
       if (componentCache.has(component)) component.$destroy()
     },
-    ...getQueriesForElement(container, queries)
+    ...getQueriesForElement(container, queries),
   }
 }
 

types/index.d.ts

@@ -2,25 +2,42 @@
 // Project: https://github.com/testing-library/svelte-testing-library
 // Definitions by: Rahim Alwer <https://github.com/mihar-22>
 
-import {BoundFunction, EventType,Queries, queries} from '@testing-library/dom'
-import { ComponentConstructorOptions,ComponentProps, SvelteComponent  } from 'svelte'
+import {
+  BoundFunction,
+  EventType,
+  Queries,
+  queries,
+} from '@testing-library/dom'
+import {
+  ComponentConstructorOptions,
+  ComponentProps,
+  SvelteComponent,
+} from 'svelte'
 
 export * from '@testing-library/dom'
 
-type SvelteComponentOptions<C extends SvelteComponent> = ComponentProps<C> | Pick<ComponentConstructorOptions<ComponentProps<C>>, "anchor" | "props" | "hydrate" | "intro" | "context">
+type SvelteComponentOptions<C extends SvelteComponent> =
+  | ComponentProps<C>
+  | Pick<
+      ComponentConstructorOptions<ComponentProps<C>>,
+      'anchor' | 'props' | 'hydrate' | 'intro' | 'context'
+    >
 
 type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>
 
-type Constructor<T> = new (...args: any[]) => T;
+type Constructor<T> = new (...args: any[]) => T
 
 /**
  * Render a Component into the Document.
  */
-export type RenderResult<C extends SvelteComponent, Q extends Queries = typeof queries> = {
+export type RenderResult<
+  C extends SvelteComponent,
+  Q extends Queries = typeof queries,
+> = {
   container: HTMLElement
   component: C
   debug: (el?: HTMLElement | DocumentFragment) => void
-  rerender: (options: SvelteComponentOptions<C>) => void
+  rerender: (props: ComponentProps<C>) => Promise<void>
   unmount: () => void
 } & { [P in keyof Q]: BoundFunction<Q[P]> }
 
@@ -38,7 +55,7 @@ export function render<C extends SvelteComponent>(
 export function render<C extends SvelteComponent, Q extends Queries>(
   component: Constructor<C>,
   componentOptions?: SvelteComponentOptions<C>,
-  renderOptions?: RenderOptions<Q>,
+  renderOptions?: RenderOptions<Q>
 ): RenderResult<C, Q>
 
 /**
@@ -50,13 +67,19 @@ export function cleanup(): void
  * Fires DOM events on an element provided by @testing-library/dom. Since Svelte needs to flush
  * pending state changes via `tick`, these methods have been override and now return a promise.
  */
-export type FireFunction = (element: Document | Element | Window, event: Event) => Promise<boolean>;
+export type FireFunction = (
+  element: Document | Element | Window,
+  event: Event
+) => Promise<boolean>
 
 export type FireObject = {
-  [K in EventType]: (element: Document | Element | Window, options?: {}) => Promise<boolean>;
-};
+  [K in EventType]: (
+    element: Document | Element | Window,
+    options?: {}
+  ) => Promise<boolean>
+}
 
-export const fireEvent: FireFunction & FireObject;
+export const fireEvent: FireFunction & FireObject
 
 /**
  * Calls a function and notifies Svelte to flush any pending state changes.