Fixing 'Component Instantiation' Error in Svelte 5 Tests: A Quick Guide

So, you’ve just upgraded to Svelte 5 and are excited to test out your components. But wait, what’s this? You run your tests and hit an error that’s throwing a spanner in the works:

Error: Instantiating a component with 'new' is no longer valid in
Svelte 5. See
https://svelte-5-preview.vercel.app/docs/breaking-changes#components-are-no-longer-classes
for more information.

Annoying, right? The solution? Good ol’ patch-package. After a bit of searching I found an issue on the svelte-testing-library repo that fixes the issue. The solution patch-package is a handy tool that allows you to patch a package and keep the patch in your repo.

Install and configure patch-package

Install patch-package as a dev dependency:

pnpm i -D patch-package

Then to configure it, add patch-package to run on your project on postinstall, it’s a script to add to your package.json:

{
  "scripts": {
    "postinstall": "patch-package"
  }
}

Create a patches directory in the root of your project, this is where the patch file is going to go.

# create the patches directory
mkdir patches
# create the patch file
touch patches/@testing-library+svelte+4.0.5.patch

Patching the Package

The patch is a diff of the changes you want to make to the package, full credit to Stefan Hoelzl for the patch on the svelte-testing-library repo.

diff --git a/node_modules/@testing-library/svelte/src/pure.js b/node_modules/@testing-library/svelte/src/pure.js
index 04d3cb0..2f041e0 100644
--- a/node_modules/@testing-library/svelte/src/pure.js
+++ b/node_modules/@testing-library/svelte/src/pure.js
@@ -3,7 +3,7 @@ import {
   getQueriesForElement,
   prettyDOM
 } from '@testing-library/dom'
-import { tick } from 'svelte'
+import { tick, createRoot } from 'svelte'

 const containerCache = new Set()
 const componentCache = new Set()
@@ -54,18 +54,15 @@ const render = (
     return { props: options }
   }

-  let component = new ComponentConstructor({
+  let component = createRoot(ComponentConstructor, {
     target,
-    ...checkProps(options)
+    ...checkProps(options),
+    ondestroy: () => componentCache.delete(component)
   })

   containerCache.add({ container, target, component })
   componentCache.add(component)

-  component.$$.on_destroy.push(() => {
-    componentCache.delete(component)
-  })
-
   return {
     container,
     component,
@@ -73,18 +70,14 @@ const render = (
     rerender: (options) => {
       if (componentCache.has(component)) component.$destroy()

-      // eslint-disable-next-line no-new
-      component = new ComponentConstructor({
+      component = createRoot(ComponentConstructor, {
         target,
-        ...checkProps(options)
+        ...checkProps(options),
+        ondestroy: () => componentCache.delete(component)
       })

       containerCache.add({ container, target, component })
       componentCache.add(component)
-
-      component.$$.on_destroy.push(() => {
-        componentCache.delete(component)
-      })
     },
     unmount: () => {
       if (componentCache.has(component)) component.$destroy()

Now, to run the patch all you need to do is run the install script on your project, whatever that is:

pnpm i

The postinstall script will run patch-package and apply the patch to the package. Now you can run your tests and they should work as expected.

Update

So, 2 months ago there was an update to the svelte-testing-library package which added support for Svelte 5 on the @next tag for the package.

So, install the package at the @next tag:

pnpm i -D @testing-library/svelte@next

Then in any tests that use the svelte-testing-library package, I can update the import to use svelte5:

+import { cleanup, fireEvent, render } from '@testing-library/svelte/svelte5'
-import { cleanup, fireEvent, render } from '@testing-library/svelte'

That’s it, no need to patch the package anymore.

References

• https://github.com/testing-library/svelte-testing-library/issues/284
• https://scottspence.com/posts/patching-packages
• https://scottspence.com/posts/patching-gatsby-react-router-scroll