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Svelte Error Boundary

This package provides a simple error boundary component for Svelte that can be can be used with both DOM and SSR targets. The default error boundary component provides an optional onError callback that can be used to log the error to e.g. Sentry.

This package also provides a createBoundary function that can be used to monkey-patch an existing Svelte component in order to create custom error state UIs.

Monkey-patching is obviously less than ideal since this might break without warning in future versions of Svelte. This library should be considered merely as a stop-gap solution for those using Svelte in production today.

Relevant Svelte issues: svelte#1096 svelte#3587 svelte#3733



npm i -D @crownframework/svelte-error-boundary

Use default error boundary

  import { Boundary } from '@crownframework/svelte-error-boundary';
  let a;

<!-- The code inside the boundary will throw -->
<Boundary onError={console.error}>

Create custom error boundary

You can use the createBoundary function to monkey-patch any ordinary Svelte component in to an error boundary.

The component needs to meet the following criteria:

  1. Have one unnamed slot (this is what will be "enhanced" with an error boundary)
  2. Accept an error prop which will contain a writable store with the last error

Feel free to use the default error boundary component as inspiration.


import { createBoundary } from '@crownframework/svelte-error-boundary';
import Component from './CustomBoundaryComponent.svelte';
export default createBoundary(Component);


   // You might need to add .js extension depending on your bundler config
  import Boundary from './CustomBoundary';
  let a;

<!-- The code inside the boundary will throw -->
<Boundary onError={console.error}>


  • Catch client side errors after initial mount
  • Allow client side recovery if error condition goes away
  • Find a way to reliablty catch errors originating from local state changes in the subtree (#3)


The initial version of this package was based on a proof of concept by @halfnelson: