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trueadmdummdidummRich-Harris
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feat: add error boundaries (#14211)
* feat: add error boundary support tweak tweak again retry -> reset tweaks add tests tweaks tweaks tweaks more tests more tests and tweaks comments tweak tweak tweak tweak tweak * tweak tweak tweak tweak more fixes tweak tweak more fixes changeset * Update packages/svelte/elements.d.ts Co-authored-by: Simon H <[email protected]> * Update .changeset/polite-peaches-do.md Co-authored-by: Simon H <[email protected]> * fix issue with rethrowing * handle fallback error * handle fallback error * add more test coverage * more tests * more bug fixes * guard against non-errors * add component_stack to error * alternative approach * remove spread support * lint * add to legacy ast * add to svelte-html * disallow anything but attributes on the boundary element * fix error * more validation * only create block when necessary * swap argument order - results in nicer-looking code in many cases * Update .changeset/polite-peaches-do.md * simplify a bit * simplify * move declaration closer to usage * push once * unused * tweaks * consistent naming * simplify * add a couple newlines * tweak comments * simplify * newlines * placeholder documentation * add some docs * Update packages/svelte/src/internal/client/dom/blocks/boundary.js Co-authored-by: Rich Harris <[email protected]> * Update packages/svelte/src/internal/client/dom/blocks/boundary.js Co-authored-by: Rich Harris <[email protected]> * Update packages/svelte/src/internal/client/dom/blocks/boundary.js Co-authored-by: Rich Harris <[email protected]> * fix type * fix link * explain what happens if onerror throws --------- Co-authored-by: Simon H <[email protected]> Co-authored-by: Simon Holthausen <[email protected]> Co-authored-by: Rich Harris <[email protected]>
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‎.changeset/polite-peaches-do.md

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---
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'svelte': minor
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---
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feat: add error boundaries with `<svelte:boundary>`

‎documentation/docs/05-special-elements/01-svelte-boundary.md

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---
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title: <svelte:boundary>
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---
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```svelte
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<svelte:boundary onerror={handler}>...</svelte:boundary>
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```
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Boundaries allow you to guard against errors in part of your app from breaking the app as a whole, and to recover from those errors.
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If an error occurs while rendering or updating the children of a `<svelte:boundary>`, or running any [`$effect`]($effect) functions contained therein, the contents will be removed.
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Errors occurring outside the rendering process (for example, in event handlers) are _not_ caught by error boundaries.
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## Properties
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For the boundary to do anything, one or both of `failed` and `onerror` must be provided.
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### `failed`
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If an `failed` snippet is provided, it will be rendered with the error that was thrown, and a `reset` function that recreates the contents ([demo](/playground/hello-world#H4sIAAAAAAAAE3VRy26DMBD8lS2tFCIh6JkAUlWp39Cq9EBg06CAbdlLArL87zWGKk8ORnhmd3ZnrD1WtOjFXqKO2BDGW96xqpBD5gXerm5QefG39mgQY9EIWHxueRMinLosti0UPsJLzggZKTeilLWgLGc51a3gkuCjKQ7DO7cXZotgJ3kLqzC6hmex1SZnSXTWYHcrj8LJjWTk0PHoZ8VqIdCOKayPykcpuQxAokJaG1dGybYj4gw4K5u6PKTasSbjXKgnIDlA8VvUdo-pzonraBY2bsH7HAl78mKSHZpgIcuHjq9jXSpZSLixRlveKYQUXhQVhL6GPobXAAb7BbNeyvNUs4qfRg3OnELLj5hqH9eQZqCnoBwR9lYcQxuVXeBzc8kMF8yXY4yNJ5oGiUzP_aaf_waTRGJib5_Ad3P_vbCuaYxzeNpbU0eUMPAOKh7Yw1YErgtoXyuYlPLzc10_xo_5A91zkQL_AgAA)):
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```svelte
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<svelte:boundary>
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<FlakyComponent />
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{#snippet failed(error, reset)}
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<button onclick={reset}>oops! try again</button>
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{/snippet}
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</svelte:boundary>
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```
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> [!NOTE]
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> As with [snippets passed to components](snippet#Passing-snippets-to-components), the `failed` snippet can be passed explicitly as a property...
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>
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> ```svelte
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> <svelte:boundary {failed}>...</svelte:boundary>
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> ```
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>
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> ...or implicitly by declaring it directly inside the boundary, as in the example above.
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### `onerror`
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If an `onerror` function is provided, it will be called with the same two `error` and `reset` arguments. This is useful for tracking the error with an error reporting service...
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```svelte
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<svelte:boundary onerror={(e) => report(e)}>
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...
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</svelte:boundary>
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```
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...or using `error` and `reset` outside the boundary itself:
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```svelte
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<script>
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let error = $state(null);
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let reset = $state(() => {});
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function onerror(e, r) {
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error = e;
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reset = r;
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}
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</script>
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<svelte:boundary {onerror}>
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<FlakyComponent />
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</svelte:boundary>
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{#if error}
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<button onclick={() => {
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error = null;
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reset();
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}}>
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oops! try again
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</button>
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{/if}
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```
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If an error occurs inside the `onerror` function (or if you rethrow the error), it will be handled by a parent boundary if such exists.

‎documentation/docs/05-special-elements/01-svelte-window.md renamed to ‎documentation/docs/05-special-elements/02-svelte-window.md

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‎documentation/docs/05-special-elements/02-svelte-document.md renamed to ‎documentation/docs/05-special-elements/03-svelte-document.md

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‎documentation/docs/05-special-elements/03-svelte-body.md renamed to ‎documentation/docs/05-special-elements/04-svelte-body.md

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‎documentation/docs/05-special-elements/06-svelte-options.md renamed to ‎documentation/docs/05-special-elements/07-svelte-options.md

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

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`<svelte:body>` does not support non-event attributes or spread attributes
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```
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### svelte_boundary_invalid_attribute
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```
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Valid attributes on `<svelte:boundary>` are `onerror` and `failed`
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```
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### svelte_boundary_invalid_attribute_value
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```
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Attribute value must be a non-string expression
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```
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### svelte_component_invalid_this
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```

‎packages/svelte/elements.d.ts

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[name: string]: any;
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};
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'svelte:head': { [name: string]: any };
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'svelte:boundary': {
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onerror?: (error: unknown, reset: () => void) => void;
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failed?: import('svelte').Snippet<[error: unknown, reset: () => void]>;
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};
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[name: string]: { [name: string]: any };
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}

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