We recommend using [SvelteKit](../kit), which lets you [build almost anything](../kit/project-types). It's the official application framework from the Svelte team and powered by [Vite](https://vite.dev/). Create a new project with:
@ -135,7 +135,7 @@ An effect only reruns when the object it reads changes, not when a property insi
An effect only depends on the values that it read the last time it ran. This has interesting implications for effects that have conditional code.
For instance, if `condition` is `true` in the code snippet below, the code inside the `if` block will run and `color` will be evaluated. As such, changes to either `condition` or `color` [will cause the effect to re-run](/playground/untitled#H4sIAAAAAAAAE21RQW6DMBD8ytaNBJHaJFLViwNIVZ8RcnBgXVk1xsILTYT4e20TQg89IOPZ2fHM7siMaJBx9tmaWpFqjQNlAKXEihx7YVJpdIyfRkY3G4gB8Pi97cPanRtQU8AuwuF_eNUaQuPlOMtc1SlLRWlKUo1tOwJflUikQHZtA0klzCDc64Imx0ANn8bInV1CDhtHgjClrsftcSXotluLybOUb3g4JJHhOZs5WZpuIS9gjNqkJKQP5e2ClrR4SMdZ13E4xZ8zTPOTJU2A2uE_PQ9COCI926_hTVarIU4hu_REPlBrKq2q73ycrf1N-vS4TMUsulaVg3EtR8H9rFgsg8uUsT1B2F9eshigZHBRpuaD0D3mY8Qm2BfB5N2YyRzdNEYVDy0Ja-WsFjcOUuP1HvFLWA6H3XuHTUSmmDV2--0TXonxsKbp7G9C6R__NONS-MFNvxj_d6mBAgAA).
For instance, if `condition` is `true` in the code snippet below, the code inside the `if` block will run and `color` will be evaluated. This means that changes to either `condition` or `color` [will cause the effect to re-run](/playground/untitled#H4sIAAAAAAAAE21RQW6DMBD8ytaNBJHaJFLViwNIVZ8RcnBgXVk1xsILTYT4e20TQg89IOPZ2fHM7siMaJBx9tmaWpFqjQNlAKXEihx7YVJpdIyfRkY3G4gB8Pi97cPanRtQU8AuwuF_eNUaQuPlOMtc1SlLRWlKUo1tOwJflUikQHZtA0klzCDc64Imx0ANn8bInV1CDhtHgjClrsftcSXotluLybOUb3g4JJHhOZs5WZpuIS9gjNqkJKQP5e2ClrR4SMdZ13E4xZ8zTPOTJU2A2uE_PQ9COCI926_hTVarIU4hu_REPlBrKq2q73ycrf1N-vS4TMUsulaVg3EtR8H9rFgsg8uUsT1B2F9eshigZHBRpuaD0D3mY8Qm2BfB5N2YyRzdNEYVDy0Ja-WsFjcOUuP1HvFLWA6H3XuHTUSmmDV2--0TXonxsKbp7G9C6R__NONS-MFNvxj_d6mBAgAA).
Conversely, if `condition` is `false`, `color` will not be evaluated, and the effect will _only_ re-run again when `condition` changes.
@ -277,4 +277,4 @@ Snippets can be created programmatically with the [`createRawSnippet`](svelte#cr
## Snippets and slots
In Svelte 4, content can be passed to components using [slots](legacy-slots). Snippets are more powerful and flexible, and as such slots are deprecated in Svelte 5.
In Svelte 4, content can be passed to components using [slots](legacy-slots). Snippets are more powerful and flexible, and so slots have been deprecated in Svelte 5.
@ -22,7 +22,7 @@ It also will not compile Svelte code.
## Styling
Content rendered this way is 'invisible' to Svelte and as such will not receive [scoped styles](scoped-styles) — in other words, this will not work, and the `a` and `img` styles will be regarded as unused:
Content rendered this way is 'invisible' to Svelte and as such will not receive [scoped styles](scoped-styles). In other words, this will not work, and the `a` and `img` styles will be regarded as unused:
@ -71,7 +71,7 @@ The user of this component has the same flexibility to use a mixture of objects,
</Button>
```
Svelte also exposes the `ClassValue` type, which is the type of value that the `class` attribute on elements accept. This is useful if you want to use a type-safe class name in component props:
Since Svelte 5.19, Svelte also exposes the `ClassValue` type, which is the type of value that the `class` attribute on elements accept. This is useful if you want to use a type-safe class name in component props:
@ -12,7 +12,7 @@ The `<svelte:options>` element provides a place to specify per-component compile
- `runes={false}` — forces a component into _legacy mode_
- `namespace="..."` — the namespace where this component will be used, can be "html" (the default), "svg" or "mathml"
- `customElement={...}` — the [options](custom-elements#Component-options) to use when compiling this component as a custom element. If a string is passed, it is used as the `tag` option
- `css="injected"` — the component will inject its styles inline: During serverside rendering, it's injected as a `<style>` tag in the `head`, during client side rendering, it's loaded via JavaScript
- `css="injected"` — the component will inject its styles inline: During server-side rendering, it's injected as a `<style>` tag in the `head`, during client side rendering, it's loaded via JavaScript
> [!LEGACY] Deprecated options
> Svelte 4 also included the following options. They are deprecated in Svelte 5 and non-functional in runes mode.
@ -41,7 +41,7 @@ If a function is returned from `onMount`, it will be called when the component i
</script>
```
> [!NOTE] This behaviour will only work when the function passed to `onMount`_synchronously_ returns a value. `async` functions always return a `Promise`, and as such cannot _synchronously_ return a function.
> [!NOTE] This behaviour will only work when the function passed to `onMount`is _synchronous_. `async` functions always return a `Promise`.
Modifiers are specific to `on:` and as such do not work with modern event handlers. Adding things like `event.preventDefault()` inside the handler itself is preferable, since all the logic lives in one place rather than being split between handler and modifiers.
Modifiers are specific to `on:` and so do not work with modern event handlers. Adding things like `event.preventDefault()` inside the handler itself is preferable, since all the logic lives in one place rather than being split between handler and modifiers.
Since event handlers are just functions, you can create your own wrappers as necessary:
@ -340,7 +340,7 @@ When spreading props, local event handlers must go _after_ the spread, or they r
## Snippets instead of slots
In Svelte 4, content can be passed to components using slots. Svelte 5 replaces them with snippets which are more powerful and flexible, and as such slots are deprecated in Svelte 5.
In Svelte 4, content can be passed to components using slots. Svelte 5 replaces them with snippets, which are more powerful and flexible, and so slots are deprecated in Svelte 5.
They continue to work, however, and you can pass snippets to a component that uses slots:
@ -599,7 +599,7 @@ Note that `mount` and `hydrate` are _not_ synchronous, so things like `onMount`
### Server API changes
Similarly, components no longer have a `render` method when compiled for serverside rendering. Instead, pass the function to `render` from `svelte/server`:
Similarly, components no longer have a `render` method when compiled for server-side rendering. Instead, pass the function to `render` from `svelte/server`:
```js
+++import { render } from 'svelte/server';+++
@ -803,7 +803,7 @@ Note that Svelte 5 will also warn if you have a single expression wrapped in quo
### HTML structure is stricter
In Svelte 4, you were allowed to write HTML code that would be repaired by the browser when serverside rendering it. For example you could write this...
In Svelte 4, you were allowed to write HTML code that would be repaired by the browser when server-side rendering it. For example you could write this...
```svelte
<table>
@ -835,7 +835,7 @@ Assignments to destructured parts of a `@const` declaration are no longer allowe
### :is(...), :has(...), and :where(...) are scoped
Previously, Svelte did not analyse selectors inside `:is(...)`, `:has(...)`, and `:where(...)`, effectively treating them as global. Svelte 5 analyses them in the context of the current component. As such, some selectors may now be treated as unused if they were relying on this treatment. To fix this, use `:global(...)` inside the `:is(...)/:has(...)/:where(...)` selectors.
Previously, Svelte did not analyse selectors inside `:is(...)`, `:has(...)`, and `:where(...)`, effectively treating them as global. Svelte 5 analyses them in the context of the current component. Some selectors may now therefore be treated as unused if they were relying on this treatment. To fix this, use `:global(...)` inside the `:is(...)/:has(...)/:where(...)` selectors.
When using Tailwind's `@apply` directive, add a `:global` selector to preserve rules that use Tailwind-generated `:is(...)` selectors:
@ -964,7 +964,7 @@ Since these mismatches are extremely rare, Svelte 5 assumes that the values are
### Hydration works differently
Svelte 5 makes use of comments during server side rendering which are used for more robust and efficient hydration on the client. As such, you shouldn't remove comments from your HTML output if you intend to hydrate it, and if you manually authored HTML to be hydrated by a Svelte component, you need to adjust that HTML to include said comments at the correct positions.
Svelte 5 makes use of comments during server-side rendering which are used for more robust and efficient hydration on the client. You therefore should not remove comments from your HTML output if you intend to hydrate it, and if you manually authored HTML to be hydrated by a Svelte component, you need to adjust that HTML to include said comments at the correct positions.
`{@const}` must be the immediate child of `{#snippet}`, `{#if}`, `{:else if}`, `{:else}`, `{#each}`, `{:then}`, `{:catch}`, `<svelte:fragment>`, `<svelte:boundary` or `<Component>`
```
### const_tag_invalid_reference
```
The `{@const %name% = ...}` declaration is not available in this snippet
```
The following is an error:
```svelte
<svelte:boundary>
{@const foo = 'bar'}
{#snippet failed()}
{foo}
{/snippet}
</svelte:boundary>
```
Here, `foo` is not available inside `failed`. The top level code inside `<svelte:boundary>` becomes part of the implicit `children` snippet, in other words the above code is equivalent to this:
```svelte
<svelte:boundary>
{#snippet children()}
{@const foo = 'bar'}
{/snippet}
{#snippet failed()}
{foo}
{/snippet}
</svelte:boundary>
```
The same applies to components:
```svelte
<Component>
{@const foo = 'bar'}
{#snippet someProp()}
<!-- error -->
{foo}
{/snippet}
</Component>
```
### constant_assignment
```
@ -364,6 +409,12 @@ The $ name is reserved, and cannot be used for variables and imports
The $ prefix is reserved, and cannot be used for variables and imports
@ -679,11 +679,11 @@ In HTML, there's [no such thing as a self-closing tag](https://jakearchibald.com
</div>
```
Some templating languages (including Svelte) will 'fix' HTML by turning `<span />` into `<span></span>`. Others adhere to the spec. Both result in ambiguity and confusion when copy-pasting code between different contexts, and as such Svelte prompts you to resolve the ambiguity directly by having an explicit closing tag.
Some templating languages (including Svelte) will 'fix' HTML by turning `<span />` into `<span></span>`. Others adhere to the spec. Both result in ambiguity and confusion when copy-pasting code between different contexts, so Svelte prompts you to resolve the ambiguity directly by having an explicit closing tag.
> The $ prefix is reserved, and cannot be used for variables and imports
## duplicate_class_field
> `%name%` has already been declared
## each_item_invalid_assignment
> Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. `array[i] = value` instead of `entry = value`, or `bind:value={array[i]}` instead of `bind:value={entry}`)
> `{@const}` must be the immediate child of `{#snippet}`, `{#if}`, `{:else if}`, `{:else}`, `{#each}`, `{:then}`, `{:catch}`, `<svelte:fragment>`, `<svelte:boundary` or `<Component>`
## const_tag_invalid_reference
> The `{@const %name% = ...}` declaration is not available in this snippet
The following is an error:
```svelte
<svelte:boundary>
{@const foo = 'bar'}
{#snippet failed()}
{foo}
{/snippet}
</svelte:boundary>
```
Here, `foo` is not available inside `failed`. The top level code inside `<svelte:boundary>` becomes part of the implicit `children` snippet, in other words the above code is equivalent to this:
```svelte
<svelte:boundary>
{#snippet children()}
{@const foo = 'bar'}
{/snippet}
{#snippet failed()}
{foo}
{/snippet}
</svelte:boundary>
```
The same applies to components:
```svelte
<Component>
{@const foo = 'bar'}
{#snippet someProp()}
<!-- error -->
{foo}
{/snippet}
</Component>
```
## debug_tag_invalid_arguments
> {@debug ...} arguments must be identifiers, not arbitrary expressions
@ -67,11 +67,11 @@ In HTML, there's [no such thing as a self-closing tag](https://jakearchibald.com
</div>
```
Some templating languages (including Svelte) will 'fix' HTML by turning `<span />` into `<span></span>`. Others adhere to the spec. Both result in ambiguity and confusion when copy-pasting code between different contexts, and as such Svelte prompts you to resolve the ambiguity directly by having an explicit closing tag.
Some templating languages (including Svelte) will 'fix' HTML by turning `<span />` into `<span></span>`. Others adhere to the spec. Both result in ambiguity and confusion when copy-pasting code between different contexts, so Svelte prompts you to resolve the ambiguity directly by having an explicit closing tag.
@ -975,6 +985,16 @@ export function const_tag_invalid_placement(node) {
e(node,'const_tag_invalid_placement',`\`{@const}\` must be the immediate child of \`{#snippet}\`, \`{#if}\`, \`{:else if}\`, \`{:else}\`, \`{#each}\`, \`{:then}\`, \`{:catch}\`, \`<svelte:fragment>\`, \`<svelte:boundary\` or \`<Component>\`\nhttps://svelte.dev/e/const_tag_invalid_placement`);
e(node,'const_tag_invalid_reference',`The \`{@const ${name} = ...}\` declaration is not available in this snippet \nhttps://svelte.dev/e/const_tag_invalid_reference`);
ComputerGuy
4 weeks ago
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GitHub