portal/docs/ai/svelte-patterns.md

Svelte 5 Patterns and Rules

This document contains Svelte-specific implementation rules.

Only read this file when modifying Svelte components, routes, or Svelte-specific logic.

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Svelte Version

This project uses:

• SvelteKit 2
• Svelte 5
• runes mode

Always use:

• $state()
• $derived()
• $effect()
• $props()

Do not use legacy stores:

• writable()
• readable()
• derived()

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Component Props

Always destructure $props() with defaults.

Preferred pattern:

let { record = {} as Record<string, unknown>, fieldErrors = {} as Record<string, string>, isSuper = false }: { record?: Record<string, unknown>; fieldErrors?: Record<string, string>; isSuper?: boolean; } = $props();

Do not assume props exist.

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State Synchronization

IMPORTANT:

Do not initialize $state from props when the prop can change.

Incorrect:

let name = $state(record.name);

This only captures the initial value.

Correct:

let name = $state('');

$effect(() => { name = String(record.name ?? ''); });

Use $effect whenever component state needs to follow changing props.

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Effect Ordering

$effect blocks execute in declaration order.

When multiple effects modify related state:

1. Reset state first.
2. Populate state second.
3. Perform dependent calculations afterward.

Example:

let dirty = $state(false);

$effect(() => { dirty = false; });

$effect(() => { name = String(record.name ?? ''); });

This ensures a new record starts clean.

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Select Inputs

IMPORTANT:

Do not use bind:value on select elements whose values are synchronized through $effect.

Incorrect:

One

This can fail to update correctly.

Use:

<select onchange={(e) => frequency = e.currentTarget.value}> <option value="1" selected={frequency === '1'}> One

Use selected on options.

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Input Elements

Text inputs are different.

This is acceptable:

bind:value works correctly with normal input elements.

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Forms

Admin forms use:

• $state for field values
• $effect for prop synchronization
• oninput for dirty tracking
• use:enhance for submission

Example:

Save

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Avoid Unnecessary State

Prefer:

$derived()

for values computed from other state.

Use:

$state()

only for values that change independently.

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Component Changes

Before modifying a component:

1. Read the entire component.
2. Find similar components.
3. Follow existing patterns.
4. Avoid introducing new abstractions unless necessary.

Small, consistent changes are preferred.