Svelte Migrate

2. State: let → $state()

Only reactive variables that are reassigned or mutated need $state().

Before:

<script lang="ts">
  let count = 0;
  let items = [];
</script>

After:

<script lang="ts">
  let count = $state(0);
  let items = $state<string[]>([]);
</script>

3. Derived Values: $: → $derived()

Before:

<script lang="ts">
  $: doubled = count * 2;
  $: filtered = items.filter((i) => i.active);
</script>

After:

<script lang="ts">
  const doubled = $derived(count * 2);
  const filtered = $derived(items.filter((i) => i.active));
</script>

For complex derivations use $derived.by():

Before:

<script lang="ts">
  $: {
    let total = 0;
    for (const item of items) total += item.value;
    sum = total;
  }
</script>

After:

<script lang="ts">
  const sum = $derived.by(() => {
    let total = 0;
    for (const item of items) total += item.value;
    return total;
  });
</script>

4. Side Effects: $: statements → $effect()

Before:

<script lang="ts">
  $: console.log('count changed:', count);
  $: if (count > 10) alert('High count!');
  $: document.title = `Count: ${count}`;
</script>

After:

<script lang="ts">
  $effect(() => {
    console.log('count changed:', count);
  });

  $effect(() => {
    if (count > 10) alert('High count!');
  });

  $effect(() => {
    document.title = `Count: ${count}`;
  });
</script>

5. Event Handlers: on:event → onevent

Before:

<button on:click={handleClick}>Click</button>
<button on:click={() => count++}>Increment</button>
<button on:click|preventDefault={submit}>Submit</button>
<input on:input={handleInput} on:focus={handleFocus} />

After:

<button onclick={handleClick}>Click</button>
<button onclick={() => count++}>Increment</button>
<button
  onclick={(e) => {
    e.preventDefault();
    submit(e);
  }}>Submit</button
>
<input oninput={handleInput} onfocus={handleFocus} />

Event modifiers like |preventDefault, |stopPropagation must be handled manually in the handler.

6. Component Events: createEventDispatcher → callback props

Before:

<script lang="ts">
  import { createEventDispatcher } from 'svelte';

  const dispatch = createEventDispatcher<{
    change: string;
    submit: { data: FormData };
  }>();

  function handleChange(value: string) {
    dispatch('change', value);
  }
</script>

After:

<script lang="ts">
  interface Props {
    onchange?: (value: string) => void;
    onsubmit?: (data: { data: FormData }) => void;
  }

  let { onchange, onsubmit }: Props = $props();

  function handleChange(value: string) {
    onchange?.(value);
  }
</script>

Parent component changes:

Before: <Child on:change={handler} /> After: <Child onchange={handler} />

7. Forwarding Events: on:event → explicit handler

Before:

<button on:click>Click me</button>

After:

<script lang="ts">
  interface Props {
    onclick?: (e: MouseEvent) => void;
  }

  let { onclick }: Props = $props();
</script>

<button {onclick}>Click me</button>

8. Slots → Snippets

Default slot:

Before:

<slot />

After:

<script lang="ts">
  import type { Snippet } from 'svelte';

  interface Props {
    children?: Snippet;
  }

  let { children }: Props = $props();
</script>

{@render children?.()}

Named slots:

Before:

<slot name="header" />
<slot />
<slot name="footer" />

After:

<script lang="ts">
  import type { Snippet } from 'svelte';

  interface Props {
    header?: Snippet;
    children?: Snippet;
    footer?: Snippet;
  }

  let { header, children, footer }: Props = $props();
</script>

{@render header?.()}
{@render children?.()}
{@render footer?.()}

Slots with props (let:):

Before:

<slot item={currentItem} index={i} />

<!-- Usage -->
<List {items} let:item let:index>
  <span>{index}: {item.name}</span>
</List>

After:

<script lang="ts">
  import type { Snippet } from 'svelte';

  interface Props {
    children?: Snippet<[{ item: Item; index: number }]>;
  }

  let { children }: Props = $props();
</script>

{@render children?.({ item: currentItem, index: i })}

<!-- Usage -->
<List {items}>
  {#snippet children({ item, index })}
    <span>{index}: {item.name}</span>
  {/snippet}
</List>

9. Bindable Props: bind: → $bindable()

Before:

<script lang="ts">
  export let value = '';
</script>

<!-- Parent -->
<Input bind:value={name} />

After:

<script lang="ts">
  interface Props {
    value?: string;
  }

  let { value = $bindable('') }: Props = $props();
</script>

<!-- Parent stays the same -->
<Input bind:value={name} />

10. Lifecycle: beforeUpdate/afterUpdate → $effect

Before:

<script lang="ts">
  import { beforeUpdate, afterUpdate } from 'svelte';

  beforeUpdate(() => {
    console.log('before update');
  });

  afterUpdate(() => {
    console.log('after update');
  });
</script>

After:

<script lang="ts">
  $effect.pre(() => {
    console.log('before update');
  });

  $effect(() => {
    console.log('after update');
  });
</script>

IMPORTANT: onMount and onDestroy remain valid and should be preserved!

• Use onMount for initialization logic that should only run once when the component first mounts
• Use $effect for reactive side effects that should re-run when dependencies change
• Never convert onMount to $effect unless the logic genuinely needs to be reactive

Examples:

Keep as onMount:

// ✅ One-time initialization - keep as onMount
onMount(() => {
  if (query) {
    $filters = toListWorkflowFilters(query, $searchAttributes);
  }
});

Convert to $effect:

// ✅ Reactive side effect - convert to $effect
$effect(() => {
  document.title = `Count: ${count}`; // re-runs when count changes
});

11. Dynamic Components: <svelte:component> → direct usage

Before:

<svelte:component this={DynamicComponent} {prop} />

After:

<DynamicComponent {prop} />

Or with conditional:

{#if Component}
  <Component {prop} />
{/if}

12. class: Directive → Conditional Classes

The class: directive still works but consider using conditional expressions:

Before:

<div class:active={isActive} class:disabled>

After (either works):

<div class:active={isActive} class:disabled>
<!-- or -->
<div class={`${isActive ? 'active' : ''} ${disabled ? 'disabled' : ''}`}>

13. App State

Before:

import {page} from '$app/stores'

After:

import {page} from '$app/state'

Workflow

1. Read the file to understand its current structure
2. Identify all Svelte 4 patterns that need migration
3. Apply transformations systematically:
   • Props first (export let → $props)
   • State variables (let → $state where needed)
   • Derived values ($: assignments → $derived)
   • Side effects ($: statements → $effect)
   • Event handlers (on: → on)
   • Component events (dispatch → callback props)
   • Slots (slot → snippets)
4. Update imports (add Snippet type if needed, remove createEventDispatcher if no longer used)
5. Ensure TypeScript types are correct
6. Run pnpm check to verify no type errors

Important Notes

• Keep onMount and onDestroy - they're still valid
• context="module" scripts are now <script module>
• CSS/styles don't change
• {#if}, {#each}, {#await} blocks remain unchanged
• bind: directives on elements remain unchanged
• use: action directives remain unchanged
• transition:, in:, out:, animate: directives remain unchanged