Concepts

Basics

Variables

let state_var = $state(start_value)
let derived_var = $derived(state_var * 2)
let derived_var_by_func = $derived.by(() => {
  // For more complicated logic
  return state_var * 2
)

Apparently you should prefer to use $derived over $effect because DOM changes should not be done in $effect.

Bind values to variables

https://svelte.dev/docs/svelte/bind

You can bind values directly to input elements, so that the variable changes when the input value changes.

This can be done on text inputs, number inputs, checkbox inputs.

let value = $state(1)

<input type="number" bind:value={message} />

You can also use function bindings to use getters and setters.

Props

https://svelte.dev/docs/svelte/$props

Props are used on children components. Parent components can instanciate children components with arguments.

Values should always be changed in the parent element (TODO why?), so pass down functions to the children to allow value mutation. Sometimes, the displayed values are mapped or filtered values, so the mutation should not be done on the props directly.

let {my_prop1, my_prop2}: {my_prop1: my_prop1_type, my_prop2: my_prop2_type} = $props()

Use components like this:

<MyComponent my_prop1="value" my_prop2={1+1} />
<MyComponent {...my_prop1, my_prop2} />

If else

https://svelte.dev/docs/svelte/if

TODO

Each

https://svelte.dev/docs/svelte/each

TODO

Intermediate

Snippets

Define a snippet with

{#snippet snippet_name(snippet_arg: snippet_arg_type, ...)}
  <div>{snippet_arg}</div>
{/snippet}

Use snippet with

{@render snippet_name('my_value')}

Classes on condition

https://svelte.dev/docs/svelte/class

You can use classes on elements conditionally

<script>
	let my_boolean = $state(true)
</script>

<div 
  class={["this class is always applied", {"this class is applied if the condition is true": my_boolean}]}
>
  My Text
</div>

Advanced

Debugging

You can use https://svelte.dev/docs/svelte/$inspect to console log values when they change.

let my_var = $state(1)
$inspect(my_var)

Transition and animation

A transition is triggered by an element entering or leaving the DOM as a result of a state change.

https://svelte.dev/docs/svelte/transition

<div transition:fade>fades in and out</div>

https://svelte.dev/docs/svelte/in-and-out

If you want more control over the transition, you can directly define the in and out transition, instead of using one transition for in and the reverse for out transition

<div in:fly={{ y: 200 }} out:fade>flies in, fades out</div>

https://svelte.dev/docs/svelte/animate

An animation is triggered when the contents of a keyed each block are re-ordered. Animations do not run when an element is added or removed, only when the index of an existing data item within the each block changes.