Svelte Actions Make Svelte The Best JavaScript Framework

Published Oct 6, 2023

Table of Contents

Svelte Actions

Svelte actions are just regular JavaScript functions that are called when an element is created, and give you a reference to the element itself, so you can attach any behavior to that element using regular JavaScript.

You can find the repo with the examples on GitHub.

example.svelte
<script lang="ts">
	function greet(element: HTMLElement) {
		// logs when element is created
		console.log('hi')

    // do whatever you want
    element.animate([{ opacity: 0 }, { opacity: 1 }], {
      duration: 300,
      fill: 'forwards',
    })

		return {
			destroy() {
				// logs when element is removed
				console.log('bye')
			}
		}
	}

	let show = false
</script>

<input bind:checked={show} type="checkbox" />

{#if show}
	<div use:greet>
		Action
	</div>
{/if}

You can optionally return a destroy method after the element is removed for cleanup, and an update method for updating the parameters.

example.svelte
<script lang="ts">
	function greet(element: HTMLElement, content: string) {
    // ...
		return {
			update(content: string) {
				// the value of content has changed
				console.log({ content })
			},
		}
	}

	let content = ''
	let show = false
</script>

<!-- bind input value to `content` -->
<input bind:checked={show} type="checkbox" />

<!-- run `update` when `content` updates  -->
<div use:greet={content}>
	Action
</div>

You can also dispatch custom events using Svelte actions.

.example.svelte
<script lang="ts">
	function greet(element: HTMLElement) {
		const greetEvent = new CustomEvent('greet', { detail: 'hi' })
		element.dispatchEvent(greetEvent)
	}

	function handleGreet(event) {
		console.log(event.detail) // "hi"
	}
</script>

<div
  on:greet={handleGreet}
  use:greet
>
	Action
</div>

🐿️ The order is important if you want to dispatch the event when the element is created. You have to add the on:greet event listener first, and then use the use:greet action.

You can use Svelte actions for anything from using JavaScript libraries in Svelte to Sveltifying existing JavaScript libraries.

Svelte actions only work with JavaScript enabled, and don’t run on the server.

Let’s go over some examples first, and then I’m also going to show you how to type your actions if you’re using TypeScript.

Tooltip Action

The next example uses Tippy.js to make a reusable tooltip with Svelte actions.

src/routes/tooltip/+page.svelte
<script lang="ts">
  import tippy, { type Props } from 'tippy.js'
  import 'tippy.js/dist/tippy.css'

  type Options = Partial<Props>

  let content = 'Tooltip'

  function tooltip(element: HTMLElement, options: Options) {
    // create tooltip
    const tooltip = tippy(element, options)

    return {
      update(options: Options) {
        // update options
        tooltip.setProps(options)
      },
      destroy() {
        // cleanup
        tooltip.destroy()
      },
    }
  }
</script>

<input bind:value={content} />

<button use:tooltip={{ content }}>Hover</button>

There is nothing wrong with using a <Tooltip /> component if you want but in this case a Svelte action makes more sense because it gives you a direct reference to the element and avoids the onMount and bind directive boilerplate.

Click Outside Action

Most of the time you’re going to need a simple action where you need a bit of JavaScript to do something like knowing when a user clicks outside of an element.

src/routes/modal/+page.svelte
<script lang="ts">
  import { scale } from 'svelte/transition'
  import type { Action } from 'svelte/action'

  type Attributes = {
    'on:outside'?: (event: CustomEvent) => void
  }
  type clickOutsideAction = Action<HTMLElement, any, Attributes>

  let open = false

  function openModal() {
    open = true
  }

  function closeModal() {
    open = false
  }

  const clickOutside: clickOutsideAction = (element) => {
    function handleClick(event: MouseEvent) {
      const targetEl = event.target as HTMLElement

      if (element && !element.contains(targetEl)) {
        const clickOutsideEvent = new CustomEvent('outside')
        element.dispatchEvent(clickOutsideEvent)
      }
    }

    document.addEventListener('click', handleClick, true)

    return {
      destroy() {
        document.removeEventListener('click', handleClick, true)
      },
    }
  }
</script>

{#if open}
  <div class="background">
    <div
      class="modal"
      on:outside={closeModal}
      use:clickOutside
      transition:scale
    >
      <h2>Modal</h2>
      <p>What's up?</p>
    </div>
  </div>
{/if}

<button on:click={openModal}>
  Open
</button>

<style>
  .background {
    position: absolute;
    inset: 0;
    display: grid;
    place-content: center;

    & .modal {
      width: 400px;
      min-height: 300px;
      padding: 2rem;
      background-color: hsl(200 10% 10% / 80%);
      backdrop-filter: blur(20px);
      border: 1px solid hsl(200 10% 12%);
      border-radius: 8px;
      box-shadow: 1px 1px 10px hsl(0 0% 0% / 20%);

      & p {
        margin-top: 1rem;
      }
    }
  }
</style>

🐿️ If you’re making a modal you should use the dialog element instead.

You can close the modal ny dispatching and listening for a custom on:outside event on the modal, and the logic isn’t tied to that specific modal, so we can reuse the use:clickOutside action on any element.

Knowing how to make a quick Svelte action like this is always useful, regardless if you need to know if a user clicked outside of an element, or if you need to know if an element is sticky.

Text Animation Action

The next example uses the JavaScript animation library Motion One to create a reusable use:text Svelte action to create elements as letters from any text, and animate it on the screen.

src/routes/motion/+page.svelte
<script lang="ts">
  import { animate, stagger, type AnimationOptions, type AnimationControls } from 'motion'
  import type { Action } from 'svelte/action'

  type Options = {
    options?: AnimationOptions
    action?: ({ animation }: { animation: AnimationControls }) => void
  }
  type Attributes = {
    'on:finished'?: (event: CustomEvent) => void
  }
  type TextAction = Action<HTMLElement, Options, Attributes>

  const text: TextAction = (element, { options, action }) => {
    // remove whitespace and split letters
    const letters = element.innerText.trim().split('')

    // remove any text
    element.innerHTML = ''

    // create an element for every letter
    letters.forEach((letter) => {
      element.innerHTML += `
        <span class="letter">
          ${letter}
        </span>
      `
    })

    // animate the letters
    const animation = animate(
      [...element.children],
      {
        color: 'orangered',
        y: [0, 30, -60, 0],
        rotate: 360,
      },
      { duration: 1, delay: stagger(0.1), ...options }
    )

    // invoke callback
    if (action) {
      action({ animation })
    }

    // dispatch event when animation is finished
    animation.finished.then(() => {
      element.dispatchEvent(new CustomEvent('finished'))
    })
  }

  let controls: AnimationControls
  let time = 0

  $: if (controls) {
    controls.currentTime = time
  }
</script>

<h1
  on:finished={() => console.log('finished')}
  use:text={{
    options: { duration: 2 },
    action({ animation }) {
      animation.stop()
      controls = animation
    },
  }}
>
  Svelte
</h1>

{#if controls}
  <input
    bind:value={time}
    type="range"
    min={0}
    max={controls.duration + 0.6}
    step={0.01}
  />
{/if}

Here we use a action callback to get a reference to animation which we use to scrub through the animation.

Progressive Form Enhancement Action

Svelte actions are great for progressive enhancement.

SvelteKit already has an action for progressive form enhancement but here is how it works.

src/routes/forms/+page.svelte
<script lang="ts">
  // please use this instead
  // import { enhance } from '$app/forms'

  type Submit = (params: SubmitParams) => void
  type SubmitParams = {
    formElement: HTMLFormElement
    formData: FormData
  }

  function enhance(formEl: HTMLFormElement, submit: Submit) {
    async function handleSubmit(e: SubmitEvent) {
      // prevent default form submit
      e.preventDefault()

      // get the submitted form data
      const data = new FormData(formEl)

      // hit the endpoint at forms/+page.server.ts
      const response = await fetch(formEl.action, {
        method: formEl.method,
        body: data,
      })

      // get the response as JSON
      console.log(await response.json())

      // invoke callback
      submit({ formElement: formEl, formData: data })
    }

    // add event listener
    formEl.addEventListener('submit', handleSubmit)

    return {
      destroy() {
        // cleanup
        formEl.removeEventListener('submit', handleSubmit)
      },
    }
  }
</script>

<form
  method="POST"
  use:enhance={({ formElement, formData }) => {
    console.log(formElement, formData)
  }}
>
  <input name="value" type="text" />
  <button>Submit</button>
</form>

<style>
  form {
    display: grid;
    gap: 1rem;
  }
</style>

Thanks to passing the submit callback to the Svelte action, we can get the current <form> element and form data, or whatever else you want.

Svelte Actions Examples

Here is a list of some great libraries and set of utilities that use Svelte actions you can use and learn from.

Libraries:

Utils:

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