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Actions are functions that are called when an element is created. They provide a way to add custom behavior to DOM elements, such as tooltips, click-outside detection, or custom event handlers.

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Action Interface

interface Action<
  Element = HTMLElement,
  Parameter = undefined,
  Attributes extends Record<string, any> = Record<never, any>
> {
  <Node extends Element>(
    ...args: undefined extends Parameter
      ? [node: Node, parameter?: Parameter]
      : [node: Node, parameter: Parameter]
  ): void | ActionReturn<Parameter, Attributes>;
}
​
Element
HTMLElement
default:"HTMLElement"
The type of element this action works on (e.g., HTMLDivElement, HTMLButtonElement)
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Parameter
any
default:"undefined"
The type of parameter the action accepts. Use undefined if the action takes no parameters
​
Attributes
Record<string, any>
default:"Record<never, any>"
Additional attributes and events the action enables on the element (TypeScript only)

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ActionReturn Interface

interface ActionReturn<
  Parameter = undefined,
  Attributes extends Record<string, any> = Record<never, any>
> {
  update?: (parameter: Parameter) => void;
  destroy?: () => void;
}
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update
(parameter: Parameter) => void
Called whenever the action’s parameter changes, immediately after Svelte applies markup updates
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destroy
() => void
Called when the element is unmounted from the DOM

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Usage

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Basic Action

Create a simple action that runs when an element is added to the DOM: 
<script>
  function greet(node) {
    console.log(`Hello from ${node.tagName}`);
  }
</script>

<div use:greet>
  I will greet you in the console
</div>

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Action with Parameters

Pass parameters to customize the action’s behavior: 
<script>
  function tooltip(node, text) {
    const tooltip = document.createElement('div');
    tooltip.textContent = text;
    tooltip.className = 'tooltip';

    function showTooltip() {
      document.body.appendChild(tooltip);
      const rect = node.getBoundingClientRect();
      tooltip.style.left = rect.left + 'px';
      tooltip.style.top = rect.bottom + 'px';
    }

    function hideTooltip() {
      tooltip.remove();
    }

    node.addEventListener('mouseenter', showTooltip);
    node.addEventListener('mouseleave', hideTooltip);

    return {
      destroy() {
        node.removeEventListener('mouseenter', showTooltip);
        node.removeEventListener('mouseleave', hideTooltip);
        tooltip.remove();
      }
    };
  }
</script>

<button use:tooltip="Click me!">
  Hover for tooltip
</button>

​
Action with Update

Handle parameter changes with the update method: 
<script>
  function tooltip(node, text) {
    let tooltipElement;

    function createTooltip() {
      tooltipElement = document.createElement('div');
      tooltipElement.textContent = text;
      tooltipElement.className = 'tooltip';
    }

    createTooltip();

    return {
      update(newText) {
        // Called when the parameter changes
        text = newText;
        if (tooltipElement) {
          tooltipElement.textContent = newText;
        }
      },
      destroy() {
        tooltipElement?.remove();
      }
    };
  }

  let tooltipText = $state('Initial text');
</script>

<button use:tooltip={tooltipText}>
  Hover me
</button>

<input bind:value={tooltipText} />

​
TypeScript Example

Type your actions for better IDE support: 
import type { Action } from 'svelte/action';

// Action that only works on div elements
export const myAction: Action<HTMLDivElement, { someProperty: boolean } | undefined> = (
  node,
  param = { someProperty: true }
) => {
  // Implementation
  console.log(node, param);

  return {
    update(newParam) {
      console.log('Updated:', newParam);
    },
    destroy() {
      console.log('Cleaned up');
    }
  };
};

​
Adding Type-Safe Attributes

Define custom attributes and events that the action enables: 
import type { Action } from 'svelte/action';

interface Attributes {
  'data-custom'?: string;
  'on:customEvent'?: (e: CustomEvent<boolean>) => void;
}

export const myAction: Action<HTMLElement, string, Attributes> = (node, param) => {
  // This enables TypeScript to recognize these attributes
  // when the action is used on an element
  node.setAttribute('data-custom', param);

  node.dispatchEvent(new CustomEvent('customEvent', { detail: true }));

  return {
    update(newParam) {
      node.setAttribute('data-custom', newParam);
    }
  };
};
Usage: 
<script>
  import { myAction } from './actions';
</script>

<!-- TypeScript recognizes these attributes -->
<div
  use:myAction="value"
  data-custom="also works"
  on:customEvent={(e) => console.log(e.detail)}
>
  Content
</div>

​
Common Patterns

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Click Outside

import type { Action } from 'svelte/action';

export const clickOutside: Action<HTMLElement, () => void> = (node, callback) => {
  function handleClick(event: MouseEvent) {
    if (!node.contains(event.target as Node)) {
      callback();
    }
  }

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

  return {
    destroy() {
      document.removeEventListener('click', handleClick, true);
    }
  };
};

​
Focus Trap

import type { Action } from 'svelte/action';

export const focusTrap: Action<HTMLElement> = (node) => {
  const focusableElements = node.querySelectorAll(
    'button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])'
  );
  const firstElement = focusableElements[0] as HTMLElement;
  const lastElement = focusableElements[focusableElements.length - 1] as HTMLElement;

  function handleKeydown(e: KeyboardEvent) {
    if (e.key === 'Tab') {
      if (e.shiftKey && document.activeElement === firstElement) {
        e.preventDefault();
        lastElement.focus();
      } else if (!e.shiftKey && document.activeElement === lastElement) {
        e.preventDefault();
        firstElement.focus();
      }
    }
  }

  node.addEventListener('keydown', handleKeydown);

  return {
    destroy() {
      node.removeEventListener('keydown', handleKeydown);
    }
  };
};

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Notes

  • Actions are called when the element is created
  • The destroy method is called when the element is removed from the DOM
  • The update method is called whenever the parameter changes
  • Multiple actions can be used on the same element
  • Actions run after the element is added to the DOM
  • Actions are useful for integrating third-party libraries
  • The Attributes generic is TypeScript-only and has no runtime effect

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See Also