Named actions in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a named action in Svelte?

A named action in Svelte is a reusable function that you can apply to HTML elements to add behavior, like event handling or animations, by using the use: directive with a specific name.

beginner

How do you apply a named action to an element in Svelte?

You apply a named action by adding use:actionName to the HTML element, where actionName is the function you want to run on that element.

intermediate

What arguments does a Svelte action function receive?

A Svelte action function receives two arguments: the HTML element it is applied to, and an optional parameter object for configuration.

intermediate

What should a Svelte action return if it needs to clean up?

If cleanup is needed, the action should return an object with a destroy method that runs when the element is removed or the action is changed.

intermediate

Can named actions in Svelte accept parameters? How?

Yes, named actions can accept parameters by passing them like use:actionName={params}. The action function receives these parameters as the second argument.

How do you attach a named action called highlight to a <div> in Svelte?

A <div use:highlight></div>

B <div action:highlight></div>

C <div on:highlight></div>

D <div highlight></div>

In Svelte, named actions are attached using the use: directive followed by the action name.

What does a Svelte action function receive as its first argument?

A An event object

B The HTML element it is applied to

C The component instance

D The action's name

The first argument to a Svelte action function is always the HTML element the action is applied to.

What should a Svelte action return if it needs to clean up when the element is removed?

A A promise

B A boolean true

C Nothing, cleanup is automatic

D An object with a destroy() method

Returning an object with a destroy method lets Svelte know how to clean up when the element is removed.

How do you pass parameters to a named action in Svelte?

A use:actionName={params}

B use:actionName(params)

C actionName={params}

D on:actionName={params}

Parameters are passed to named actions using curly braces after the action name, like use:actionName={params}.

Which of these is NOT a valid use of a named action in Svelte?

A use:fade

B use:tooltip={text}

C use:onClick

D use:draggable

use:onClick is invalid because event handlers use on:, not use:.

Explain how named actions work in Svelte and how you would create and use one.

Think about how you add behavior to elements with functions.

Describe how to pass parameters to a named action and how the action function handles them.

Consider how you customize the action's effect on the element.

Practice

1. What is the main purpose of named actions in Svelte?

easy

A. To style elements with CSS

B. To define new components

C. To add reusable behavior to HTML elements

D. To manage application state

Solution

1. Step 1: Understand what named actions do

   Named actions in Svelte are used to add behavior to HTML elements that can be reused across components.

2. Step 2: Compare with other options

   Defining components, styling, and state management are handled differently in Svelte, not by named actions.

3. Final Answer:

   To add reusable behavior to HTML elements -> Option C

4. Quick Check:

   Named actions = reusable element behavior [OK]

Hint: Named actions add behavior to elements, not styles or state [OK]

Common Mistakes:

• Confusing actions with components
• Thinking actions handle styling
• Assuming actions manage state

2. Which of the following is the correct syntax to define a named action in Svelte?

easy

A. function myAction(node) { /* code */ }

B. const myAction = () => { /* code */ }

C. let myAction = node => { /* code */ }

D. action myAction(node) { /* code */ }

Solution

1. Step 1: Recall named action syntax

   Named actions are functions that receive the element node as the first argument, typically declared as function myAction(node) {}.

2. Step 2: Check other options

   Arrow functions can work but must return an object for cleanup; action myAction(node) { /* code */ } uses invalid syntax.

3. Final Answer:

   function myAction(node) { /* code */ } -> Option A

4. Quick Check:

   Named action = function with node parameter [OK]

Hint: Named actions are functions with node parameter, not keywords [OK]

Common Mistakes:

• Using invalid 'action' keyword
• Defining without node parameter
• Using arrow function without return object

3. Given this named action and usage:

function highlight(node) {
  node.style.backgroundColor = 'yellow';
  return {
    destroy() {
      node.style.backgroundColor = '';
    }
  };
}

<div use:highlight>Hello</div>

What happens when the component is removed from the page?

medium

A. The background color resets to default

B. The element is removed from the DOM immediately

C. An error occurs because destroy is missing

D. The background color stays yellow

Solution

1. Step 1: Understand the action's destroy method

   The destroy method resets the background color to empty string when the element is removed.

2. Step 2: Effect on component removal

   When the component unmounts, destroy runs, so the background color resets to default.

3. Final Answer:

   The background color resets to default -> Option A

4. Quick Check:

   destroy resets styles on removal [OK]

Hint: Destroy cleans up styles when element is removed [OK]

Common Mistakes:

• Assuming styles persist after removal
• Thinking destroy is optional
• Confusing destroy with element removal

4. What is wrong with this named action code?

function focus(node) {
  node.focus();
  return {
    destroy() {
      node.blur();
    }
  }
}

<input use:focus>

medium

A. Calling node.blur() in destroy may cause errors if node is removed

B. The action should not call node.focus() immediately

C. The action does not return an object

D. The action must be async to work properly

Solution

1. Step 1: Analyze the focus call timing

   Calling node.focus() immediately may not work reliably because the element might not be fully rendered, visible, or ready (e.g., during CSS transitions or animations).

2. Step 2: Best practice for focus actions

   In Svelte, it's often necessary to delay the focus call using tick(), requestAnimationFrame, or setTimeout(..., 0) to ensure DOM readiness.

3. Final Answer:

   The action should not call node.focus() immediately -> Option B

4. Quick Check:

   Delay focus with tick() or RAF [OK]

Hint: Delay node.focus() with tick() for reliability [OK]

Common Mistakes:

• Calling focus immediately without delay
• Ignoring DOM/transition timing issues
• Assuming focus always succeeds synchronously

5. You want to create a named action that changes an element's text color based on a parameter and updates it when the parameter changes. Which approach is correct?

hard

A. Use a reactive statement inside the action function to watch parameters

B. Set the color once in the action and ignore parameter changes

C. Recreate the action every time parameters change instead of updating

D. Return an update function from the action that sets the color when parameters change

Solution

1. Step 1: Understand parameter updates in named actions

   Named actions can return an update function to handle parameter changes dynamically.

2. Step 2: Evaluate options for handling parameter changes

   Setting color once ignores updates; reactive statements inside action function don't track parameters; recreating action is inefficient.

3. Final Answer:

   Return an update function from the action that sets the color when parameters change -> Option D

4. Quick Check:

   Use update() to handle parameter changes [OK]

Hint: Use update() function to react to parameter changes [OK]

Common Mistakes:

• Ignoring parameter updates
• Misusing reactive statements inside actions
• Recreating actions unnecessarily