Action parameters in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are action parameters in Svelte?

Action parameters are extra values you can pass to a Svelte action to customize its behavior when you use it on an element.

beginner

How do you pass parameters to a Svelte action?

You pass parameters by adding a second argument to the action in the template, like <div use:myAction={param}>.

intermediate

What is the shape of a Svelte action function that uses parameters?

It is a function that takes two arguments: the node (element) and the parameters object, like function myAction(node, params) { ... }.

intermediate

How can an action respond to parameter changes in Svelte?

The action can return an object with an update function that Svelte calls when parameters change, allowing the action to adjust behavior.

beginner

Why is it useful to use action parameters in Svelte?

They let you reuse the same action with different settings on different elements, making your code cleaner and more flexible.

How do you pass parameters to a Svelte action?

A By adding a second argument in the use directive, like use:action={params}

B By calling the action function manually in script

C By setting a global variable

D By using a special Svelte store

You pass parameters directly in the template with use:action={params}.

What arguments does a Svelte action function receive?

A Node and parameters

B Parameters only

C Only the node (element)

D No arguments

The action function receives the node and the parameters as arguments.

What should an action return to handle parameter updates?

A An update function only

B An object with update and destroy functions

C A cleanup function only

D Nothing

An action returns an object with update and destroy functions to handle changes and cleanup.

If you want to change an action’s behavior when parameters change, what do you do?

A Recreate the element

B Ignore parameter changes

C Use the update function returned by the action

D Use a reactive statement

The update function lets the action respond to parameter changes.

Why use parameters with Svelte actions?

A To avoid writing any JavaScript

B To create new components

C To style elements

D To make actions reusable with different settings

Parameters let you reuse the same action with different options on different elements.

Explain how to pass and use parameters in a Svelte action.

Think about how you customize behavior for different elements.

Describe why action parameters improve code reuse in Svelte.

Imagine using one tool for many jobs by changing its settings.

Practice

1. What is the purpose of passing parameters to a Svelte action using use:action={parameter}?

easy

A. To define a new component inside the action

B. To customize the behavior of the action with extra data

C. To import external libraries into the action

D. To style the element directly from the action

Solution

1. Step 1: Understand action parameters

   Action parameters allow you to send extra data to a Svelte action to customize its behavior.

2. Step 2: Identify the purpose of use:action={parameter}

   This syntax passes the parameter to the action so it can use it internally.

3. Final Answer:

   To customize the behavior of the action with extra data -> Option B

4. Quick Check:

   Action parameters customize behavior = A [OK]

Hint: Parameters customize actions with extra data [OK]

Common Mistakes:

• Thinking parameters define components
• Confusing parameters with styling
• Assuming parameters import libraries

2. Which of the following is the correct syntax to pass a parameter named color with value 'red' to a Svelte action called highlight?

easy

A. use:highlight={{color: 'red'}}

B. use:highlight={'red'}

C. use:highlight={color} with let color = 'red'

D. use:highlight(color='red')

Solution

1. Step 1: Recall correct parameter passing syntax

   Svelte actions receive parameters using use:action={parameter} where parameter can be any value or object.

2. Step 2: Identify correct object syntax for passing color

   To pass an object with a key color and value 'red', use {color: 'red'} inside curly braces.

3. Final Answer:

   use:highlight={{color: 'red'}} -> Option A

4. Quick Check:

   Pass object with color key = D [OK]

Hint: Use curly braces with object for named parameters [OK]

Common Mistakes:

• Using parentheses instead of curly braces
• Passing string directly without braces
• Passing variable without defining it

3. Given this Svelte action:

function border(node, { color }) {
  node.style.border = `2px solid ${color}`;
  return {
    update({ color }) {
      node.style.border = `2px solid ${color}`;
    }
  };
}

What will be the border color after this code runs?

<div use:border={{ color: 'blue' }}>Box</div>

medium

A. A 2px solid blue border

B. No border will appear

C. A 2px solid red border

D. A 2px solid black border

Solution

1. Step 1: Understand initial border style setting

   The action sets the border style to '2px solid' plus the passed color, which is 'blue'.

2. Step 2: Confirm the border color after applying the action

   The div will have a blue border because the parameter color is 'blue' and the style is applied immediately.

3. Final Answer:

   A 2px solid blue border -> Option A

4. Quick Check:

   Passed color 'blue' sets border color = B [OK]

Hint: Action sets style using passed color parameter [OK]

Common Mistakes:

• Assuming default color instead of passed color
• Ignoring the update method
• Thinking no border is set initially

4. Consider this Svelte action code:

function scale(node, factor) {
  node.style.transform = `scale(${factor})`;
  return {
    update(newFactor) {
      node.style.transform = `scale(${newFactor})`;
    }
  };
}

Which of the following will result in an invalid CSS transform value when used as use:scale={...}?

medium

A. use:scale={2}

B. use:scale={'2'}

C. use:scale={{ factor: 2 }}

D. use:scale={0.5}

Solution

1. Step 1: Analyze expected parameter type

   The action expects a single number parameter factor, not an object.

2. Step 2: Identify which option passes an object instead of a number

   use:scale={{ factor: 2 }} passes an object { factor: 2 }, which will cause scale(${newFactor}) to become scale([object Object]), an invalid CSS transform value.

3. Final Answer:

   use:scale={{ factor: 2 }} -> Option C

4. Quick Check:

   Passing object instead of number causes invalid transform = C [OK]

Hint: Match parameter type expected by action exactly [OK]

Common Mistakes:

• Passing object when number expected
• Passing string instead of number (allowed but may cause visual issues)
• Passing objects when primitive expected

5. You want to create a Svelte action tooltip that shows a message passed as a parameter and updates the message if it changes. Which of these implementations correctly handles parameter updates?

hard

A. function tooltip(node, message) {
const tip = document.createElement('div');
tip.textContent = message;
node.appendChild(tip);
}

B. function tooltip(node, message) {
node.title = message;
return {};
}

C. function tooltip(node, message) {
node.setAttribute('tooltip', message);
}

D. function tooltip(node, message) {
const tip = document.createElement('span');
tip.textContent = message;
node.appendChild(tip);
return {
update(newMessage) {
tip.textContent = newMessage;
},
destroy() {
node.removeChild(tip);
}
};
}

Solution

1. Step 1: Check for update method handling parameter changes

   function tooltip(node, message) {
     const tip = document.createElement('span');
     tip.textContent = message;
     node.appendChild(tip);
     return {
       update(newMessage) {
         tip.textContent = newMessage;
       },
       destroy() {
         node.removeChild(tip);
       }
     };
   }

   defines an update method that updates the tooltip text when the parameter changes.

2. Step 2: Verify cleanup with destroy method

   This implementation also defines a destroy method to remove the tooltip element when the action is removed, which is good practice.

3. Final Answer:

   The implementation with update and destroy methods -> Option D

4. Quick Check:

   Update method handles parameter changes = A [OK]

Hint: Use update and destroy methods to handle changes and cleanup [OK]

Common Mistakes:

• Not implementing update method for parameter changes
• Not cleaning up DOM elements on destroy
• Using incorrect element types or missing return object