Action update and destroy in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a Svelte action that logs when an element is mounted.

Svelte

function logAction(node) {
  console.log('Element mounted');
  return {
    [1]() {
      console.log('Element removed');
    }
  };
}

Drag options to blanks, or click blank then click option'

Aremove

Bupdate

Cinit

Ddestroy

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' instead of 'destroy' for cleanup.

Using a non-existent method like 'remove'.

The destroy method runs when the element is removed from the DOM.

2fill in blank

medium

Complete the code to update the action when the parameter changes.

Svelte

function colorAction(node, color) {
  node.style.color = color;
  return {
    [1](newColor) {
      node.style.color = newColor;
    }
  };
}

Drag options to blanks, or click blank then click option'

Achange

Bdestroy

Cupdate

Drefresh

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'destroy' instead of 'update' to handle parameter changes.

Using non-standard method names like 'change' or 'refresh'.

The update method runs when the parameter changes, allowing the action to update the element.

3fill in blank

hard

Fix the error in the action to properly clean up when the element is removed.

Svelte

function focusAction(node) {
  node.focus();
  return {
    [1]() {
      node.blur();
    }
  };
}

Drag options to blanks, or click blank then click option'

Adestroy

Bupdate

Ccleanup

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' for cleanup instead of 'destroy'.

Using non-existent methods like 'cleanup' or 'remove'.

The destroy method is the correct place to clean up when the element is removed.

4fill in blank

hard

Fill both blanks to create an action that updates color and cleans up style on destroy.

Svelte

function styleAction(node, color) {
  node.style.color = color;
  return {
    [1](newColor) {
      node.style.color = newColor;
    },
    [2]() {
      node.style.color = '';
    }
  };
}

Drag options to blanks, or click blank then click option'

Aupdate

Bdestroy

Cremove

Dreset

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the roles of update and destroy methods.

Using non-standard method names like 'remove' or 'reset'.

update changes the color when parameters change, and destroy cleans up the style when the element is removed.

5fill in blank

hard

Fill all three blanks to create an action that sets font size, updates it, and cleans up on destroy.

Svelte

function fontSizeAction(node, size) {
  node.style.fontSize = [1];
  return {
    [2](newSize) {
      node.style.fontSize = newSize;
    },
    [3]() {
      node.style.fontSize = '';
    }
  };
}

Drag options to blanks, or click blank then click option'

Asize

Bupdate

Cdestroy

DfontSize

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect variable names for the size.

Mixing up update and destroy method names.

The initial font size is set using the size parameter. The update method changes it when parameters change, and destroy cleans up when the element is removed.

Practice

1. What is the main purpose of the destroy function in a Svelte action?

easy

A. To update the element's attributes when parameters change

B. To initialize the action when the element is created

C. To clean up resources when the element is removed from the DOM

D. To render the element on the page

Solution

1. Step 1: Understand the role of destroy in Svelte actions

   The destroy function is called when the element using the action is removed from the DOM to clean up any side effects or event listeners.

2. Step 2: Differentiate from other lifecycle functions

   update handles parameter changes, and initialization happens in the main action function, not in destroy.

3. Final Answer:

   To clean up resources when the element is removed from the DOM -> Option C

4. Quick Check:

   destroy cleans up on removal = A [OK]

Hint: Destroy cleans up when element leaves the page [OK]

Common Mistakes:

• Confusing destroy with update
• Thinking destroy initializes the action
• Assuming destroy runs on parameter change

2. Which of the following is the correct syntax for returning an update and destroy function from a Svelte action?

easy

A. return [ update(params) {}, destroy() {} ];

B. return { update: (params) => {}, destroy: () => {} };

C. return function update(params) {}, function destroy() {};

D. return { onUpdate: () => {}, onDestroy: () => {} };

Solution

1. Step 1: Recall the Svelte action return structure

   A Svelte action returns an object with optional update and destroy methods as properties.

2. Step 2: Match the correct syntax

   return { update: (params) => {}, destroy: () => {} }; correctly returns an object with update and destroy as arrow functions. Other options use invalid syntax or wrong property names.

3. Final Answer:

   return { update: (params) => {}, destroy: () => {} }; -> Option B

4. Quick Check:

   Return object with update and destroy keys = B [OK]

Hint: Return an object with update and destroy keys [OK]

Common Mistakes:

• Using arrays instead of objects
• Wrong function syntax inside return
• Using incorrect property names like onUpdate

3. Given this Svelte action code:

function highlight(node, color) {
  node.style.backgroundColor = color;
  return {
    update(newColor) {
      node.style.backgroundColor = newColor;
    },
    destroy() {
      node.style.backgroundColor = '';
    }
  };
}

What happens when the action's parameter changes from 'yellow' to 'lightblue'?

medium

A. The background color changes to 'lightblue'

B. The background color resets to empty string

C. Nothing changes because update is not called

D. The action throws an error

Solution

1. Step 1: Understand the update function role

   The update method runs when the action's parameter changes, here it sets the background color to the new value.

2. Step 2: Trace the parameter change effect

   Changing from 'yellow' to 'lightblue' triggers update, which sets node.style.backgroundColor to 'lightblue'.

3. Final Answer:

   The background color changes to 'lightblue' -> Option A

4. Quick Check:

   Parameter change triggers update = D [OK]

Hint: Update runs on parameter change to update styles [OK]

Common Mistakes:

• Thinking destroy runs on parameter change
• Assuming update does nothing
• Confusing initial setup with update

4. Identify the error in this Svelte action code:

function logClicks(node) {
  function handleClick() {
    console.log('Clicked');
  }
  node.addEventListener('click', handleClick);
  return {
    update() {
      node.removeEventListener('click', handleClick);
    }
  };
}

medium

A. The update function incorrectly removes the event listener on every parameter change

B. The handleClick function is not defined properly

C. The event listener is added multiple times causing duplicates

D. The destroy function is missing to remove the event listener on element removal

Solution

1. Step 1: Analyze the event listener management

   The action adds a click listener but does not clean it up when the element is removed.

2. Step 2: Check lifecycle methods usage

   There is no destroy method to remove the listener on element removal, which can cause memory leaks.

3. Final Answer:

   The destroy function is missing to remove the event listener on element removal -> Option D

4. Quick Check:

   Missing destroy cleanup = C [OK]

Hint: Always clean event listeners in destroy [OK]

Common Mistakes:

• Removing listeners in update instead of destroy
• Not removing listeners at all
• Confusing update with destroy lifecycle

5. You want to create a Svelte action that changes an element's text color based on a parameter and logs a message when the element is removed. Which code correctly implements update and destroy?

hard

A. function colorText(node, color) {
node.style.color = color;
return {
update(newColor) {
node.style.color = newColor;
},
destroy() {
console.log('Element removed');
}
};
}

B. function colorText(node, color) {
node.style.color = color;
return {
update() {
console.log('Color updated');
},
destroy() {
node.style.color = '';
}
};
}

C. function colorText(node, color) {
node.style.color = color;
return {
update(newColor) {
node.style.backgroundColor = newColor;
},
destroy() {
console.log('Element removed');
}
};
}

D. function colorText(node, color) {
node.style.color = color;
return {
update(newColor) {
node.style.color = newColor;
}
};
}

Solution

1. Step 1: Check update function correctness

   The update method should take a newColor parameter and set node.style.color = newColor.

2. Step 2: Verify destroy function behavior

   The destroy method should console.log('Element removed').

3. Final Answer:

   function colorText(node, color) { node.style.color = color; return { update(newColor) { node.style.color = newColor; }, destroy() { console.log('Element removed'); } }; } -> Option A

4. Quick Check:

   Update changes color, destroy logs message = A [OK]

Hint: Update changes style, destroy cleans or logs [OK]

Common Mistakes:

• Updating wrong style property
• Missing destroy method
• Destroy resets style instead of logging