Array and object reactivity gotchas in Svelte - Mini Project: Build & Apply

Array and Object Reactivity Gotchas in Svelte

📖 Scenario: You are building a simple Svelte app to manage a list of tasks. You want to update the UI reactively when tasks are added or modified.

🎯 Goal: Learn how to properly update arrays and objects in Svelte so the UI updates automatically without manual refresh tricks.

📋 What You'll Learn

Create an array of task objects with exact properties

Add a helper variable to track new task input

Update the array reactively when adding a new task

Modify an object property reactively and update the UI

💡 Why This Matters

🌍 Real World

Managing lists of items like tasks, shopping lists, or messages in a web app requires reactive updates to show changes immediately.

💼 Career

Understanding Svelte's reactivity with arrays and objects is essential for building responsive user interfaces in modern web development jobs.

1

Create the initial tasks array

Create a variable called tasks that is an array with these exact objects: { id: 1, text: 'Buy milk', done: false } and { id: 2, text: 'Walk dog', done: true }

Svelte

# Create the tasks array with two objects

// Your code here

Hint

Use let tasks = [ ... ] with two objects inside the array.

2

Add a new task input variable

Add a variable called newTaskText and set it to an empty string '' to hold the new task text input.

Svelte

let tasks = [
  { id: 1, text: 'Buy milk', done: false },
  { id: 2, text: 'Walk dog', done: true }
];

// Add newTaskText variable here

// Your code here

Hint

Declare let newTaskText = ''; below the tasks array.

3

Add a new task reactively

Add a function called addTask that pushes a new object with id as tasks.length + 1, text as newTaskText, and done as false into tasks. Then reassign tasks to itself to trigger reactivity. Finally, reset newTaskText to an empty string.

Svelte

let tasks = [
  { id: 1, text: 'Buy milk', done: false },
  { id: 2, text: 'Walk dog', done: true }
];

let newTaskText = '';

// Add addTask function here

// Your code here

Hint

Remember to reassign tasks = tasks; after pushing to update the UI.

4

Toggle task done status reactively

Add a function called toggleDone that takes a parameter id. Inside, find the task in tasks with matching id and toggle its done property. Then reassign tasks to itself to trigger reactivity.

Svelte

let tasks = [
  { id: 1, text: 'Buy milk', done: false },
  { id: 2, text: 'Walk dog', done: true }
];

let newTaskText = '';

function addTask() {
  tasks.push({ id: tasks.length + 1, text: newTaskText, done: false });
  tasks = tasks;
  newTaskText = '';
}

// Add toggleDone function here

// Your code here

Hint

Use tasks.find() to locate the task and toggle done. Then reassign tasks to trigger UI update.

Practice

1. In Svelte, why does directly modifying an array element like arr[0] = 10 not update the UI?

easy

A. Because Svelte does not support arrays at all.

B. Because arrays are immutable in Svelte.

C. Because you must use a special method to update arrays in Svelte.

D. Because Svelte only tracks assignments to the whole variable, not internal mutations.

Solution

1. Step 1: Understand Svelte's reactivity model

   Svelte updates the UI only when a variable is assigned a new value, not when its internal parts change.

2. Step 2: Analyze direct array element modification

   Changing arr[0] = 10 mutates the array but does not assign a new array to arr, so Svelte does not detect the change.

3. Final Answer:

   Because Svelte only tracks assignments to the whole variable, not internal mutations. -> Option D

4. Quick Check:

   Direct mutation ≠ UI update [OK]

Hint: Always reassign arrays to trigger updates in Svelte [OK]

Common Mistakes:

• Thinking direct mutations trigger updates
• Believing arrays are immutable in Svelte
• Using methods without reassignment

2. Which of the following is the correct way to update an object property reactively in Svelte?

easy

A. obj = { ...obj, key: 'value' };

B. obj.key = 'value';

C. obj.key = 'value'; obj.update();

D. Object.assign(obj, { key: 'value' });

Solution

1. Step 1: Recall Svelte's reactivity for objects

   Svelte detects changes only when the whole object variable is reassigned.

2. Step 2: Evaluate each option

   obj = { ...obj, key: 'value' }; uses spread syntax to create a new object and assign it to obj, triggering reactivity. Direct property assignment like obj.key = 'value'; or Object.assign(obj, { key: 'value' }); mutate the object without reassignment, and obj.key = 'value'; obj.update(); calls a non-existent method.

3. Final Answer:

   obj = { ...obj, key: 'value' }; -> Option A

4. Quick Check:

   Reassign object with spread = reactive update [OK]

Hint: Use spread syntax and reassign objects for reactivity [OK]

Common Mistakes:

• Mutating object properties directly
• Expecting methods like obj.update() to exist
• Using Object.assign without reassignment

3. Given the code:

let numbers = [1, 2, 3];
numbers.push(4);

What will Svelte render if you display {numbers.join(',')} immediately after?

medium

A. 1,2,3,4

B. 1,2,3

C. Error: numbers is not reactive

D. 4

Solution

1. Step 1: Understand array mutation with push()

   The push() method changes the array but does not reassign it.

2. Step 2: Check Svelte's reactivity trigger

   Since numbers is not reassigned, Svelte does not detect the change and the UI remains with the old array.

3. Final Answer:

   1,2,3 -> Option B

4. Quick Check:

   Array mutation without reassignment = no UI update [OK]

Hint: Reassign array after mutation to update UI [OK]

Common Mistakes:

• Assuming push triggers UI update
• Expecting automatic reactivity on mutations
• Confusing errors with silent no-update

4. You have this Svelte code:

let user = { name: 'Alice', age: 25 };

function birthday() {
  user.age += 1;
}

Why does calling birthday() not update the UI showing {user.age}?

medium

A. Because the age property is read-only in Svelte.

B. Because Svelte does not support objects with numeric properties.

C. Because the function does not reassign the user object after mutation.

D. Because the function is missing an await keyword.

Solution

1. Step 1: Analyze mutation inside birthday()

   The function increments user.age directly, mutating the object.

2. Step 2: Check reactivity requirements

   Svelte requires reassignment of the whole object variable to detect changes; direct mutation alone does not trigger UI updates.

3. Final Answer:

   Because the function does not reassign the user object after mutation. -> Option C

4. Quick Check:

   Direct mutation without reassignment = no UI update [OK]

Hint: Always reassign objects after changing properties [OK]

Common Mistakes:

• Thinking numeric properties are special
• Assuming await is needed for sync code
• Believing direct mutation triggers UI update

5. How can you correctly update the first element of an array items reactively in Svelte?

hard

A. items = [...items]; items[0] = 'new';

B. items = ['new', ...items.slice(1)];

C. items[0] = 'new'; items = items;

D. items = items; items[0] = 'new';

Solution

1. Step 1: Understand reactivity with arrays

   Direct mutation like items[0] = 'new' does not trigger updates unless followed by reassignment.

2. Step 2: Evaluate each option

   items = ['new', ...items.slice(1)]; creates a new array with the first element replaced, triggering reactivity correctly. items = [...items]; items[0] = 'new'; creates a shallow copy of the array first, then mutates the first element, which is reactive because the reassignment happens before mutation and the array reference changes. items[0] = 'new'; items = items; mutates first then reassigns the same array reference, which may not trigger reactivity reliably. items = items; items[0] = 'new'; reassigns the same array reference then mutates, which does not trigger reactivity.

3. Final Answer:

   items = [...items]; items[0] = 'new'; -> Option A

4. Quick Check:

   Reassign array with spread then mutate = reactive update [OK]

Hint: Reassign array with spread before mutation for reactivity [OK]

Common Mistakes:

• Mutating after reassignment
• Reassigning without changes
• Mutating before reassignment expecting update