Custom transitions in Svelte - Mini Project: Build & Apply

Custom transitions

📖 Scenario: You are building a simple Svelte app where a message appears and disappears with a smooth custom animation. This helps make the app feel lively and friendly.

🎯 Goal: Create a Svelte component that shows a message with a custom fade and slide transition when it appears and disappears.

📋 What You'll Learn

Create a boolean variable visible to control message visibility

Create a custom transition function called fadeSlide

Use the transition:fadeSlide directive on the message element

Add a button to toggle the visible variable

💡 Why This Matters

🌍 Real World

Custom transitions make web apps feel more dynamic and polished by animating elements smoothly when they appear or disappear.

💼 Career

Knowing how to create and apply custom transitions is valuable for frontend developers to improve user experience and interface polish.

1

DATA SETUP: Create a visible variable

Create a boolean variable called visible and set it to true inside the <script> tag.

Svelte

  // Create a boolean variable called visible and set it to true
  // Your code here


Toggle Message

{#if visible}
  Hello, welcome to Svelte!
{/if}

Hint

Use let visible = true; inside the <script> tag.

2

CONFIGURATION: Create a custom transition function fadeSlide

Inside the <script> tag, create a function called fadeSlide that takes a parameter node and returns an object with duration: 400 and a css function that returns a string combining opacity and transform: translateX based on t. Use t to fade from 0 to 1 and slide from 20px to 0px.

Svelte

  let visible = true;

  // Create a custom transition function called fadeSlide
  // Your code here


Toggle Message

{#if visible}
  Hello, welcome to Svelte!
{/if}

Hint

Define fadeSlide returning an object with duration and css properties. Use t to control opacity and horizontal slide.

3

CORE LOGIC: Use the fadeSlide transition on the message

Add the directive transition:fadeSlide to the <p> element inside the {#if visible} block.

Svelte

  let visible = true;

  function fadeSlide(node) {
    return {
      duration: 400,
      css: t => `opacity: ${t}; transform: translateX(${(1 - t) * 20}px);`
    };
  }


Toggle Message

{#if visible}
   element -->
  Hello, welcome to Svelte!
{/if}

Hint

Use transition:fadeSlide on the <p> tag.

4

COMPLETION: Add a button click to toggle visible

Add an on:click event to the <button> that toggles the visible variable using visible = !visible.

Svelte

  let visible = true;

  function fadeSlide(node) {
    return {
      duration: 400,
      css: t => `opacity: ${t}; transform: translateX(${(1 - t) * 20}px);`
    };
  }



Toggle Message

{#if visible}
  Hello, welcome to Svelte!
{/if}

Hint

Add on:click={() => visible = !visible} to the <button> tag.

Practice

1. What is the main purpose of a custom transition in Svelte?

easy

A. To create unique animations when elements enter or leave the page

B. To fetch data from an API

C. To manage component state

D. To handle user input events

Solution

1. Step 1: Understand what transitions do in Svelte

   Transitions animate elements as they appear or disappear from the DOM.

2. Step 2: Identify the role of custom transitions

   Custom transitions let you define your own animation styles for these changes.

3. Final Answer:

   To create unique animations when elements enter or leave the page -> Option A

4. Quick Check:

   Custom transitions = unique enter/leave animations [OK]

Hint: Custom transitions animate element entry/exit smoothly [OK]

Common Mistakes:

• Confusing transitions with data fetching
• Thinking transitions manage state
• Mixing transitions with event handling

2. Which of the following is the correct way to define a custom transition function in Svelte?

easy

A. function fade(node) { return { delay: 300, css: t => `opacity: ${t}` }; }

B. function fade(params) { return { duration: 300, css: t => `opacity: ${t}` }; }

C. function fade() { return { duration: 300, css: t => `opacity: ${t}` }; }

D. function fade(node, params) { return { duration: 300, css: t => `opacity: ${t}` }; }

Solution

1. Step 1: Recall the custom transition function signature

   It must accept node and optionally params.

2. Step 2: Check the returned object structure

   The object should include duration and a css function using t for progress.

3. Final Answer:

   function fade(node, params) { return { duration: 300, css: t => `opacity: ${t}` }; } -> Option D

4. Quick Check:

   Function(node, params) + {duration, css} = C [OK]

Hint: Custom transition needs node param and returns duration + css [OK]

Common Mistakes:

• Omitting the node parameter
• Returning delay instead of duration
• Not using a css function with progress t

3. Given this custom transition function:

function slide(node) {
  return {
    duration: 400,
    css: t => `transform: translateX(${(1 - t) * 100}%)`
  };
}

What happens to the element when it enters the DOM using transition:slide?

medium

A. It fades in with increasing opacity

B. It slides out to the right when leaving

C. It slides in from the right to its original position

D. It instantly appears without animation

Solution

1. Step 1: Analyze the css function for entry

   The transform moves from translateX(100%) at t=0 to translateX(0%) at t=1.

2. Step 2: Understand the visual effect

   The element starts off-screen right (100%) and slides left to original place (0%).

3. Final Answer:

   It slides in from the right to its original position -> Option C

4. Quick Check:

   translateX from 100% to 0% = slide in right to left [OK]

Hint: Check transform values at t=0 and t=1 for motion direction [OK]

Common Mistakes:

• Confusing enter and leave animations
• Assuming opacity changes instead of transform
• Thinking it slides out instead of in

4. This custom transition code causes an error:

function grow(node) {
  return {
    duration: 500,
    css: t => `scale(${t})`
  };
}

What is the error and how to fix it?

medium

A. Missing 'transform:' prefix in css string; fix by returning `transform: scale(${t})`

B. Duration must be a string; fix by changing 500 to '500ms'

C. Function must return a number, not a string; fix by removing css

D. Node parameter is unused; fix by removing it

Solution

1. Step 1: Identify the CSS syntax error

   The css string lacks the CSS property name; 'scale' alone is invalid.

2. Step 2: Correct the CSS string

   It should be 'transform: scale(...)' to apply scaling properly.

3. Final Answer:

   Missing 'transform:' prefix in css string; fix by returning `transform: scale(${t})` -> Option A

4. Quick Check:

   CSS property needed before scale() [OK]

Hint: CSS string must include property name like 'transform:' [OK]

Common Mistakes:

• Returning only scale() without 'transform:'
• Using wrong duration type
• Removing needed node parameter

5. You want a custom transition that fades in and grows an element from half size to full size over 600ms. Which function correctly implements this?

hard

A. function fadeGrow(node) { return { duration: 600, css: t => `opacity: ${1 - t}; transform: scale(${t})` }; }

B. function fadeGrow(node) { return { duration: 600, css: t => `opacity: ${t}; transform: scale(${0.5 + 0.5 * t})` }; }

C. function fadeGrow(node) { return { duration: 600, css: t => `opacity: ${t}; transform: scale(${t / 2})` }; }

D. function fadeGrow(node) { return { duration: 600, css: t => `opacity: ${t}; transform: scale(${1 - t})` }; }

Solution

1. Step 1: Understand the fade and grow requirements

   Opacity should go from 0 to 1 (fade in), scale from 0.5 to 1 (grow).

2. Step 2: Check each option's css function

   function fadeGrow(node) { return { duration: 600, css: t => `opacity: ${t}; transform: scale(${0.5 + 0.5 * t})` }; } uses opacity = t (0 to 1) and scale = 0.5 + 0.5 * t (0.5 to 1), matching requirements.

3. Final Answer:

   function fadeGrow(node) { return { duration: 600, css: t => `opacity: ${t}; transform: scale(${0.5 + 0.5 * t})` }; } -> Option B

4. Quick Check:

   Opacity 0->1 and scale 0.5->1 = function fadeGrow(node) { return { duration: 600, css: t => `opacity: ${t}; transform: scale(${0.5 + 0.5 * t})` }; } [OK]

Hint: Map t from 0-1 to desired opacity and scale ranges [OK]

Common Mistakes:

• Reversing opacity or scale ranges
• Using scale starting at 0 instead of 0.5
• Mixing up increasing and decreasing values