Custom transitions in Svelte

Introduction

Custom transitions let you create smooth animations when elements appear or disappear. They make your app feel lively and polished.

When you want to animate a box fading in and sliding from the side.

When you want to add a bounce effect to a button when it appears.

When you want to smoothly change an element's size while it enters or leaves.

When you want to combine multiple animation effects for a unique look.

When you want to control how long and how an element transitions on screen.

Syntax

Svelte

import { cubicOut } from 'svelte/easing';

function customTransition(node, { duration = 400 }) {
  return {
    duration,
    css: t => `opacity: ${t}; transform: translateX(${(1 - t) * 100}px);`,
    easing: cubicOut
  };
}

The node is the HTML element to animate.

The function returns an object with duration, css for styles, and optional easing.

Examples

A simple fade and slide from left effect.

Svelte

function fadeAndSlide(node, { duration = 500 }) {
  return {
    duration,
    css: t => `opacity: ${t}; transform: translateX(${(1 - t) * 50}px);`
  };
}

Scales the element up from 0 to full size while fading in.

Svelte

function scaleUp(node, { duration = 300 }) {
  return {
    duration,
    css: t => `transform: scale(${t}); opacity: ${t};`
  };
}

Adds a bounce effect by moving the element up and down while fading in.

Svelte

function bounce(node, { duration = 600 }) {
  return {
    duration,
    css: t => `transform: translateY(${Math.sin(t * Math.PI) * -20}px); opacity: ${t};`
  };
}

Sample Program

This Svelte component shows a green box that appears and disappears with a custom fade and slide transition. The button toggles the box. The transition moves the box from right to left while fading it in or out.

Accessibility is included with aria-pressed, aria-live, and keyboard focus.

Svelte

<script>
  import { cubicOut } from 'svelte/easing';
  import { onMount } from 'svelte';

  let visible = false;

  function customFadeSlide(node, { duration = 400 }) {
    return {
      duration,
      easing: cubicOut,
      css: t => `opacity: ${t}; transform: translateX(${(1 - t) * 100}px);`
    };
  }

  onMount(() => {
    visible = true;
  });
</script>

<button on:click={() => visible = !visible} aria-pressed={visible} aria-label="Toggle box">
  Toggle Box
</button>

{#if visible}
  <div
    transition:customFadeSlide={{ duration: 600 }}
    role="region"
    aria-live="polite"
    tabindex="0"
    style="width: 200px; height: 100px; background-color: #4caf50; color: white; display: flex; align-items: center; justify-content: center; margin-top: 1rem; border-radius: 0.5rem;"
  >
    Hello with custom transition!
  </div>
{/if}

Important Notes

Custom transitions let you control exactly how elements animate.

Use css function to write styles that change over time from 0 to 1.

Remember to add accessibility attributes like aria-live and keyboard focus for dynamic content.

Summary

Custom transitions create smooth, unique animations for elements entering or leaving.

You write a function that returns styles changing over time.

They help make your app feel friendly and polished.

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