Writing a Custom Transition Function
Return CSS or tick functions from a custom transition to animate DOM entry/exit.
Writing a Custom Transition Function is a free Sveltejs Academy lesson on CoddyKit. This is lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Sveltejs Academy learning path, and your progress syncs across the web and the CoddyKit app. The Sveltejs Academy course includes 4 lessons in total.
Custom Transitions
Beyond built-ins, you can write fully custom transition functions.
Function Signature
A transition function takes (node, params) and returns an options object.
function spin(node) {
return {
duration: 600,
css: (t) => `transform: rotate(${t * 360}deg); opacity: ${t};`
};
}Use Your Transition
Apply with transition:spin on any element.
CSS Function
Return a css(t, u) function. t goes 0..1; u goes 1..0.
JS Function (tick)
For values not animatable by CSS, return a tick function instead.
tick: (t) => node.textContent = Math.floor(t * 100) + "%";When to Use CSS vs Tick
CSS is faster (GPU-accelerated). Use tick only when CSS cannot express the change.
Easing
Apply easing inside your function or accept it as a parameter.
Delay and Duration
Return delay and duration in the options object.
Reusable Library
Build a transitions module for your design system.
Direction Awareness
The framework calls your function on enter; an opposite direction is used on exit.
Combine With in/out
You can use one custom function for enter and another for exit via in: and out:.
Quick Check
What does the css function in a transition receive?
Recap
Define custom transitions by returning { duration, css } or { duration, tick } from a function.
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Frequently Asked Questions
Is the “Writing a Custom Transition Function” lesson free?
Yes — the full text of “Writing a Custom Transition Function” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Sveltejs Academy course, upgrade to CoddyKit PRO. The Sveltejs Academy course includes 4 lessons in total.
What will I learn in “Writing a Custom Transition Function”?
Return CSS or tick functions from a custom transition to animate DOM entry/exit. You practise Sveltejs Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Sveltejs Academy?
No prior experience is required. Sveltejs Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.
How long does the “Writing a Custom Transition Function” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Sveltejs Academy lesson?
Yes. Every Sveltejs Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Writing a Custom Transition Function
- Transition Parameters: node duration delay
- Creating a morphing Transition
- CSS vs JS Transitions