Default Prop Values
Assign fallback values to props when the parent does not supply them.
Default Prop Values is a free Sveltejs Academy lesson on CoddyKit. This is lesson 3 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.
Why Defaults
Defaults make a component usable without every consumer passing every prop. They define the fallback behavior.
Inline Default
Assign a value when declaring the prop.
export let label = "Click me";Function Defaults
Default values may be expressions or function literals.
export let onClick = () => {};Object Defaults
Use an object literal for complex defaults.
export let options = { color: "blue", size: "md" };Watch the Shared Reference
Object literal defaults are evaluated per instance, so each component gets its own object. Safe to mutate.
Override at Parent
When the parent provides a value, it replaces the default for that instance.
<Button label="Save" />Required Detection
Without a default, the prop is conceptually required. TypeScript can enforce this with the required hint.
null and undefined
Passing undefined as a value causes the default to apply; passing null does not.
Composing Defaults
Compose defaults from other props with reactive declarations.
export let title = "";
export let subtitle;
$: effectiveSubtitle = subtitle ?? title;Documentation
Document defaults in JSDoc comments to help IDE tooltips show them.
/** @param {string} label */Stable Defaults
Avoid defaults that change between renders. Stable defaults reduce re-renders and surprise behavior.
Quick Check
How do you set a default prop value?
Recap
Provide defaults with plain assignment. They make components self-sufficient and clarify which props are optional.
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Frequently Asked Questions
Is the “Default Prop Values” lesson free?
Yes — the full text of “Default Prop Values” 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 “Default Prop Values”?
Assign fallback values to props when the parent does not supply them. 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 3 of 4.
How long does the “Default Prop Values” 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
- Declaring Props with export let
- Passing Props from Parent
- Default Prop Values
- Forwarding Props with $$props and $$restProps