Derived Stores
All Svelte topics∙ Svelte
Derived Stores explains a store calculated from one or more source stores for this derived, stores lesson. You will learn its exact Svelte rule, failure mode, verification plan, and production evidence.
Syntax
const doubled = derived(count, value => value * 2);Example
// Topic: Derived Stores
const count = 4;
const doubled = count * 2;
console.log(doubled);
// Expected Output: 8Expected Output
8Line-by-line
| Line | Meaning |
|---|---|
const count = 4; | Defines state, behavior, or output for this Svelte example. |
const doubled = count * 2; | Defines state, behavior, or output for this Svelte example. |
console.log(doubled); | Prints the expected result for this Svelte lesson. |
Real-World Uses
- 1Derived Stores is used for authentication, carts, preferences, and cross-route state.
- 2Its mechanism is a store calculated from one or more source stores for this derived, stores lesson.
- 3Keep derivation pure and avoid duplicating source state manually. Keep decisions specific to derived, stores.
- 4Production code must account for Writing back to source stores from derivation can create loops. Do not copy assumptions from a neighboring topic into derived, stores.
- 5Teams evaluate it using derived consistency measured for derived, stores.
Common Mistakes
- 1Writing back to source stores from derivation can create loops. Do not copy assumptions from a neighboring topic into derived, stores.
- 2Implementing Derived Stores without understanding a store calculated from one or more source stores for this derived, stores lesson.
- 3Choosing Derived Stores where simpler local Svelte code is clearer.
- 4Skipping Update every source and verify output, cleanup, and asynchronous derivation. Include an assertion that directly exercises derived, stores.
- 5Optimizing before measuring derived consistency measured for derived, stores.
Best Practices
- 1Keep derivation pure and avoid duplicating source state manually. Keep decisions specific to derived, stores.
- 2Document a store calculated from one or more source stores for this derived, stores lesson in the smallest useful component, store, action, route, or service.
- 3Represent every relevant loading, success, empty, denied, and failure state.
- 4Update every source and verify output, cleanup, and asynchronous derivation. Include an assertion that directly exercises derived, stores.
- 5Use derived consistency measured for derived, stores to guide improvements.
How it works
- 1Derived Stores relies on a store calculated from one or more source stores for this derived, stores lesson.
- 2Keep derivation pure and avoid duplicating source state manually. Keep decisions specific to derived, stores.
- 3Its main failure mode is Writing back to source stores from derivation can create loops. Do not copy assumptions from a neighboring topic into derived, stores.
- 4Useful evidence is derived consistency measured for derived, stores.
Implementation decisions
- 1Identify the owning component, store, action, route, load function, or server handler.
- 2Keep state local until multiple owners genuinely need it.
- 3Keep server secrets and validation outside browser components.
- 4Define cleanup for subscriptions, actions, timers, and requests.
Verification plan
- 1Update every source and verify output, cleanup, and asynchronous derivation. Include an assertion that directly exercises derived, stores.
- 2Check initial render, assignment-driven updates, user interaction, and cleanup.
- 3Confirm keyboard and screen-reader behavior for visible UI.
- 4Measure production output only after correctness passes.
Practice task
- 1Build the smallest Derived Stores example.
- 2Introduce this failure: Writing back to source stores from derivation can create loops. Do not copy assumptions from a neighboring topic into derived, stores.
- 3Correct it using this rule: Keep derivation pure and avoid duplicating source state manually. Keep decisions specific to derived, stores.
- 4Record derived consistency measured for derived, stores before and after the change.
Quick Summary
- Derived Stores works through a store calculated from one or more source stores for this derived, stores lesson.
- Keep derivation pure and avoid duplicating source state manually. Keep decisions specific to derived, stores.
- Avoid Writing back to source stores from derivation can create loops. Do not copy assumptions from a neighboring topic into derived, stores.
- Update every source and verify output, cleanup, and asynchronous derivation. Include an assertion that directly exercises derived, stores.
- Measure success with derived consistency measured for derived, stores.
Interview Questions
Q1. What is Derived Stores used for?
Answer: It is used for authentication, carts, preferences, and cross-route state.
Q2. How does Derived Stores work in Svelte?
Answer: It works through a store calculated from one or more source stores for this derived, stores lesson.
Q3. What rule matters most?
Answer: Keep derivation pure and avoid duplicating source state manually. Keep decisions specific to derived, stores.
Q4. What failure is common?
Answer: Writing back to source stores from derivation can create loops. Do not copy assumptions from a neighboring topic into derived, stores.
Q5. How should it be verified?
Answer: Update every source and verify output, cleanup, and asynchronous derivation. Include an assertion that directly exercises derived, stores. Evaluate derived consistency measured for derived, stores.
Quiz
Which practice best supports Derived Stores?