Isolated Logic Validation for Runes

Snippet

Isolated Logic Validation for Runes

Expert Svelte testing involves validating reactive logic in isolation. Because runes like $derived and $effect update asynchronously for performance, flushSync must be used in unit tests to force microtask execution before asserting the results.

snippet.js
javascript
// logic.test.ts
import { flushSync } from 'svelte';
 
test('reactive increment', () => {
  let count = $state(0);
  let double = $derived(count * 2);
  
  count = 5;
  flushSync();
  expect(double).toBe(10);
});

svelte

Breakdown

import { flushSync } from 'svelte'

Imports the utility required to synchronize reactive updates in a non-component environment.

flushSync()

Immediately processes all pending state changes and effects.

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Isolated Logic Validation for Runes

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