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Synchronously flush any pending state updates and their effects. If a callback function is provided, it will be called first, and then all resulting updates will be flushed synchronously. 
function flushSync<T>(fn?: () => T): T

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Parameters

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fn
() => T
Optional callback function to execute before flushing. Any state changes made in this function will be flushed synchronously before flushSync returns

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Returns

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result
T | void
If a callback is provided, returns the result of calling the callback. Otherwise returns void

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Examples

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Basic synchronous updates

<script>
  import { flushSync } from 'svelte';

  let count = $state(0);

  function increment() {
    flushSync(() => {
      count++;
    });
    // DOM is guaranteed to be updated here
    console.log('Count in DOM:', document.getElementById('count').textContent);
  }
</script>

<p id="count">{count}</p>
<button onclick={increment}>Increment</button>

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Measuring DOM after update

<script>
  import { flushSync } from 'svelte';

  let items = $state([1, 2, 3]);
  let listHeight = $state(0);

  function addItem() {
    flushSync(() => {
      items.push(items.length + 1);
    });
    
    // Measure the new height immediately after update
    const list = document.getElementById('list');
    listHeight = list.offsetHeight;
  }
</script>

<ul id="list">
  {#each items as item}
    <li>{item}</li>
  {/each}
</ul>
<button onclick={addItem}>Add item</button>
<p>List height: {listHeight}px</p>

​
Focusing elements after render

<script>
  import { flushSync } from 'svelte';

  let showInput = $state(false);

  function reveal() {
    flushSync(() => {
      showInput = true;
    });
    
    // Input is now in the DOM and can be focused
    document.getElementById('username').focus();
  }
</script>

<button onclick={reveal}>Show input</button>
{#if showInput}
  <input id="username" type="text" />
{/if}

​
Synchronizing with external libraries

<script>
  import { flushSync } from 'svelte';
  import { initializeChart } from './chart-library';

  let data = $state([1, 2, 3, 4, 5]);
  let chartContainer;

  function updateChart() {
    flushSync(() => {
      data = data.map(x => x * 2);
    });
    
    // DOM is updated, safe to reinitialize chart
    initializeChart(chartContainer, data);
  }
</script>

<div bind:this={chartContainer}></div>
<button onclick={updateChart}>Update chart</button>

​
Sequential updates

<script>
  import { flushSync } from 'svelte';

  let step = $state(1);

  function runSteps() {
    flushSync(() => step = 1);
    console.log('Step 1 rendered');
    
    flushSync(() => step = 2);
    console.log('Step 2 rendered');
    
    flushSync(() => step = 3);
    console.log('Step 3 rendered');
  }
</script>

<p>Current step: {step}</p>
<button onclick={runSteps}>Run all steps</button>

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Without callback

import { flushSync } from 'svelte';

let count = $state(0);

// Update state
count++;

// Flush all pending updates
flushSync();

// DOM is now updated
console.log('Updated!');

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Use cases

  • DOM measurements: When you need to measure element dimensions immediately after a state change
  • Focus management: Setting focus on elements that are conditionally rendered
  • Third-party integrations: Synchronizing Svelte state updates with external libraries that expect synchronous DOM updates
  • Testing: Ensuring state updates complete before assertions run
  • Animations: Starting animations that depend on the final rendered state

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Performance considerations

  • flushSync() is synchronous and can block the main thread if there are many updates
  • Use sparingly, as it bypasses Svelte’s optimized batching of updates
  • In most cases, tick() is preferred for waiting on asynchronous updates
  • Multiple rapid calls to flushSync() can hurt performance

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Notes

  • Unlike tick(), which returns a Promise, flushSync() is completely synchronous
  • All effects and DOM updates are guaranteed to complete before flushSync() returns
  • If called without a callback, it flushes any pending updates that were scheduled before the call
  • This is useful when you need immediate DOM updates, but should be used judiciously