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chore: avoid microtasks when flushing sync #15855

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chore: avoid microtasks when flushing sync #15855

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eb80b77
chore: avoid microtasks when flushing sync
trueadm Apr 30, 2025
479cf5d
chore: avoid microtasks when flushing sync
trueadm Apr 30, 2025
359dc16
chore: avoid microtasks when flushing sync
trueadm Apr 30, 2025
425a8d8
Merge branch 'main' into flush-sync
Rich-Harris May 12, 2025
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5 changes: 5 additions & 0 deletions .changeset/polite-melons-tickle.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
'svelte': patch
---

chore: avoid microtasks when flushing sync
36 changes: 23 additions & 13 deletions packages/svelte/src/internal/client/runtime.js
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Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,7 @@ const handled_errors = new WeakSet();
let is_throwing_error = false;

let is_flushing = false;
let is_flushing_sync = false;

/** @type {Effect | null} */
let last_scheduled_effect = null;
Expand Down Expand Up @@ -734,7 +735,9 @@ function flush_queued_effects(effects) {
export function schedule_effect(signal) {
if (!is_flushing) {
is_flushing = true;
queueMicrotask(flush_queued_root_effects);
if (!is_flushing_sync) {
queueMicrotask(flush_queued_root_effects);
}
}

var effect = (last_scheduled_effect = signal);
Expand Down Expand Up @@ -818,23 +821,30 @@ function process_effects(root) {
* @returns {T}
*/
export function flushSync(fn) {
var result;
var previously_flushing_sync = is_flushing_sync;

if (fn) {
is_flushing = true;
flush_queued_root_effects();
result = fn();
}
try {
var result;
is_flushing_sync = true;

flush_tasks();
if (fn) {
is_flushing = true;
flush_queued_root_effects();
result = fn();
}

while (queued_root_effects.length > 0) {
is_flushing = true;
flush_queued_root_effects();
flush_tasks();
}

return /** @type {T} */ (result);
while (queued_root_effects.length > 0) {
is_flushing = true;
flush_queued_root_effects();
flush_tasks();
}

return /** @type {T} */ (result);
} finally {
is_flushing_sync = previously_flushing_sync;
}
}

/**
Expand Down