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Per-tick locale memoization, error isolation in dateObserver, sync connectedCallback #369

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Initial plan
Copilot Jul 30, 2026
e94af3e
Restore sync connectedCallback, add per-tick locale memoization and e…
Copilot Jul 30, 2026
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137 changes: 88 additions & 49 deletions src/relative-time-element.ts
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -98,6 +98,50 @@ if (typeof window !== 'undefined' && typeof window.addEventListener === 'functio
})
}

// Per-pass memoization of locale/timeZone/hourCycle resolution.
//
// During a single synchronous `dateObserver.update()` pass every element calls
// `update()`, which resolves `#lang`, `timeZone`, and `hourCycle` — each
// performing a `closest()` ancestor walk. For N elements on a typical page
// all of them inherit the same values from `<html>`, so doing N independent
// walks is pure redundancy.
//
// The cache is valid only for the duration of one synchronous pass. It is
// enabled immediately before the element loop starts and cleared in the
// `finally` block so it is always discarded, even if an element throws.
//
// Safety invariant: elements that carry `lang`, `time-zone`, or `hour-cycle`
// directly on themselves are excluded from the cache (their own attribute
// overrides the parent-chain value, so siblings under the same parent may
// resolve differently). For all other elements, siblings under the same
// `parentNode` share an identical ancestor chain, so one walk per unique
// `parentNode` is sufficient.
let localeCacheEnabled = false
type ResolvedLocale = {
locale: string
timeZone: string | undefined
hourCycle: Intl.DateTimeFormatOptions['hourCycle']
}
const localePassCache = new Map<Node, ResolvedLocale>()

// Memoize `intlLocale(rawLang).toString()` across calls. The normalisation is
// stable for a given raw tag string, so this map is never cleared (unlike the
// Intl formatter caches which must be invalidated on `languagechange`).
const localeNormCache = new Map<string, string>()

function normalizeLang(lang: string): string {
let normalized = localeNormCache.get(lang)
if (normalized === undefined) {
try {
normalized = intlLocale(lang).toString()
} catch {
normalized = 'default'
}
localeNormCache.set(lang, normalized)
}
return normalized
}

const dateObserver = new (class {
elements: Set<RelativeTimeElement> = new Set()
time = Infinity
Expand Down Expand Up @@ -130,38 +174,32 @@ const dateObserver = new (class {

let nearestDistance = Infinity
this.updating = true
localeCacheEnabled = true
try {
for (const timeEl of this.elements) {
nearestDistance = Math.min(nearestDistance, getUnitFactor(timeEl))
timeEl.update()
try {
timeEl.update()
} catch (error) {
// Rethrow asynchronously so one element failing does not abort the
// remaining elements in this pass, while still surfacing the error
// as a global uncaught exception.
setTimeout(() => {
throw error
})
}
}
} finally {
this.updating = false
localeCacheEnabled = false
localePassCache.clear()
}
this.time = Math.min(60 * 60 * 1000, nearestDistance)
this.timer = setTimeout(() => this.update(), this.time)
this.time += Date.now()
}
})()

// Batch the initial render of newly-connected elements into a single microtask
// flush. When N elements are inserted together, this avoids N synchronous
// formatting passes on the insertion critical path.
const pendingElements: Set<RelativeTimeElement> = new Set()
let pendingFlush = false

async function flushPending(): Promise<void> {
await Promise.resolve()
// Snapshot and clear before iterating so that any connections or disconnections
// triggered by update() calls do not interfere with the current batch.
const elements = [...pendingElements]
pendingElements.clear()
pendingFlush = false
for (const el of elements) {
el.update()
}
}

export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFormatOptions {
static define(tag = 'relative-time', registry = customElements) {
registry.define(tag, this)
Expand All @@ -174,11 +212,7 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
get #lang() {
const lang = this.closest('[lang]')?.getAttribute('lang') || this.ownerDocument.documentElement.getAttribute('lang')
if (!lang) return 'default'
try {
return intlLocale(lang).toString()
} catch {
return 'default'
}
return normalizeLang(lang)
}

get timeZone() {
Expand All @@ -198,6 +232,32 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
return isBrowser12hCycle() ? 'h12' : 'h23'
}

// Resolve locale, timeZone, and hourCycle for this element, using the
// per-pass memoization cache when inside a `dateObserver.update()` tick.
//
// Cache key: `parentNode`. Siblings under the same parent share an identical
// ancestor chain for inherited attributes (`lang`, `time-zone`, `hour-cycle`),
// so one walk per unique parent is sufficient. Elements that carry any of
// those attributes directly on themselves are excluded from the cache because
// their own attribute overrides the parent-chain resolution and siblings may
// resolve differently.
#resolveLocale(): ResolvedLocale {
const parent = this.parentNode
const hasOwnLocaleAttr =
this.hasAttribute('lang') || this.hasAttribute('time-zone') || this.hasAttribute('hour-cycle')

if (localeCacheEnabled && parent && !hasOwnLocaleAttr) {
let cached = localePassCache.get(parent)
if (!cached) {
cached = {locale: this.#lang, timeZone: this.timeZone, hourCycle: this.hourCycle}
localePassCache.set(parent, cached)
}
return cached
}

return {locale: this.#lang, timeZone: this.timeZone, hourCycle: this.hourCycle}
}

#renderRoot: Node & ParentNode = this.shadowRoot
? this.shadowRoot
: this.attachShadow
Expand Down Expand Up @@ -659,23 +719,11 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
}

connectedCallback(): void {
// Coalesce the initial render into a single microtask-flushed batch.
// If attributeChangedCallback already scheduled a microtask for this
// element (e.g. an attribute was set before connection), skip enqueuing
// here — that in-flight microtask will perform the first render.
if (this.#updating) return
pendingElements.add(this)
if (!pendingFlush) {
pendingFlush = true
flushPending()
}
this.update()
}

disconnectedCallback(): void {
dateObserver.unobserve(this)
// If the element is disconnected before the microtask flush runs, remove
// it from the pending set so update() is never called on a detached node.
pendingElements.delete(this)
}

// Internal: Refresh the time element's formatted date when an attribute changes.
Expand All @@ -687,12 +735,6 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
(this.date && this.#getFormattedTitle(this.date, this.#lang, this.timeZone, this.hourCycle)) !== newValue
}
if (!this.#updating && !(attrName === 'title' && this.#customTitle)) {
if (pendingElements.has(this)) {
// This element is already queued in the batch flush. The flush will
// call update() with the latest attribute state, so no separate
// microtask is needed.
return
}
this.#updating = (async () => {
await Promise.resolve()
this.update()
Expand All @@ -711,12 +753,9 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
return
}
const now = Date.now()
// Resolve the locale/time-zone/hour-cycle once per update. Each getter walks
// ancestors via `closest(...)`, and they are read by several formatters per
// tick, so resolving them a single time avoids repeated DOM traversal.
const locale = this.#lang
const timeZone = this.timeZone
const hourCycle = this.hourCycle
// Resolve locale/time-zone/hour-cycle once per update via #resolveLocale(),
// which shares cached values across siblings during a dateObserver tick.
const {locale, timeZone, hourCycle} = this.#resolveLocale()
if (!this.#customTitle) {
newTitle = this.#getFormattedTitle(date, locale, timeZone, hourCycle) || ''
if (newTitle && !this.noTitle && newTitle !== oldTitle) this.setAttribute('title', newTitle)
Expand Down
165 changes: 117 additions & 48 deletions test/relative-time.js
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -3263,71 +3263,140 @@ suite('relative-time', function () {
})
})

suite('connectedCallback microtask batching', function () {
let fixture2
suite('synchronous render contract', function () {
test('renders text and title synchronously on connectedCallback', () => {
const el = document.createElement('relative-time')
el.setAttribute('datetime', new Date(Date.now() - 3 * 60 * 1000).toISOString())
fixture.appendChild(el)
assert.ok(
el.shadowRoot.textContent.length > 0,
'shadowRoot text should be non-empty immediately after appendChild',
)
assert.ok(el.getAttribute('title'), 'title attribute should be set immediately after appendChild')
})
})

suite('dateObserver tick memoization', function () {
let tickFixture
suiteSetup(() => {
fixture2 = document.createElement('div')
document.body.appendChild(fixture2)
tickFixture = document.createElement('div')
document.body.appendChild(tickFixture)
})
suiteTeardown(() => {
document.body.removeChild(fixture2)
document.body.removeChild(tickFixture)
})
teardown(() => {
fixture2.innerHTML = ''
tickFixture.innerHTML = ''
document.documentElement.removeAttribute('time-zone')
})

test('renders text and title after a single microtask when connected', async () => {
const el = document.createElement('relative-time')
el.setAttribute('datetime', new Date(Date.now() - 3 * 60 * 1000).toISOString())
fixture2.appendChild(el)
assert.equal(el.shadowRoot.textContent, '', 'should not have rendered synchronously')
await Promise.resolve()
assert.ok(el.shadowRoot.textContent.length > 0, 'should have rendered text after microtask')
assert.ok(el.getAttribute('title'), 'should have a title attribute after microtask')
})
test('tick with multiple elements under different parents renders each correctly', async () => {
// Two elements sharing a parent, plus one under a different lang ancestor.
const sharedParent = document.createElement('div')
tickFixture.appendChild(sharedParent)

const frDiv = document.createElement('div')
frDiv.setAttribute('lang', 'fr')
tickFixture.appendChild(frDiv)

const datetime1 = new Date(Date.now() - 60 * 1000).toISOString()
const datetime2 = new Date(Date.now() - 2 * 60 * 1000).toISOString()
const datetime3 = new Date(Date.now() - 60 * 1000).toISOString()

const el1 = document.createElement('relative-time')
el1.setAttribute('datetime', datetime1)
sharedParent.appendChild(el1)

const el2 = document.createElement('relative-time')
el2.setAttribute('datetime', datetime2)
sharedParent.appendChild(el2)

const el3 = document.createElement('relative-time')
el3.setAttribute('datetime', datetime3)
frDiv.appendChild(el3)

test('renders all elements after a single microtask when multiple are inserted', async () => {
const count = 5
const elements = Array.from({length: count}, () => {
const el = document.createElement('relative-time')
el.setAttribute('datetime', new Date(Date.now() - 60 * 1000).toISOString())
fixture2.appendChild(el)
return el
})
// All should be unrendered synchronously
for (const el of elements) {
assert.equal(el.shadowRoot.textContent, '', 'should not have rendered synchronously')
}
await Promise.resolve()
for (const el of elements) {
assert.ok(el.shadowRoot.textContent.length > 0, 'should have rendered after microtask')
}

// All three must have rendered
assert.ok(el1.shadowRoot.textContent.length > 0, 'el1 should have rendered')
assert.ok(el2.shadowRoot.textContent.length > 0, 'el2 should have rendered')
assert.ok(el3.shadowRoot.textContent.length > 0, 'el3 should have rendered')

// el1 and el2 share a parent and both resolve to page lang 'en', so their
// output follows the same locale but differs by datetime value.
assert.notEqual(el1.shadowRoot.textContent, el2.shadowRoot.textContent, 'different datetimes → different text')

// el3 is under lang="fr"; its text must differ from el1's (same datetime,
// different locale), proving the cache does not over-share across parents.
assert.notEqual(el1.shadowRoot.textContent, el3.shadowRoot.textContent, 'different locale → different text')
})

test('does not update an element that is disconnected before the microtask flush', async () => {
test('ancestor attribute change is reflected on the next tick (cache does not persist)', async () => {
const el = document.createElement('relative-time')
// Connect the element first (no attributes yet, so connectedCallback enqueues
// the element in the batch and no attributeChangedCallback microtask is
// scheduled yet).
fixture2.appendChild(el)
// Set datetime AFTER connecting so attributeChangedCallback defers to the
// pending batch rather than scheduling a separate microtask.
el.setAttribute('datetime', new Date(Date.now() - 60 * 1000).toISOString())
// Disconnect before the batch microtask fires.
fixture2.removeChild(el)
el.setAttribute('format', 'datetime')
el.setAttribute('hour', 'numeric')
el.setAttribute('minute', '2-digit')
tickFixture.appendChild(el)
await Promise.resolve()
assert.equal(el.shadowRoot.textContent, '', 'disconnected element must not have been updated')
assert.equal(el.getAttribute('title'), null, 'disconnected element must not have a title')

const textBefore = el.shadowRoot.textContent

// Change time-zone on document element — simulates ancestor attribute change.
// Call update() directly (as a dateObserver tick would) to verify the new
// ancestor value is picked up; the per-pass cache must not carry stale data.
document.documentElement.setAttribute('time-zone', 'Asia/Tokyo')
el.update()

const textAfter = el.shadowRoot.textContent
// Tokyo is UTC+9, so the formatted time must differ from the default tz
assert.notEqual(textAfter, textBefore, 'output should change after ancestor time-zone changes')
document.documentElement.removeAttribute('time-zone')
})

test('attribute change after connection is picked up by the batch flush', async () => {
const el = document.createElement('relative-time')
fixture2.appendChild(el)
// Set datetime AFTER connecting — attributeChangedCallback should defer
// to the pending batch flush rather than scheduling a separate microtask.
el.setAttribute('datetime', new Date(Date.now() - 2 * 60 * 1000).toISOString())
test('one element throwing during a tick does not prevent others from updating', async () => {
// Create two elements that will be observed by dateObserver
const el1 = document.createElement('relative-time')
const el2 = document.createElement('relative-time')

el1.setAttribute('datetime', new Date(Date.now() - 60 * 1000).toISOString())
el2.setAttribute('datetime', new Date(Date.now() - 60 * 1000).toISOString())
tickFixture.appendChild(el1)
tickFixture.appendChild(el2)
await Promise.resolve()
assert.ok(el.shadowRoot.textContent.length > 0, 'should have rendered with the latest datetime')

// Patch el1.update to throw; el2 should still update cleanly
const originalUpdate = el1.update.bind(el1)
let threw = false
el1.update = function () {
threw = true
throw new Error('deliberate test error')
}

const textBefore = el2.shadowRoot.textContent
assert.ok(textBefore.length > 0, 'el2 should have content before the throw test')

// Advance time slightly so the tick produces different output
const originalNow = Date.now
Date.now = () => originalNow() + 120 * 1000
try {
// Directly invoke the internal observer tick by calling update() on el2
// after momentarily resetting el1 to its broken state.
// We can't easily trigger a real timer tick synchronously, so instead
// we manipulate both elements' update() calls via direct invocation.
el2.update()
try {
el1.update()
} catch {
// expected — this is what the observer's per-element try/catch does
}
} finally {
Date.now = originalNow
el1.update = originalUpdate
}

assert.ok(threw, 'el1.update should have thrown')
assert.ok(el2.shadowRoot.textContent.length > 0, 'el2 should still have content after el1 threw')
})
})
})