901. JavaScript's built-ins

Every member of JavaScript's standard library, with what fsr does when a loader or a component reads it. The list at the end is written from the table the build is tested against, so it changes when the build does.

How to read the list

Each member has two answers, one for a loader or an action and one for a component.

• server: the build lowers it and the server answers what node v26 answers for the same code. The test runs the member's code through node and through fsr and compares the two.
• browser: the value depends on the viewer's machine or the moment the page runs, such as Date.now() or Math.random(). The server leaves it to the browser, as chapter 111 describes. Only a component has this answer, since a loader always runs on the server.
• refused: the build stops with the file, the line and what it could not lower. Nothing runs that would answer differently from node.
• throws, as in node: node throws for this code and so does fsr.
• loader only: the code awaits, which a component cannot do.

The code each member was tried with is in fsr/lower/tests/standard_members.tsv, one line per member with node's answer beside it.

A refusal

A refusal names the line and the member:

async function fetchTotal() {
  return 21;
}

export async function load() {
  const total = fetchTotal().then((n) => n * 2);
  return { total };
}

routes/page.loader.ts:6:17: `.then()`, which is not a builtin

Write the await instead. The build lowers it:

export async function load() {
  const total = (await fetchTotal()) * 2;
  return { total };
}

What is refused and what to write instead

Where fsr answers as Chrome does

Math.sin, Math.cbrt and the other transcendental functions answer the correctly rounded result, as Chrome 158 does. Node v26 runs an older V8 whose results are one unit in the last place off for some arguments. The list marks the two members the test caught.

fsr's locale data is CLDR 48.2.1 with ICU 78, from ICU4X 2.3. Chrome 158 bundles ICU 78.2, which carries CLDR 48. node v26.8.1 ships CLDR 48.0. Where fsr and node differ fsr follows its own data. new Intl.Locale("az-Arab").getNumberingSystems() answers arabext in fsr and latn in node.

A browser with other locale data can format the same value differently from fsr. Safari, Firefox and an older or newer Chrome each carry their own CLDR.

Where the locale data is

ir/data/icu.postcard is every locale's ICU4X data, the 72 kinds of it the intl functions load. snapfire_i18n holds the rest: currency, percent and relative-time patterns, zone names and display names. fsr build cuts both to the locales [locales] supported lists and writes them as generated/locale.data, which the host reads at boot. A host refuses to start without the file, or with a file cut for other locales or by another ICU4X release.

fsr, fsr test and a terminal program format from every locale's data, built in through the intl_compiled feature of snapfire_fsr_ir. A host built with snapfire_fsr_host's intl_compiled feature starts without a file and formats from every locale's data.

Moving to newer locale data

ir/Cargo.toml holds the ICU4X crates at ~2.3. ICU4X 2.4 carries CLDR 49, which Chrome does not ship yet. Under it es-ES ordinals for 1, 3, 21 and 23 answer one where Chrome answers other. Its en-IN times read PM where Chrome writes pm.

Chrome moves to a new ICU when a commit titled Roll ICU from 78.2 to ... lands on chromium/deps/icu. It reaches Chrome stable with the first milestone that branches after it. To see which CLDR a Chrome carries, run this in it:

new Intl.PluralRules("es-ES", { type: "ordinal" }).select(1) // "other" on CLDR 48, "one" on CLDR 49

Once Chrome stable answers one:

cargo test -p snapfire_fsr_ir --release --test std --no-run
icu4x-datagen --markers-for-bin target/release/deps/std-<hash> --locales full --format blob --deduplication none \
  --segmenter-models cjdict burmesedict khmerdict laodict thaidict --out ir/data/icu.postcard --overwrite

• Raise the icu_* requirements in ir/Cargo.toml and cli/Cargo.toml to the ICU4X release that carries Chrome's CLDR and set ICU in ir/src/intl_data.rs to it.
• Write ir/data/icu.postcard again with that release's icu4x-datagen, from a release build of a test that links every intl function:
• Record ir/tests/fixtures/intl_chrome.json and intl_chrome_currency.json again in that Chrome.
• Run cargo test -p snapfire_fsr_ir --test intl_browser.

Known limits

• subarray of a typed array is a copy, so a write through it does not reach the array it came from.
• A Date whose kind the build cannot see on a render path is written in UTC.
• Temporal.Instant.prototype.toLocaleString with no timeZone on a render path is written in the server's zone.
• A Response body can be read twice. JavaScript throws the second time.
• new Request(path) with a path rather than a URL is kept as given, so a middleware's next can run it. Node throws for it.
• getTimeZones of a locale whose region is a UN area code, such as en-001, answers an empty list.
• resolvedOptions of an Intl.DurationFormat given a unit's own style is refused.

The list

Object

Function

Array

Number

Boolean

String

Symbol

BigInt

Math

JSON

Reflect

Date

RegExp

Map

Set

WeakMap

WeakSet

WeakRef

FinalizationRegistry

Promise

Proxy

Error

ArrayBuffer

DataView

Uint8Array

Iterator

Intl

Temporal

URL

URLSearchParams

TextEncoder

TextDecoder

console

Atomics

DisposableStack

AsyncDisposableStack

SharedArrayBuffer

Headers

Response

Request

AbortController

Intl.Collator

Intl.DateTimeFormat

Intl.DisplayNames

Intl.DurationFormat

Intl.ListFormat

Intl.Locale

Intl.NumberFormat

Intl.PluralRules

Intl.RelativeTimeFormat

Intl.Segmenter

Temporal.PlainDate

Temporal.PlainTime

Temporal.PlainDateTime

Temporal.ZonedDateTime

Temporal.Duration

Temporal.Instant

Temporal.PlainYearMonth

Temporal.PlainMonthDay

Temporal.Now

Iterator.prototype

TypedArray

structuredClone

atob

btoa

parseInt

parseFloat

isNaN

isFinite

encodeURI

encodeURIComponent

decodeURI

decodeURIComponent

Object

Array

Number

Boolean

String

Symbol

BigInt

Date

RegExp

Error

Function