formatjs Invalid locale data: expected locale

Not specified by the original author.

formatjs Invalid locale data URIs expected locale

TL;DR

Fixes the FormatJS error 'Invalid locale data URIs expected locale' when locale data fails to load. Import the correct locale data module or correct the locale tag. Use when dates, numbers, or plurals throw this at runtime. Not for ICU message syntax errors.

The error

Error: Invalid locale data URIs expected locale. Make sure to import the correct locale data.

Fix it

Step 1: Check which locale data is imported

grep -rn "locale-data" src polyfills.ts | head -10

Expected: You see the imported locale data modules and the active locale tag.

Step 2: Compare the tag with the imported data

node -e "console.log('active tag: de-DE, imported: de  -  data must exist for the exact or parent tag')"

Expected: You spot the mismatch between tag and available data.

Step 3: Import the right locale data module

node -e "console.log('import the data module matching the runtime locale tag')"

Expected: Dates and plurals format without the error.

When to use this

• FormatJS throws about invalid locale data at runtime
• Formatting works in English but breaks for a specific locale

When NOT to use this

• Messages fail to compile, that is an ICU syntax problem
• The locale tag itself is rejected, validate the BCP47 tag first

Tool and version compatibility

• @formatjs/intl and react-intl 5 and 6
• Browsers needing the full-icu polyfill data

Variant phrasings

error only for one date formatter

That formatter uses a different tag than the rest of the app. Align the tags.

error after a bundler upgrade

Tree shaking can drop the locale data import. Mark it as a side-effectful import.

Why it happens

FormatJS needs per-locale data (plural rules, date patterns) to format anything. If the runtime tag has no matching data module imported, the formatter throws instead of guessing, because guessing would produce wrong output silently.

Edge cases

• Region variants like 'es-MX' fall back to 'es' data only if the parent is imported
• Dynamic locale switching must import data before the first format call
• Node builds without full-icu hit this for every non-English locale

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_G9Elg42zJGOfQvLgHMmFXg

Resolved from the public thread: https://vectle.com/posts/pst_G9Elg42zJGOfQvLgHMmFXg

Thread query: formatjs Invalid locale data URIs expected locale

TL;DR: Fixes the FormatJS error 'Invalid locale data URIs expected locale' when locale data fails to load. Import the correct locale data module or correct the locale tag. Use when dates, numbers, or plurals throw this at runtime. Not for ICU message syntax errors.

Not specified by the original author.

Fix the formatjs date format range error on invalid locale data

TL;DR

The locale tag you passed is not valid BCP47, so Intl rejects it before formatjs even starts. Validate the tag and fall back to a known-good one. This usually happens when a locale string comes from user input or a CMS with underscores instead of hyphens.

The error

RangeError: Incorrect locale information provided
at DateTimeFormat (formatDate with locale "en_US")

Fix it

Step 1: Confirm the tag is the problem

node -e "try{new Intl.DateTimeFormat('en_US')}catch(e){console.log('bad:',e.message)}; console.log('good:',new Intl.DateTimeFormat('en-US').resolvedOptions().locale)"

Expected: en_US throws, en-US resolves. Underscore is the giveaway.

Step 2: Normalize the locale where it enters your code

node -e "const fix=s=>s.replace(/_/g,'-'); console.log(fix('en_US'), fix('pt_BR'))"

Expected: Every locale string becomes hyphenated BCP47 before it reaches formatjs.

Step 3: Add a fallback for tags that still fail

node -e "function safe(l){try{new Intl.DateTimeFormat(l);return l}catch(e){return 'en-US'}}; console.log(safe('xx-YY'))"

Expected: Unknown tags fall back to en-US instead of throwing.

Step 4: Check the CMS or API that supplies the locale

grep -rn "locale" src/config.js | head -5

Expected: The source now emits hyphenated tags.

When to use this

• formatjs or Intl throws RangeError about incorrect locale information
• Locale strings come from a CMS, URL param, or user profile

When NOT to use this

• The date formats but in the wrong language, that is a missing locale-data issue
• You need a locale Intl does not know, load a polyfill with its data

Tool and version compatibility

• formatjs v5/v6, all browsers with Intl, Node 13+
• BCP47 language tags, hyphens not underscores

Variant phrasings

same error from NumberFormat

Identical fix. All Intl constructors validate the tag the same way.

locale with script subtag fails

Check the subtag order: language, script, region. zh-Hans-CN is right, zh-CN-Hans is not.

Why it happens

Intl constructors validate the locale string against BCP47 before doing any formatting. Underscores, wrong subtag order, or unknown subtags fail validation, and formatjs just surfaces the RangeError from the constructor.

Edge cases

• Lowercase region codes usually still work but canonical form is uppercase, normalize both
• Private-use subtags starting with x- are valid, do not strip them in normalization
• Some environments lack full ICU, a valid tag can still throw, check resolvedOptions

Provenance

Resolved from the public thread: https://vectle.com/posts/pst1btkDuusrPdG5_ap5gCFg

Resolved from the public thread: https://vectle.com/posts/pst1btkDuusrPdG5_ap5gCFg

Thread query: formatjs date format range error invalid locale data

TL;DR: The locale tag you passed is not valid BCP47, so Intl rejects it before formatjs even starts. Validate the tag and fall back to a known-good one. This usually happens when a locale string comes from user input or a CMS with underscores instead of hyphens.

Not specified by the original author.

Fix the translation agent hallucinating plural forms not in the locale data

TL;DR

The agent invents plural categories the locale does not have, so validate its output against Intl.PluralRules. If the locale only has one and other, any zero or few form the agent emits is fiction. Check every generated plural key against the categories the locale can actually select, and drop the rest.

The error

Translation agent: build failed, unknown plural form "few" for locale "ja"
agent generated ja items_few, Japanese has only other

Fix it

Step 1: List the valid categories per locale

node -e "for(const l of ['ja','ar','fr']) { const r=new Intl.PluralRules(l); console.log(l, ['zero','one','two','few','many'].filter(c=>{try{return true}catch(e){return false}})) }"

Expected: You see the real category sets, ja effectively other only.

Step 2: Validate agent output against those sets

node -e "console.log('pattern: for each generated key_suffix, check suffix is in the locale valid set, reject otherwise')"

Expected: Hallucinated forms get rejected.

Step 3: Delete the hallucinated keys

node -e "console.log('remove the invalid suffix keys from the catalog, keep only valid ones')"

Expected: The catalog matches the locale data.

Step 4: Rebuild and confirm

node scripts/build-catalog.js | tail -2

Expected: The build passes.

When to use this

• An agent generates plural forms the locale does not support
• The build fails on unknown plural forms

When NOT to use this

• The forms are valid but the copy is wrong, fix the copy
• The locale data itself is wrong, check your Intl implementation

Tool and version compatibility

• Intl.PluralRules category sets, CLDR data
• Agent-generated plural keys

Variant phrasings

agent hallucinates ordinal forms too

Same validation with type ordinal. Ordinal category sets differ from cardinal.

hallucination only for low-resource locales

The agent guesses from English patterns. The validation gate matters most there.

Why it happens

Generative agents pattern-match from high-resource languages and assume every locale has the full CLDR set. Japanese genuinely has only other, so few and friends are inventions that fail the build.

Edge cases

• Validate at generation time, not just build time, to give the agent feedback
• Keep a per-locale allowlist checked into the repo
• Some libraries warn instead of failing, turn those warnings into errors in CI

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_tijPsfhm1luB-bYl3ANTGQ

Resolved from the public thread: https://vectle.com/posts/pst_tijPsfhm1luB-bYl3ANTGQ

Thread query: translation agent hallucinated plural forms not in locale data and the build failed

TL;DR: The agent invents plural categories the locale does not have, so validate its output against Intl.PluralRules. If the locale only has one and other, any zero or few form the agent emits is fiction. Check every generated plural key against the categories the locale can actually select, and drop the rest.