intl.datetimeformat invalid language tag bcp47 error

Not specified by the original author.

Fix Intl.DateTimeFormat invalid language tag BCP47 error

TL;DR

The tag you passed is not valid BCP47, so canonicalize it before constructing the formatter. Intl validates strictly and throws RangeError on anything malformed. Normalize underscores to hyphens, fix subtag order, and fall back to a default when the tag is beyond repair.

The error

RangeError: Invalid language tag: en__US
new Intl.DateTimeFormat(tag) throws

Fix it

Step 1: Reproduce with the exact tag

node -e "try{new Intl.DateTimeFormat('en__US')}catch(e){console.log('throws:',e.message)}"

Expected: You confirm the tag is the problem.

Step 2: Canonicalize the tag

node -e "const t='en__US'.replace(/_+/g,'-').replace(/-{2,}/g,'-'); console.log(t); console.log(Intl.getCanonicalLocales(t))"

Expected: The tag becomes valid BCP47 or the canonicalizer throws honestly.

Step 3: Add a safe fallback

node -e "function safeTag(t,fb='en-US'){try{return Intl.getCanonicalLocales(t)[0]}catch(e){return fb}}; console.log(safeTag('en__US'), safeTag('!!!'))"

Expected: Bad tags fall back instead of throwing.

Step 4: Fix the source that emits the bad tags

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

Expected: The source now emits clean tags.

When to use this

• Intl.DateTimeFormat throws RangeError invalid language tag
• Locale tags come from user input or a CMS

When NOT to use this

• The tag is valid but the language is wrong, check the tag value
• The formatter works but output looks off, check the options

Tool and version compatibility

• Intl.getCanonicalLocales, all modern runtimes
• BCP47 tag structure

Variant phrasings

double underscore from a join bug

Classic. Joining [lang, region] with when one part is empty gives en. Trim empties before joining.

tag with the right parts in the wrong order

Order is language, script, region, variants. zh-Hans-CN is valid, zh-CN-Hans is not.

Why it happens

BCP47 has a strict grammar: subtags separated by single hyphens in a fixed order. Double separators, underscores, or misplaced subtags fail validation, and Intl constructors throw RangeError rather than guess.

Edge cases

• Intl.getCanonicalLocales throws on invalid input, wrap it in try/catch
• Grandfathered tags like i-klingon are valid, do not reject them in validation
• Private-use x- subtags are valid, keep them when present

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_VelybfcRLB61q-xlMaicig

Resolved from the public thread: https://vectle.com/posts/pst_VelybfcRLB61q-xlMaicig

Thread query: intl.datetimeformat invalid language tag bcp47 error

TL;DR: The tag you passed is not valid BCP47, so canonicalize it before constructing the formatter. Intl validates strictly and throws RangeError on anything malformed. Normalize underscores to hyphens, fix subtag order, and fall back to a default when the tag is beyond repair.

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.

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.