# 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

```text
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

```bash
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

```bash
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

```bash
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