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doc: improve documentation about ICU data fallback
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This patch:

- Documents `--with-icu-default-data-dir` and its precedence
- Elaborates a bit more about the format of the name of the expected
  data file.

PR-URL: nodejs#49666
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: James M Snell <[email protected]>
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joyeecheung authored and nodejs-github-bot committed Sep 22, 2023
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32 changes: 26 additions & 6 deletions doc/api/intl.md
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Expand Up @@ -113,27 +113,47 @@ This mode provides a balance between features and binary size.

If the `small-icu` option is used, one can still provide additional locale data
at runtime so that the JS methods would work for all ICU locales. Assuming the
data file is stored at `/some/directory`, it can be made available to ICU
through either:
data file is stored at `/runtime/directory/with/dat/file`, it can be made
available to ICU through either:

* The `--with-icu-default-data-dir` configure option:

```bash
./configure --with-icu-default-data-dir=/runtime/directory/with/dat/file --with-intl=small-icu
```

This only embeds the default data directory path into the binary.
The actual data file is going to be loaded at runtime from this directory
path.

* The [`NODE_ICU_DATA`][] environment variable:

```bash
env NODE_ICU_DATA=/some/directory node
env NODE_ICU_DATA=/runtime/directory/with/dat/file node
```

* The [`--icu-data-dir`][] CLI parameter:

```bash
node --icu-data-dir=/some/directory
node --icu-data-dir=/runtime/directory/with/dat/file
```

(If both are specified, the `--icu-data-dir` CLI parameter takes precedence.)
When more than one of them is specified, the `--icu-data-dir` CLI parameter has
the highest precedence, then the `NODE_ICU_DATA` environment variable, then
the `--with-icu-default-data-dir` configure option.

ICU is able to automatically find and load a variety of data formats, but the
data must be appropriate for the ICU version, and the file correctly named.
The most common name for the data file is `icudt6X[bl].dat`, where `6X` denotes
The most common name for the data file is `icudtX[bl].dat`, where `X` denotes
the intended ICU version, and `b` or `l` indicates the system's endianness.
Node.js would fail to load if the expected data file cannot be read from the
specified directory. The name of the data file corresponding to the current
Node.js version can be computed with:
```js
`icudt${process.versions.icu.split('.')[0]}${os.endianness()[0].toLowerCase()}.dat`;
```
Check ["ICU Data"][] article in the ICU User Guide for other supported formats
and more details on ICU data in general.
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