doc: add documentation on ICU

PR-URL: nodejs#13916
Refs: nodejs#13644 (comment)
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

doc/api/_toc.md

@@ -22,6 +22,7 @@
 * [Globals](globals.html)
 * [HTTP](http.html)
 * [HTTPS](https.html)
+* [Internationalization](intl.html)
 * [Modules](modules.html)
 * [Net](net.html)
 * [OS](os.html)

doc/api/all.md

@@ -18,6 +18,7 @@
 @include globals
 @include http
 @include https
+@include intl
 @include modules
 @include net
 @include os

doc/api/intl.md

@@ -0,0 +1,198 @@
+# Internationalization Support
+
+Node.js has many features that make it easier to write internationalized
+programs. Some of them are:
+
+- Locale-sensitive or Unicode-aware functions in the [ECMAScript Language
+  Specification][ECMA-262]:
+  - [`String.prototype.normalize()`][]
+  - [`String.prototype.toLowerCase()`][]
+  - [`String.prototype.toUpperCase()`][]
+- All functionality described in the [ECMAScript Internationalization API
+  Specification][ECMA-402] (aka ECMA-402):
+  - [`Intl`][] object
+  - Locale-sensitive methods like [`String.prototype.localeCompare()`][] and
+    [`Date.prototype.toLocaleString()`][]
+
+Node.js (and its underlying V8 engine) uses [ICU][] to implement these features
+in native C/C++ code. However, some of them require a very large ICU data file
+in order to support all locales of the world. Because it is expected that most
+Node.js users will make use of only a small portion of ICU functionality, only
+a subset of the full ICU data set is provided by Node.js by default. Several
+options are provided for customizing and expanding the ICU data set either when
+building or running Node.js.
+
+## Options for building Node.js
+
+To control how ICU is used in Node.js, four `configure` options are available
+during compilation. Additional details on how to compile Node.js are documented
+in [BUILDING.md][].
+
+- `--with-intl=none` / `--without-intl`
+- `--with-intl=system-icu`
+- `--with-intl=small-icu` (default)
+- `--with-intl=full-icu`
+
+An overview of available Node.js and JavaScript features for each `configure`
+option:
+
+|                                         | `none`                            | `system-icu`                 | `small-icu`            | `full-icu`
+|-----------------------------------------|-----------------------------------|------------------------------|------------------------|------------
+| [`String.prototype.normalize()`][]      | none (function is no-op)          | full                         | full                   | full
+| `String.prototype.to*Case()`            | full                              | full                         | full                   | full
+| [`Intl`][]                              | none (object does not exist)      | partial/full (depends on OS) | partial (English-only) | full
+| [`String.prototype.localeCompare()`][]  | partial (not locale-aware)        | full                         | full                   | full
+| `String.prototype.toLocale*Case()`      | partial (not locale-aware)        | full                         | full                   | full
+| [`Number.prototype.toLocaleString()`][] | partial (not locale-aware)        | partial/full (depends on OS) | partial (English-only) | full
+| `Date.prototype.toLocale*String()`      | partial (not locale-aware)        | partial/full (depends on OS) | partial (English-only) | full
+
+*Note*: The "(not locale-aware)" designation denotes that the function carries
+out its operation just like the non-`Locale` version of the function, if one
+exists. For example, under `none` mode, `Date.prototype.toLocaleString()`'s
+operation is identical to that of `Date.prototype.toString()`.
+
+### Disable all internationalization features (`none`)
+
+If this option is chosen, most internationalization features mentioned above
+will be **unavailable** in the resulting `node` binary.
+
+### Build with a pre-installed ICU (`system-icu`)
+
+Node.js can link against an ICU build already installed on the system. In fact,
+most Linux distributions already come with ICU installed, and this option would
+make it possible to reuse the same set of data used by other components in the
+OS.
+
+Functionalities that only require the ICU library itself, such as
+[`String.prototype.normalize()`][], are fully supported under `system-icu`.
+Features that require ICU locale data in addition, such as
+[`Intl.DateTimeFormat`][] *may* be fully or partially supported, depending on
+the completeness of the ICU data installed on the system.
+
+### Embed a limited set of ICU data (`small-icu`)
+
+This option makes the resulting binary link against the ICU library statically,
+and includes a subset of ICU data (typically only the English locale) within
+the `node` executable.
+
+Functionalities that only require the ICU library itself, such as
+[`String.prototype.normalize()`][], are fully supported under `small-icu`.
+Features that require ICU locale data in addition, such as
+[`Intl.DateTimeFormat`][], generally only work with the English locale:
+
+```js
+const january = new Date(9e8);
+const english = new Intl.DateTimeFormat('en', { month: 'long' });
+const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
+
+console.log(english.format(january));
+// Prints "January"
+console.log(spanish.format(january));
+// Prints "January" or "M01" on small-icu
+// Should print "enero"
+```
+
+This mode provides a good balance between features and binary size, and it is
+the default behavior if no `--with-intl` flag is passed. The official binaries
+are also built in this mode.
+
+#### Providing ICU data at runtime
+
+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:
+
+* The [`NODE_ICU_DATA`][] environmental variable:
+
+  ```shell
+  env NODE_ICU_DATA=/some/directory node
+  ```
+
+* The [`--icu-data-dir`][] CLI parameter:
+
+  ```shell
+  node --icu-data-dir=/some/directory
+  ```
+
+(If both are specified, the `--icu-data-dir` CLI parameter takes precedence.)
+
+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 `icudt5X[bl].dat`, where `5X` denotes
+the intended ICU version, and `b` or `l` indicates the system's endianness.
+Check ["ICU Data"][] article in the ICU User Guide for other supported formats
+and more details on ICU data in general.
+
+The [full-icu][] npm module can greatly simplify ICU data installation by
+detecting the ICU version of the running `node` executable and downloading the
+appropriate data file. After installing the module through `npm i full-icu`,
+the data file will be available at `./node_modules/full-icu`. This path can be
+then passed either to `NODE_ICU_DATA` or `--icu-data-dir` as shown above to
+enable full `Intl` support.
+
+### Embed the entire ICU (`full-icu`)
+
+This option makes the resulting binary link against ICU statically and include
+a full set of ICU data. A binary created this way has no further external
+dependencies and supports all locales, but might be rather large. See
+[BUILDING.md][BUILDING.md#full-icu] on how to compile a binary using this mode.
+
+## Detecting internationalization support
+
+To verify that ICU is enabled at all (`system-icu`, `small-icu`, or
+`full-icu`), simply checking the existence of `Intl` should suffice:
+
+```js
+const hasICU = typeof Intl === 'object';
+```
+
+Alternatively, checking for `process.versions.icu`, a property defined only
+when ICU is enabled, works too:
+
+```js
+const hasICU = typeof process.versions.icu === 'string';
+```
+
+To check for support for a non-English locale (i.e. `full-icu` or
+`system-icu`), [`Intl.DateTimeFormat`][] can be a good distinguishing factor:
+
+```js
+const hasFullICU = (() => {
+  try {
+    const january = new Date(9e8);
+    const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
+    return spanish.format(january) === 'enero';
+  } catch (err) {
+    return false;
+  }
+})();
+```
+
+For more verbose tests for `Intl` support, the following resources may be found
+to be helpful:
+
+- [btest402][]: Generally used to check whether Node.js with `Intl` support is
+  built correctly.
+- [Test262][]: ECMAScript's official conformance test suite includes a section
+  dedicated to ECMA-402.
+
+[btest402]: https://github.com/srl295/btest402
+[BUILDING.md]: https://github.com/nodejs/node/blob/master/BUILDING.md
+[BUILDING.md#full-icu]: https://github.com/nodejs/node/blob/master/BUILDING.md#build-with-full-icu-support-all-locales-supported-by-icu
+[`Date.prototype.toLocaleString()`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Date/toLocaleString
+[ECMA-262]: https://tc39.github.io/ecma262/
+[ECMA-402]: https://tc39.github.io/ecma402/
+[full-icu]: https://www.npmjs.com/package/full-icu
+[ICU]: http://icu-project.org/
+["ICU Data"]: http://userguide.icu-project.org/icudata
+[`--icu-data-dir`]: cli.html#cli_icu_data_dir_file
+[`Intl`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Intl
+[`Intl.DateTimeFormat`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat
+[`NODE_ICU_DATA`]: cli.html#cli_node_icu_data_file
+[`Number.prototype.toLocaleString()`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Number/toLocaleString
+[`String.prototype.localeCompare()`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/localeCompare
+[`String.prototype.normalize()`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/normalize
+[`String.prototype.toLowerCase()`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/toLowerCase
+[`String.prototype.toUpperCase()`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/toUpperCase
+[Test262]: https://github.com/tc39/test262/tree/master/test/intl402