jsii-rosetta
Advanced tools
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const rosetta = require('jsii-rosetta');
const translated = rosetta.translateSnippet('python', 'const x: number = 42;');
console.log(translated);Translating TypeScript to Java
This feature allows you to translate a TypeScript code snippet into Java. The `translateSnippet` function takes the target language and the TypeScript code as arguments and returns the translated code.
const rosetta = require('jsii-rosetta');
const translated = rosetta.translateSnippet('java', 'const x: number = 42;');
console.log(translated);Translating TypeScript to C#
This feature allows you to translate a TypeScript code snippet into C#. The `translateSnippet` function takes the target language and the TypeScript code as arguments and returns the translated code.
const rosetta = require('jsii-rosetta');
const translated = rosetta.translateSnippet('csharp', 'const x: number = 42;');
console.log(translated);Babel is a JavaScript compiler that allows you to use next-generation JavaScript, today. It can transform syntax, polyfill features that are missing in your target environment, and more. Unlike jsii-rosetta, Babel focuses on JavaScript and its ecosystem rather than translating between different programming languages.
ts-migrate is a tool for migrating JavaScript projects to TypeScript. It automates the process of converting JavaScript code to TypeScript, adding type annotations, and fixing type errors. While ts-migrate focuses on converting JavaScript to TypeScript, jsii-rosetta translates TypeScript to other languages.
jscodeshift is a toolkit for running codemods over JavaScript and TypeScript codebases. It is used to automate code refactoring and transformations. Unlike jsii-rosetta, which translates TypeScript to other languages, jscodeshift is used for making changes within JavaScript and TypeScript codebases.
jsii-rosetta translates code samples contained in jsii libraries from TypeScript to supported jsii target languages.
This is what enables the AWS Cloud Development Kit to deliver polyglot documentation from a single codebase!
jsii-rosetta leverages knowledge about jsii language translation conventions in order to produce translations. It only
supports a limited set of TypeScript language features (which can be reliably represented in other languages).
Head over to our documentation website!
The jsii toolchain spreads out on multiple repositories:
jsii compiler is maintained (except releases
in the 1.x line)jsii-rosetta sample code transliteration tool
is maintained (except releases in the 1.x line)@jsii/spec, the package that defines the .jsii assembly specificationjsii-config, an interactive tool to help configure your jsii packagejsii-pacmak, the bindings generator for jsii packagesjsii-reflect, a higher-level way to process .jsii assemblies1.x release lines of jsii and jsii-rosettaThe applicable Maintenance & Support policy can be reviewed in SUPPORT.md.
The current status of jsii-rosetta releases is:
| Release | Status | EOS | Comment |
|---|---|---|---|
5.7.x | Current | TBD |  |
5.6.x | Maintenance | 2025-07-01 |  |
5.5.x | Maintenance | 2025-05-15 |  |
5.4.x | Maintenance | 2025-02-28 |  |
See CONTRIBUTING.
This section describes what to pay attention to when writing examples that will be converted by Rosetta.
The translator can translate both code that completely compiles and typechecks, as well as code that doesn't.
In case of non-compiling samples the translations will be based off of grammatical parsing only. This has the downside that we do not have the type information available to the exact thing in all instances. Specifically struct types will not be able to be inferred from object literals. Have a look at the following piece of code:
someObject.someMethod('foo', {
bar: 3,
});
In non-TypeScript languages, it is important to know the type of the second
argument to the method here. However, without access to the definition of
someMethod(), it's impossible for Rosetta to know the type, and hence
it cannot translate the example. It is therefore important to include necessary
imports, variable declarations, etc, to give Rosetta enough information to figure
out what's going on in this code, and the example should read like this:
import * as myLib from 'some-library';
declare const someObject: myLib.SomeClass;
someObject.someMethod('foo', {
bar: 3,
});
By default, Rosetta will accept non-compiling examples. If you set
jsiiRosetta.strict to true in your package.json,
the Rosetta command will fail if any example contains an error:
/// package.json
{
"jsiiRosetta": {
"strict": true
}
}
To avoid having to repeat common setup every time, code samples can use
"fixtures": a source template where the example is inserted. A fixture must
contain the text /// here and typically looks like this:
const * as module from '@some/dependency';
class MyClass {
constructor() {
const obj = new MyObject();
/// here
}
}
The example will be inserted at the location marked as /// here and will have
access to module, obj and this. Any import statements found in the
example will automatically be hoisted at the top of the fixture, where they are
guaranteed to be syntactically valid.
The default file loaded as a fixture is called rosetta/default.ts-fixture in
the package directory (if it exists).
Examples can request an alternative fixture by specifying a fixture parameter
as part of the code block fence:
```ts fixture=some-fixture
Or opt out of using the default fixture by specifying nofixture:
```ts nofixture
To specify fixtures in an @example block, use an accompanying @exampleMetadata tag:
/**
* My cool class
*
* @exampleMetadata fixture=with-setup
* @example
*
* new MyCoolClass();
*/
When compiling examples, Rosetta will make sure your package itself and all of
its dependencies and peerDependencies are available in the dependency
closure that your examples will be compiled in.
If there are packages you want to use in an example that should not be part
of your package's dependencies, declare them in jsiiRosetta.exampleDependencies
in your package.json:
/// package.json
{
"jsiiRosetta": {
"exampleDependencies": {
"@some-other/package": "^1.2.3",
"@yet-another/package": "*",
}
}
}
You can also set up a directory with correct dependencies yourself, and pass
--directory when running jsii-rosetta extract. We recommend using the
automatic closure building mechanism and specifying exampleDependencies though.
This section describes how Rosetta integrates into your build process.
Rosetta has a number of subcommands. The most important one is jsii-rosetta extract.
The jsii-rosetta extract command will take one or more jsii assemblies,
extract the snippets from them, will try to compile them with respect to a given
home directory, and finally store all translations in something called a
"tablet".
A couple of things to note here:
jsii to
regenerate the assembly, before re-running jsii-rosetta extract.imports. Currently, you
are responsible for building a directory with the correct node_modules
directories in there so that a TypeScript compilation step will find all
libraries referenced in the examples. This is especially revelant if your
examples include libraries that depend on the current library: it is not
uncommon to write examples in library A showing how to use it in combination
with library B, where B depends on A. However, since by definition B
cannot be in the set of dependencies of A, you must build a directory with
both B and A in it somewhere in your filesystem and run Rosetta in that
directory.The extract command will write a file named .jsii.tabl.json next to every
assembly, containing translations for all samples found in the assembly. You
should include this file in your NPM package when you publish, so that
downstream consumers of the package have access to the translations.
An example invocation of jsii-rosetta extract looks like this:
jsii-rosetta extract --directory some/dir $(find . -name .jsii)
Since TypeScript compilation takes a lot of time, much time can be gained by
using the CPUs in your system effectively. jsii-rosetta extract will run the
compilations in parallel.
jsii-rosetta will use a number of workers equal to half the number of CPU
cores, up to a maximum of 16 workers. This default maximum can be overridden by
setting the JSII_ROSETTA_MAX_WORKER_COUNT environment variable.
If you get out of memory errors running too many workers, run a command like this to raise the memory allowed for your workers:
/sbin/sysctl -w vm.max_map_count=2251954
Rosetta extract will translate all examples found in .jsii and write the
translations to .jsii.tabl.json. From compilation to compilation, many of these
examples won't have changed. Since TypeScript compilation is a fairly expensive
process, we would like to avoid doing unnecessary work as much as possible.
To that end, rosetta can reuse translations from a cache, and write new translations into the same cache:
jsii-rosetta extract \
--directory some/dir \
--cache cache.json \
[--trim-cache] \
$(find . -name .jsii)
The --trim-cache flag will remove any old translations from the cache that
don't exist anymore in any of the given assemblies. This prevents the cache from
growing endlessly over time (an equivalent jsii-rosetta trim-cache command is
available if your workflow involves running extract in multiple distinct
invocations and want to retain the cache between them).
The jsii-rosetta infuse command increases the coverage of examples for classes
in the assembly.
It finds classes in the assembly that don't have an example associated with them
yet (as specified via the @example tag in the doc comments), but that are used
in another example found elsewhere—in either a README or an example of another
class—it will copy the example to all classes involved. This will make sure
your handwritten examples go as far as possible.
Note that in order to do this, infuse will modify the assemblies it is
given.
rosetta infuse depends on the analysis perfomed by rosetta extract, and must
therefore be run after extract. It can also be run as part of extract, by
passing the --infuse flag:
jsii-rosetta extract \
--directory some/dir \
--infuse \
$(find . -name .jsii)
jsii-pacmak will read translation from tablets to substitute translated examples
into the generated source bindings. pacmak will automatically read individual
.jsii.tabl.json files if present, and can additionally also read from a global
tablet file.
When a translation for a code sample cannot be found, pacmak can be configured
to do one of the following:
extract command)Example:
jsii-pacmak \
[--rosetta-tablet=global.json] \
[--rosetta-unknown-snippets=verbatim|translate|fail]
The diagram below shows how data flows through the jsii tools when used together:
┌───────────┐
│ │
│ Source ├───┐
│ │ │ ╔══════════╗ ┌────────────┐ ╔═══════════════╗ ┌──────────┐
└───────────┘ │ ║ ║ │ │ ║ rosetta ║ │ │
├───▶║ jsii ║───▶│ assembly │────▶║ extract ║───▶│ tablet │
┌───────────┐ │ ║ ║ │ │ ║ ║ │ │
│ │ │ ╚══════════╝ └────────────┘ ╚═══════════════╝ └──────────┘
│ README │───┘ │ │
│ │ │ │
└───────────┘ │ ╔═══════════════╗ │
│ ║ rosetta ║ │
└──────────▶║ infuse ║◀─────────┘
║ ║
╚═══════════════╝
│
┌───────────────────┴───────────────────┐
│ │
▼ ▼
┌────────────┐ ┌──────────┐
│ │ │ │
│ assembly' │ │ tablet' │
│ │ │ │
└────────────┘ └──────────┘
│ │
│ │
│ ▼ ┌─────────────┐
│ ╔═══════════════╗ ┌┴────────────┐│
│ ║ ║ │ ││
└──────────────────────────────▶║ pacmak ║────▶│ packages ││
║ ║ │ ├┘
╚═══════════════╝ └─────────────┘
(potentially
live-translates)
In order to make examples compile, boilerplate code may need to be added that detracts from the example at hand (such as variable declarations and imports).
This package supports hiding parts of the original source after translation.
To mark special locations in the source tree, we can use one of three mechanisms:
void expression statement to mark statement locations in the AST.comma operator combined with a void expression to mark expression locations in the AST./// !hide, /// !show) to mark locations that span AST nodes. This is less reliable
(because the source location of translated syntax sometimes will have to be estimated) but the only option if you want
to mark non-contiguous nodes (such as hide part of a class declaration but show statements inside the constructor).The void expression keyword and or the comma operator feature are little-used JavaScript features that are reliably
parsed by TypeScript and do not affect the semantics of the application in which they appear (so the program executes
the same with or without them).
A handy mnemonic for this feature is that you can use it to "send your code into the void".
Statement hiding looks like this:
before(); // will be shown
void 0; // start hiding (the argument to 'void' doesn't matter)
middle(); // will not be shown
void 'show'; // stop hiding
after(); // will be shown again
For hiding expressions, we use comma expressions to attach a void statement to an expression value without changing
the meaning of the code.
Example:
foo(1, 2, (void 1, 3));
Will render as
foo(1, 2)
Also supports a visible ellipsis:
const x = (void '...', 3);
Renders to:
x = ...
Use special comment directives:
before();
/// !hide
notShown();
/// !show
after();
Thanks goes to these wonderful people (emoji key):
This project follows the all-contributors specification. Contributions of any kind welcome!
jsii is distributed under the Apache License, Version 2.0.
FAQs
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