Looks good!

Currently skipped tests:

• Error handling (will be moved to a different place)
• Nested rich text

Needs to be addressed on the icu-to-json side I think.

this "should" work 😄

you might have to define a tag and/or a baseTag formatter

Could these two properties be removed from the parsed result? Or should they be used by the consumer?

Not sure what you mean - Formatters are called here with (value, locale, formatOptions)

https://github.com/jantimon/icu-to-json/blob/83f24322bfd4ab66402e70d9727a650720501925/src/runtime.ts#L150-L152

the format options are passed through from MessageFormat

I mean e.g. these cases:

// [["price",4,"numberFmt",{"type":0,"tokens":[{"stem":"currency","options":["EUR"]}],"parsedOptions":{"style":"currency","currency":"EUR"}}]]
'{price, number, ::currency/EUR}'

// [["value",4,"numberFmt",{"type":0,"tokens":[{"stem":".#","options":[]}],"parsedOptions":{"maximumFractionDigits":1}}]]
'{value, number, ::.#}'

Is tokens necessary here? In regard to minimizing the size of the AST, could this be simplified to something like this?

[["value",4,"numberFmt",{"maximumFractionDigits":1}]]

Similarly, date skeletons seem to be a bit verbose:

// [["now",4,"date",{"type":1,"pattern":"yyyyMdHm","parsedOptions":{"year":"numeric","month":"numeric","day":"numeric","hourCycle":"h23","hour":"numeric","minute":"numeric"}}]]
'{now, date, ::yyyyMdHm}'

Could the parsedOptions be unwrapped and replace the object they're placed within?

Could this be removed from the parsed result? See also https://github.com/amannn/next-intl/pull/705/files#r1418720864

use compileToJson instead it's the same as compile just without args

args are helpful to generate types or to optimize which formatters should be included to a bundle.
e.g. if date is used you might want to add a date formatter

Got it, thanks!

Should I pass the values from the user or args from the parsed result here?

the args are not needed for formatters however the args can tell you during compile time which formatters need to be available to run the code.

in your case there is a very special formatter wich you should add for full react support

it allows processing the children before they are passed to the given tag function:

{
   tag: (children: Array<string | ReactNode>, locale: string) => 
         children.map((value, i) => typeof value === "string"
            ? value 
            : <Fragment key={`f-${i}`}>{value}</Fragment>
}

e.g. for a translation like <foo>Hello <b>{name}</b></foo>

there is also the baseTag formatter which is a fallback which kicks in if the end-user did not define a tag.

by default the tag is converted to a string but you might also wrap in a fragment or even allow some tags to be used as html:

// for a compiled message "<b>Hello {name}</b>" 
const formatters = {
  baseTag: (Tag, chilren) => {
      // allowlist:
      if (["b", "strong", "p"].include(Tag)) { 
        return <Tag>{children}</Tag>
      }
      return <>{children}</>;
  }
};
run(message, lang, { name: "Joe" }, formatters)

I tried updating your example here to use nested rich text: https://codesandbox.io/p/sandbox/icu-to-json-demo-forked-2jzngr?file=%2Fsrc%2Ficu.tsx%3A19%2C1-20%2C1

Somehow the inner part is missing. Am I doing something wrong?

there are two different ways to run the precompiled icu messages:

1. run
2. evaluateAst

the idea is that run will always return a string and evaluateAst will always return an array.

const t = (key, args) => run(messages[key], lang, args);

icu-to-json is framework independent so it would require you to wrap a react Fragment around the result:

t.rich = (key, args) => <>{evaluateAst(messages[key], lang, args)}</>;

Oh, that helps with the fragment—thanks!

I was wondering about this case:

It seems like the args determine the return value. However, the function b has already been called at this point (having returned a React.JSX.Element) and date should be turned into a string.

Therefore the returned value should have the type (string | React.JSX.Element)[], right?

Or am I missing something?