[Bug]: Typescript mapped type with implicit any results in faulty TS

[aczekajski]

💻

• Would you like to work on a fix?

How are you using Babel?

Programmatic API (babel.transform, babel.parse)

Input code

type Bar = { [key in Foo] };

Configuration file name

No response

Configuration

options passed to @babel/parser:

{
    sourceType: 'module',
    plugins: ['typescript'],
}

Current and expected behavior

When just parsed using @babel/parser and generated back using @babel/generator, with nothing else being done to AST, the mentioned code becomes:

type Bar = { [key in Foo]: };

which is an ivalid typescript, raises "Type expected" error because indeed, there is no type after :.

Expected behavior:
It should be generated back into

type Bar = { [key in Foo] };

Environment

• Babel: 7.24.3
• @babel/plugin-syntax-typescript: 7.24.1
• Node: 18.19.0
• npm: 10.2.3

Possible solution

If the MappedType node has its type field set to undefined, there should be no : printed out after the [key in Foo].

Additional context

No response

[babel-bot]

Hey @aczekajski! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite.