Add support for beforeDeclarations

🔍 Search Terms

beforeDeclarations declaration transform transformers afterDeclarations

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

The TypeScript compiler API already supports custom transformers in 3 ways:

1. Before built-in .js transformations
2. After built-in .js transformations
3. After built-in .d.ts transformations

But the API is missing a "Before built-in .d.ts transformations" variant.

📃 Motivating Example

To generate type declarations more dynamically, you often need to access AST nodes of your codebase as it is written. The built-in transformers erase already a lot of the AST making this information unavailable. With the help of this new beforeDeclarations you are able to access your full AST and use it to auto-generate your typescript declaration files.

💻 Use Cases

1. Use information from property initializers to include "default values" your JSDoc comments. (see #41486).
2. Remove declarations using information like visibility modifiers or used functions in your code to not expose implementation details.

Empieza revisando las variantes existentes de transformadores personalizados de la TypeScript compiler API, especialmente las etapas before y after de transformación de declaraciones descritas en el issue. Compara las formas de sus API públicas y determina cómo encajaría una etapa beforeDeclarations. Se considera terminado cuando los transformadores personalizados puedan acceder al AST previo a la transformación para la generación de declaraciones y los casos de uso que motivan el cambio estén admitidos.