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.

Start by reviewing the TypeScript compiler API's existing custom transformer variants, especially the before and after declaration-transform stages described in the issue. Compare their public API shapes and determine how a beforeDeclarations stage would fit. Done means custom transformers can access the pre-transformed AST for declaration generation, with the motivating use cases supported.