Enable Type Checking for Custom File Extensions

🔍 Search Terms

"xsjs", "xsjslib", "hana", "file extensions"

✅ 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

Enhance TypeScript's tsconfig.json to support type checking for files with unsupported extensions, such as "/*.xsjs" and "/*.xsjslib", when specified explicitly in the "include" config option

📃 Motivating Example

Support for SAP HANA server-side javascript and other javascript-based languages using custom file extensions

💻 Use Cases

We are using TypeScript to check our SAP HANA server-side javascript files (.xsjs) and library files(xsjslib).
To do so, we currently need to add at least two top-comments in each file:

///<reference path"./pathto/config.xsjs" />
//@ts-check

In the config.xsjs we store all other triple-slash directives (like lib=es2016) to configure the typescript project.

It is really great that there's a way to enable type-checking on non-standard file extensions (at least in VSCode), but it would be great if you'd be able to configure an entire project which includes non-standard file extensions.

Start with TypeScript's tsconfig.json include handling and the existing support for type-checking unsupported extensions in VSCode. Define done as allowing explicitly included .xsjs and .xsjslib files to be checked project-wide without per-file reference and @ts-check comments, while preserving existing JavaScript behavior.