skipLibChecks specificity

[dragomirtitian]

🔍 Search Terms

skipLibCheck specific files

✅ 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

Currently skipLibCheck is an all or nothing proposition. You either suppress all errors from declaration files or you get errors from all declaration files. This is not really an accurate reflection of how declaration files are usually used. Almost all projects use declaration files from a trusted source, like npm. Errors from these files are usually not relevant and can be safely suppressed. Many projects however also have declaration files they write by hand (usually for interop with js files, or for libraries that are not typed) - these generally should be type checked since they are actively authored as part of the project.

Ideally skipLibCheck would allow us to more granularly control which declaration files are checked and which are not. This allows us to reap the performance benefits of skipLibCheck while not sacrificing type checking on authored code.

Proposal

One possible solution would be to allow skipLibCheck to be a list of glob patterns for which type checking should be skipped. This would be a simple solution allowing users to decide what declaration files they want and don't want checked.

While this solution is probably the simplest one to implement, we should consider other ones.

📃 Motivating Example

We have a custom build tool that uses the compiler API to force some declaration files to skip checking by setting their hasDefaultLib flag and using skipDefaultLibCheck instead of skipLibCheck. While this solution worked well in the past this is not really a supported solution which we expect will break (6.0 already breaks our current implementation - we have found an alternate implementation, 7.0 will make any sort of custom solution much more difficult)

💻 Use Cases

1. What do you want to use this for?
   Improve build performance of TypeScript projects.

2. What shortcomings exist with current approaches?
   You can suppress errors from all declaration files or from none. There is no granular control over it.

3. What workarounds are you using in the meantime?
   Use the compiler API to control which declaration files are checked.

[dragomirtitian]

This is similar to #30511 but that issue is specifically for just node_modules

[guillaumebrunerie]

Many projects however also have declaration files they write by hand (usually for interop with js files, or for libraries that are not typed) - these generally should be type checked since they are actively authored as part of the project.

My understanding is that one should never write .d.ts files by hand, but rather write regular .ts files with declare global blocks. This way the code will be type checked properly even with skipLibCheck. See for instance https://www.danielfullstack.com/article/stop-using-d-ts-files-in-typescript

But maybe this doesn't work for your use case?

[RyanCavanaugh]

Yeah, I think broadly we have an implied feature from the fact that, broadly speaking, you have a 2x2 matrix of .ts/.d.ts and "want checked" / "don't want checked"

So "skipLibCheck" is probably best understood as a special case of that feature where the "is checked" bit exactly corresponds to "is not .d.ts". But it's somewhat straightforward to imagine that you could go much more custom than that.

The hard part here isn't implementation, it's just figuring out the right way to represent that user configuration.

[jakebailey]

Personally, I am wondering if we just need a new split, where we use roughly the same rules we do for determining if a file should be emitted ("root files"? those in include/files?) and say that we can check those d.ts files. I feel like there must be existing issues for this already.

I can even imagine having control of "check lib.d.ts" + "check root .d.ts files" + "check non-root lib.d.ts" files.

It seems like that would satisfy everyone without introducing glob matching or something (which, would be a true pain because our glob matcher is designed to ignore node_modules!)