Design Meeting Notes, 9/9/2025

[DanielRosenwasser]

noUncheckedSideEffectImports by default

#62421

• [[Funny aside - presenter wrote import whoops from "whoops" as an example in the TypeScript playground expecting an error, but gets none because a package named whoops actually exists on npm and we fetched its types.]]
• You can write import * as blah from "./something-that-doesnt-exist-i-swear.css, you will get an error.
• But if you write import "./something-that-doesnt-exist-i-swear.css, you will get no error.
• We added --noUncheckedSideEffectImports to catch this, but its default is false.
• For users who want it on but for certain things to work (e.g. importing .css files), they either need
  • declare module "*.css" at the top level of (a non-module) .d.ts file.
  • write a .d.css.ts file per CSS file.
• Users can also always // @ts-ignore.
• If we want it for 6.0, do this immediately on 7.0.
• The reason we didn't add the error initially was because we didn't have pattern ambient modules like *.filetype.
• Would like to special-case the error message.
• Pattern ambient modules are the least safe way to declare this, which is unfortunate. Sort of defeats the purpose.

Enable --allowImportingTsExtensions by default

#62342

• Most server-side runtimes + bundlers support .ts imports.
• Want people to just be able to run .ts files with no extra flag/
• --allowImportingTsExtensions is off by default.
  • Requires --noEmit though.
• There's also --rewriteRelativeImportExtensions which is off by default, but wouldn't you actually want this more?
• So much of this really needs to be informed by whether you have --noEmit.
• We don't want to make this behavior dependent on more flags.
• Maybe a good first step is to turn allowImportingTsExtensions on in the default editor config.
• rewriteRelativeImportExtensions being on depending on outDir seems tempting.
  • "no it isn't!"
• A lot of this is really dependent on the target system.
• This kind of relies on if you're an app author or library author?
  • App authors don't care, will likely always run directly from source.
  • Library authors care because they must emit; they can't ship .ts files.
    • But can't make rewriteRelativeImportExtensions on by default because no default outDir.
    • Wait, that's not a problem. That's why allowJs must be off by default.
• esnext is a weird target for this.
  • nodenext?
  • bundler?
  • Makes sense under these.
• We should be able to make things "just work" to run .ts files in an editor.
  • Sure, but what happens as soon as you add a tsconfig.json?
• We can fix this in the editor - what about type-checking?