Design Meeting Notes, 1/10/2025

[DanielRosenwasser]

Make lib Replacement Opt-In by Default

#60829

• Since TypeScript 4.5, we've supported loading lib files from node_modules if something is located in @typescript/lib-*.
• But this lib replacement is on for everyone by default, and that leads to us
  • doing a whole bunch of module resolution all the time at project load going up the spine
  • add file watchers for each of the directories that don't exist.
• This is a perf hit for everyone (and it's noisy to look at when diagnosing issues) so we want to make it opt-in.
• According to a rudimentary search, 352 files across all of GitHub use a custom dom. Limited impact?
• 2 major groups who originally asked for this?
  • People who need to lock on an older dom file like @types/web.
  • People who want a custom version of the built-in lib.
• How do we roll this out?
  • Introduce this in 5.8, and then make it the default in 6.0.