TypeScript 6

Upgrade as one workspace

A compiler major belongs in the root catalog and every workspace manifest together. Separate local errors from dependency declaration errors. Do not add assertions just to pass the upgrade; repair contracts and translation boundaries.

Validate external values

Frontmatter, JSON, and HTTP responses are not trustworthy because they have a TypeScript type assertion. Start with unknown, verify required fields and union values, then return an immutable internal model.

function isLocale(value: string): value is "ko" | "en" {
  return value === "ko" || value === "en";
}

Represent intentional absence with null or a discriminated union. Narrow the undefined naturally returned by lookups. noUncheckedIndexedAccess helps expose those boundaries.

Verify public APIs

Package exports and documentation drift easily. Use the Compiler API to read export symbols from source entry modules and compare them with frontmatter apiSymbols. The build can then reject missing or stale API docs.

Practical tips

• Use interface for object shapes and type for unions and mapped types.
• Default domain data and returned collections to readonly.
• Use satisfies to validate contracts without widening literals.
• Use Promise.all for independent work and explicit void for intentional fire-and-forget.

Failure cases

Spreading any during migration

Temporary any escapes cross boundaries quickly. Validate unknown in a narrow adapter and keep only checked values in the domain.

Exporting implementation types

Leaking framework or database types makes replacement expensive. Publish the smallest consumer-facing interface and explicit converters.