evolving-types

Some variables start with broad types and narrow as TypeScript sees values added.

This is an exception to the rule that types don't change. Variables initialized without a value, or as empty arrays, can have "evolving" types that narrow based on what you assign to them.

Once a variable leaves its scope or is used in a function, its type is fixed:

コードの実行中に型が変更される場合に使用します。配列が段階的に構築される場合に使用します。狭くなる any[] を操作する場合に使用します。 ソース: marius-townhouse/effective-typescript-skills。