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:

當程式碼執行時類型發生變化時使用。在增量建構數組時使用。在處理任何縮小的[] 時使用。 來源：marius-townhouse/effective-typescript-skills。