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:

当代码执行时类型发生变化时使用。在增量构建数组时使用。在处理任何缩小的[] 时使用。 来源：marius-townhouse/effective-typescript-skills。