当你不想重复自己时，有时一种类型需要基于另一种类型。

¥When you don’t want to repeat yourself, sometimes a type needs to be based on another type.

映射类型建立在索引签名的语法之上，用于声明未提前声明的属性类型：

¥Mapped types build on the syntax for index signatures, which are used to declare the types of properties which have not been declared ahead of time:

映射类型是一种泛型类型，它使用 PropertyKey 的联合（经常创建的 通过 keyof）来迭代键来创建类型：

¥A mapped type is a generic type which uses a union of PropertyKeys (frequently created via a keyof) to iterate through keys to create a type:

在此示例中，OptionsFlags 将获取 Type 类型的所有属性并将其值更改为布尔值。

¥In this example, OptionsFlags will take all the properties from the type Type and change their values to be a boolean.

¥Mapping Modifiers

在映射期间可以应用两个额外的修饰符：readonly 和 ? 分别影响可变性和可选性。

¥There are two additional modifiers which can be applied during mapping: readonly and ? which affect mutability and optionality respectively.

你可以通过添加前缀 - 或 + 来移除或添加这些修饰符。如果你不添加前缀，则假定为 +。

¥You can remove or add these modifiers by prefixing with - or +. If you don’t add a prefix, then + is assumed.

¥Key Remapping via as

在 TypeScript 4.1 及更高版本中，你可以使用映射类型中的 as 子句重新映射映射类型中的键：

¥In TypeScript 4.1 and onwards, you can re-map keys in mapped types with an as clause in a mapped type:

你可以利用 模板字面类型 等功能从以前的属性名称中创建新的属性名称：

¥You can leverage features like template literal types to create new property names from prior ones:

你可以通过条件类型生成 never 来过滤掉键：

¥You can filter out keys by producing never via a conditional type:

你可以映射任意联合，不仅是 string | number | symbol 的联合，还可以映射任何类型的联合：

¥You can map over arbitrary unions, not just unions of string | number | symbol, but unions of any type:

¥Further Exploration

映射类型与此类型操作部分中的其他功能配合得很好，例如这里是 使用条件类型的映射类型，它返回 true 或 false，具体取决于对象是否将属性 pii 设置为字面 true：

¥Mapped types work well with other features in this type manipulation section, for example here is a mapped type using a conditional type which returns either a true or false depending on whether an object has the property pii set to the literal true: