TypeScript$Concept-TopAndBottomTypes

1. Top Types

A top type (symbol: ⊤) is a type that describes any possible value allowed by the system.  { any possible value }

TypeScript provides two of these types: any and unknown.

1.1 any

any 就像普通的 JavaScript 变量。在迁移的时候常用。

console.log(window, Promise, setTimeout, "foo")
        // (method) Console.log(...data: any[]): void

1.2 unknown

和 any 不同的是，unknown 不能随意读取其属性，需要 applying a type guard。

unknown 常用在接受用户的输入。

unknown 也常用在 handling throwables in a catch block：

function doSomethingRisky() {
  if (Math.random() > 0.5)  return "ok"
  else if (Math.random() > 0.5) throw new Error("Bad luck!")
  else throw "Really bad luck"
}
 
try {
  doSomethingRisky()
} catch (e: unknown) {
  if (e instanceof Error) {
    e
   // var e: Error
  } else if (typeof e === 'string') {
    e
   // var e: string
  } else {
    // Last resort
    console.error(e)
                 //var e: unknown
  }
}

当 compiler flag useUnknownInCatchVariables 设置为 true 时，上面的 catch 部分可以简写为 catch(e)。

1.3 Almost top type: object

object 表示对象类型。 { all possible values except for primitives }

1.4 Almost top type: {}

{} 表示除了 null 和 undefined 意外的类型。{ all possible values, except for null and undefined }

{} | null | undefined 表示 { all possible values }

通过 &{}，可以清除 null 和 undefined 类型（remove nullability from another type）：

type NullableStringOrNumber = string | number | null | undefined;
type StringOrNumber = NullableStringOrNumber & {}

2. Bottom type: never

A bottom type (symbol: ⊥) is a type that describes no possible value allowed by the system. {}

应用：ExhaustivenessChecking。在 never 时抛出异常：

class UnreachableError extends Error {
  constructor(_nvr: never, message: string) {
    super(message)
  }
}
 
// The exhaustive conditional
if (myVehicle instanceof Truck) {
  myVehicle.tow() // Truck
} else if (myVehicle instanceof Car) {
  myVehicle.drive() // Car
} else {
  // NEITHER!
  throw new UnreachableError(
    myVehicle,
// Argument of type 'Boat' is not assignable to parameter of type 'never'.
    `Unexpected vehicle type: ${myVehicle}`
  )
}

3. Unit types

Unit types are types that represent a set of exactly one value.

let num: 65 = 65 // represents the set { 65 }
null & undefined
// `void` type is _almost_ a unit type, but it can check against `undefined` as well

Links

TopAndBottomTypes