ts null、undefined、strictNullChecks

1. 引用：
   1. ts any、unknown、object、void、undefined、null 和 never 的类型兼容性
2. null 表示这里有值，但是个空值
3. undefined 表示这里没有值
4. null、undefined 是一个具有意义的实际类型
5. null 值可以赋值给哪些类型？

var x1: any = null
var x2: unknown = null
var x3: object = null // 当 strictNullChecks: true 时会报错
var x4: void = null // 当 strictNullChecks: true 时会报错
var x5: undefined = null // 当 strictNullChecks: true 时会报错
var x6: null = null

1. null 类型可以被哪些类型赋值？

const a1 = 1 as unknown as any
const a2 = 1 as unknown as undefined
const a3 = 1 as unknown as null
const a4 = 1 as unknown as never
var x1: null = a1
var x2: null = a2 // 当 strictNullChecks: true 时会报错
var x3: null = a3
var x4: null = a4

1. undefined 值可以赋值给哪些类型？

var x1: any = undefined
var x2: unknown = undefined
var x3: object = undefined // 当 strictNullChecks: true 时会报错
var x4: void = undefined
var x5: undefined = undefined
var x6: null = undefined // 当 strictNullChecks: true 时会报错

1. undefined 类型可以被哪些类型赋值？

const a1 = 1 as unknown as any
const a2 = 1 as unknown as undefined
const a3 = 1 as unknown as null
const a4 = 1 as unknown as never
var x1: undefined = a1
var x2: undefined = a2
var x3: undefined = a3 // 当 strictNullChecks: true 时会报错
var x4: undefined = a4

1. strictNullChecks 表示“严格的空值检查”，它是 TypeScript 的一个编译选项。
2. 当开启 strictNullChecks 选项时，null 和 undefined 值将不再被认为是所有类型的子类型。这有助于在 ts 代码中捕获潜在的空值引用错误。
3. strictNullChecks 默认值是 false
4. 实践：请在 tsconfig.json 中开启 strictNullChecks 配置，打开更严格的 null 类型检查。

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}