(2). TypeScript 配置 —— strictNullChecks配置

一. emptyType 空类型

// 两个空类型
let u: undefined = undefined
let n: null = null

// 常见区别
Number(null)            // 0
Number(undefined)      // NaN

let age: number = null
console.log(5 + age)      //5

age = undefined
console.log(5 + age)      //NaN

console.log(undefined == null) // true

二. 类型检测

// 类型检测
let height: number
height = 100      //success
height = '100'    // fail
height = undefined //success
height = null       //success

三. 其他类型

let　weight: null | undefined
weight = undefined

// ?相当于 string | undefined
function getPeople(name?: string){
      return name || ''
}

getPeople('wangrong')      //正确
getPeople(undefined)      //正确
getPeople(null)            //错误


// any （不建议使用any）
let school: any
school = null             //success
school = undefined      //success
school = 'hello'      //success
school = 8            //success

总结： undefined 和 null 两个空类型的设计，使用上不方便，所以 通过strictNullChecks严格校验类型，让代码更安全