ts配置文件中有个选项  "strictNullChecks"  如果设置值为false，那么以下代码都不是问题 ，如果设置为true， 以下代码可以说明undefined和null在ts中的区别

// 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) // ture

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


let weight: number | undefined
weight = undefined

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

getPeople('liuyi')
getPeople(undefined)
getPeople(null)  // fail

// any
let school: any
school = null // success
school = undefined // success
school = 'hello' // success
school = 3 // success

let width: number
//width = getWidth()

if (width === undefined) {  // fail

}
function getWidth(): number {
  if (Math.random() > 0.3) {
    return undefined
  } else if (Math.random() > 0.6) {
    return null
  }
  return 0
}