//(1)null转为数值为0

console.log(Number(null));//0

console.log(null+3);//3

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

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

console.log(isNaN(null));false

//(2)undefined转为数值为NaN

console.log(Number(undefined));//NaN（not a number）

console.log(3+undefined);//NaN

console.log(isNaN(undefined));//true

//(3)null和undefined比较

console.log(null == undefined); //true,存在隐式类型转换

console.log(null === undefined);//false

/* "==="表示全等，二者类型不同，所以为false */

console.log(typeof null);//object数据类型

console.log(typeof undefined);//undefined数据类型

console.log(Object.getPrototypeOf(Object.prototype));//null

//(1)变量未赋值

let a

console.log(a);//undefined

//(2)函数参数未赋值

(function fn(x) {

    console.log(x); //undefined

})()

//(3)对象属性未赋值

let obj = {

    name:'张三'

}

console.log(obj.name);

console.log(obj.age);//undefined

//(4)函数没有返回值

function fn(){}

var x = fn();

console.log(x);//undefined