//(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