javascript中undefined与null的区别

//undefined值在布尔类型环境中会被当做false
var myArray = new Array()
if(!myArray[0]){
console.log("undefined is regared as false in the context of boolean")
}
//undefined值在数值类型环境中会被转换为NaN
var a;//NaN
console.log(a+2)//Evaluates to NaN
//null值在布尔类型环境中会被当做false
var b=null;
if(!b){
console.log("null is regared as false in the context of boolean")
}
//null值在数值类型环境中会被转换为0
var b=null;
console.log(b+2);//Evaluates to 2