出现场景

image.png
二者不同
undefined == null//true
undefined === null//false
Number(null)//0
5+null//5
Number(undefined)//NaN
5+undefined//NaN
思考
为何要有null,还要用undefined?
undefined == null//true
undefined === null//false
Number(null)//0
5+null//5
Number(undefined)//NaN
5+undefined//NaN
为何要有null,还要用undefined?