[].copyWithin.call({length:5,3:1},0,3)
call 指代this指向
[{length:5,3:1}].copyWithin(0,3)
类数组转换
[0:underfined,1:undefined,2:underfined,3:1,4:underfined,length:5].copyWithin(0,3,5)
类数组完善 和 取值范围限定修改
[0:1,1:undefined,2:underfined,3:1,4:underfined,length:5]
[0:1,3:1,length:5]
可能有点问题,但感觉这样好理解。