[].copyWithin.call({length:5,3:1},0,3)

[].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。