关于es6数组章节[].copyWithin.call({length:5,3:1},0,3)的理解

// 将3号位复制到0号位
[].copyWithin.call({
     length: 5, 3: 1}, 0, 3)
// {0: 1, 3: 1, length: 5}

// 对象转数组(建议用es6的Array.from(Obj)转换比较直观)
Array.from({
     length: 5, 3: 1})
// [undefined, undefined, undefined, 1, undefined]

// 转换完毕，进行处理
[undefined, undefined, undefined, 1, undefined].copyWithin(0,3);
// [1, undefined, undefined, 1, undefined]
// 恢复回类对象(忽略undefined,补上length)
[0:1,3:1,length:5]