SOURCE

// Console查看结果
var result = [].copyWithin.call({3: 1, length: 5}, 0, 3);
console.log(JSON.stringify(result, null, 2));

// 类似于
var data = [undefined, undefined, undefined, 1, undefined, undefined];
var res = data.copyWithin(0, 3);
console.log(JSON.stringify(res, null, 2));