下面定义的几个数组哪一个是稀疏数组?
let arr = [,,,];
let arr1 = new Array(3);
let arr2 = [undefined,undefined,undefined];
let arr3 = new Array();
arr3.length = 3;
用什么方法来检测呢?
in操作符?for-in?还是别的什么
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