稀疏数组

稀疏数组并不含有从0开始的连续索引。一般length属性比实际元素个数大

var arr = [undefinded]

0 in arr //true

var arr2 = new Array(1)

0 in arr2  //false


arr.length = 100

arr[99] = 123

99 in arr  //true

98 in arr //false


var arr3 = [, , ,]

0 in arr  //false