为什么以下代码中的map()输出有所不同?
var y = [1,2,2,1];
var t = y.map(ind => [...Array(ind)].map((_,i) => ind+""+i));
// This makes [ [ '10' ], [ '20', '21' ], [ '20', '21' ], [ '10' ] ]
var t1 = y.map(ind => Array(ind).map((_,i) => ind+""+i));
//[ [ <1 empty item> ], [ <2 empty items> ], [ <2 empty items> ], [ <1 empty item> ] ]