* [['hello'],['world','1','2']]
 * => [Array(1), Array(3)]
 * [['hello'],...[].slice.call(['world','1','2'], 0)]
 * => [Array(1), "world", "1", "2"]
 * [...['hello'],...[].slice.call(['world','1','2'], 0)]
 * => ["hello", "world", "1", "2"]
 

You can see the effect of adding syntax sugar.... But my level is limited and I can't find any information about... on the Internet. Can someone explain it to me? It’s better to have resources. Thanks.