（1）splice会改变原来是数组，splice(start,length)
splice
splice(start,length)
var a=[1,2,3]; var b=a.splice(2,1); b===[3] a===[1,2]
（2）slice会构建新数组，slice(start,end)
slice
slice(start,end)
var a =[1,2,3]; var b=a.slice(2,3); b===[3] a===[1,2,3]