const arr = [1,4,11,34,]
arr.filter(item => item<30) //[1,4,11]
arr.some(item => item>34) //false
[1,2,30,-4,0].filter(item => item<=2) //[1, 2, -4, 0]  filter函数会一直过滤数组的每一项到数组的最后一个元素