function fn(arr){
  var result = [],
      i = 0;
  result[i] = [arr[0]];
  arr.reduce(function(prev, cur){
    cur-prev === 1 ? result[i].push(cur) : result[++i] = [cur];
    return cur;
  });
  return result;
}

这是一个很难的问题么？

var oldArr = [1,2,3,7,8,9,15,17,18,19];
var pre=undefined,arr=[],newArr=[];
for(var i=0;i<oldArr.length;i++){
    if(typeof pre == "undefined") {
       arr.push(oldArr[i]);
       pre = oldArr[i];
       continue;
    }
    if(oldArr[i] === pre+1){
       pre++;
       arr.push(pre);
    }else{
       newArr.push(arr);
       arr = [oldArr[i]];
       pre = oldArr[i];
    }
}
 newArr.push(arr);

var newArr = oldArr.reduce((newArr, value, index) => {
  if (newArr.length && (value - newArr[0][0]) === 1) {
    newArr[0].unshift(value);
  } else {
    newArr.unshift([value]);
  }
  return newArr;
}, []).reverse().map(a => a.reverse());