数组扁平化

function flatten(array) {
  return array.reduce((flat, current)=>{
    if(current instanceof Array){
      return flat.concat(flatten(current));
    } else{
      return [...flat, current];
    }
  },[])
}

let a = flatten([1,2,3,[4,5,[6,7],8],9]);
console.log(a);

原生：

var newArray = arr.flat([depth]);