ES6 code to flatten array of arbitrarily nested arrays

flatten_array.js

let arr = [[1,2,[3]],4];

let flatten = (accu, e) => {
  if(!Array.isArray(e)){
    return [...accu, e];
  }else{
    return [...accu, ...e.reduce(flatten, [])];
  }
}

let result = arr.reduce(flatten, [])
console.log(result)