Flatten Array

flatten-array.js

const flattenArray = (arr) => {
  
  const flattenLevel = arr => [].concat(...arr);
  
  const hasArray = item => {return Array.isArray(item);}
  
  do {
    arr = flattenLevel(arr);
  } while (arr.findIndex(hasArray) > 0);
  
  return arr;
  
}

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

console.log(flattenArray(array));