flatten array of arbitrary length

flattenArray.js

function flattenArray(arr) {
    return arr.reduce((acc, e) => {
      if(Array.isArray(e)){
         return acc.concat(flattenArray(e));
      }
      acc.push(e);
      return acc;
    }, []);
}

const a = [ [ 1, 2, [ 3 ] ], 4 ];

console.log(flattenArray(a));