Flatten Arrays using Reducer

flatten-arrays.js

const myarr = [1,2,[3]];

function flatten(arr) {
  if (Array.isArray(arr)) {
    return arr.reduce((prev, next) => {
      return prev.concat(flatten(next));
    }, []);
  } else {
    return arr;
  }
}

console.log(flatten(myarr));