flattenArray created by Soulman - https://repl.it/@Soulman/flattenArray

flattenArray.js

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

const flatten = (array) => {
  // reduce traverses the array and we return the result
  return array.reduce((acc, b) => {
    // if is an array we use recursion to perform the same operations over the array we found 
    // else we just concat the element to the accumulator
    return acc.concat(Array.isArray(b) ? flatten(b) : b);
  }, []); // we initialize the accumulator on an empty array to collect all the elements
}

flatten(array);