unfold_array.js

function unfold(arr) {
  var result = [];
  arr.forEach(function(elem, i) {
    if ( elem instanceof Array) {
      intermediate = unfold(elem);
      result = result.concat(intermediate)
      return
    }
    result.push(elem);
  })
  
  return result;
}

console.log(unfold([[1,2,[3]],4]))