let data = [[1,2,[3]],4];
Array.prototype.isNormalized = function(){
  for(let item of this)
    if(typeof item === "object")
      return false
  return true;
}

Array.prototype.normalize = function() {
  let output = [];
  for(let item of this) {
    if(typeof item !== "object") {
      output.push(item);
    }
    else {
      if (!item.isNormalized()) {
        item = item.normalize();
      }
      output.push(...item)
    }
  }
  return output;
}
let flatArray = data.normalize();