function concat(...args) {
  const array = args[0];
  const rest = args.slice(1);

  let result = Array.isArray(array) ? [...array] : [array];
  for (const value of rest) {
    if (Array.isArray(value)) {
      result.push(...value);
    } else {
      result.push(value);
    }
  }
  return result
}

var array = [1];
var other = concat([1,2,3],[4,5,[6]]);
console.log(other);