js_code_golf_1_splice.js

const splice = (arr = [], start = 0, deleteCount = 0, ...items) => {
  if (start < 0 || arr.length - 1 < start) {
   return [[], [...arr, ...items]]; 
  }
  
  const arr1 = [];
  const arr2 = [...arr];
  const result = [];
  
  for (let i = 0; i < arr.length; i++) {
    if (i == start) break;
    arr1.push(arr2.shift());
  }
  
  if(deleteCount > 0) {
    for(let i = deleteCount; i > 0; i--) {
      result.push(arr2.shift());
    }
  }
  
  return [[...result], [...arr1, ...items, ...arr2]];
};