small immutable object merge

merge.js

let NIL = void 0,
  isObj = x => x && Object.getPrototypeOf(x) === Object.prototype;

function merge(obj, patch) {
  obj = Array.isArray(obj) ? [ ...obj ] : isObj(obj) ? { ...obj } : NIL;
  if (!obj) throw Error('You can only copy objects or arrays.');

  for (let k in patch) {
    let v = patch[k];
    if (typeof v === 'function') obj[k] = v(obj[k]);
    else if (isObj(v) && isObj(obj[k])) obj[k] = merge(obj[k], v);
    else if (v === NIL) delete obj[k];
    else obj[k] = v;
  }

  return obj;
}

multiple patches:

function merge(obj, ...patches) {
  obj = Array.isArray(obj) ? [ ...obj ] : isObj(obj) ? { ...obj } : NIL;
  if (!obj) throw Error('You can only copy objects or arrays.');

  for (let patch of patches) {
    for (let k in patch) {
      let v = patch[k];
      if (typeof v === 'function') obj[k] = v(obj[k]);
      else if (isObj(v) && isObj(obj[k])) obj[k] = merge(obj[k], v);
      else if (v === NIL) delete obj[k];
      else obj[k] = v;
    }
  }

  return obj;
}