set/update value in an object

mapper5.js

const mapper5 = (keys, v, o) => {
  if (keys.length === 0) {
    return o;
  }
  if (keys.length === 1) {
    return { ...o, [keys[0]]: v };
  }

 const key = keys[0];
 const isNextKeyForArray = /[0-9]+/.test(keys[1] || '');
 const isPlainArrayValue = isNextKeyForArray && keys.length === 2;
 const arrayKey = isNaN(parseInt(keys[1])) ? -1 : parseInt(keys[1]);
 const child = isNextKeyForArray && arrayKey >= 0 ? o[key] || new Array(arrayKey + 1).fill({}) : o[key] || {};
 return {
   ...o,
   [key]: isNextKeyForArray ?
      child.map((i, idx) => idx === arrayKey ? (isPlainArrayValue ? v : mapper5(keys.slice(2), v, i)) : i)
      : { ...child, ...mapper5(keys.slice(1), v, child) }
 };
}


// some tests:
mapper5('a.2'.split('.'), 3, {});
// "{\"a\":[{},{},3]}"

JSON.stringify(mapper5('a.2.c'.split('.'), 3, {}))
// "{\"a\":[{},{},{\"c\":3}]}"

JSON.stringify(mapper5('a.b.c'.split('.'), 3, {}))
// "{\"a\":{\"b\":{\"c\":3}}}"