Inspired by optional chaining

chain.js

const isObj = (val) => Object.prototype.toString.call(val) === '[object Object]';
const isArr = (val) => Array.isArray(val);
const isNum = (val) => val === `${+val}`;
const strToNum = (str) => (isNum(str) ? +str : str);

/**
 * Used to change undefined keys to object or array, similar effect to the 'optional chaining'.
 * Received an object and an array of keys, returns a formatted object.
 *
 * e.g.
 * list -> ['a', 'b.c']     -> a is object, b is object, b.c is object
 * list -> ['a[]', 'b.c[]'] -> a is array
 * list -> ['b[0].c']       -> b is array, b[0] is object, b[0].c is object
 * list -> ['b[0][]']       -> b is array, b[0] is array
 *
 * Default, change last key to object
 * chain({ a: 1 }, ['a', 'b.c', 'd'])   ->   { a: {}, b: { c: {} }, d: {} }   // change not object to object, add missed object
 * chain({ a: { b: 1 } }, ['a', 'd'])   ->   { a: { b: 1 } }, d: {} }         // if object exists, not change
 *
 * chain({ a: 1 }, ['b[0]'])            ->   { a: 1, b: [{}] }                // b -> array, b[0] -> object
 * chain({ a: 1 }, ['b[0].c'])          ->   { a: 1, b: [{ c: {} }] }         // b -> array, b[0] -> object, b[0].c -> object
 *
 * To change last key to array, not object, add '[]' behind.
 * chain({ a: 1 }, ['b[]'])             ->   { a: 1, b: [] }                  // b -> array
 * chain({ a: 1 }, ['b[0][]'])          ->   { a: 1, b: [[]] }                // b -> array, b[0] -> array
 *
 * @param {object} obj - source object
 * @param {array} list - needed keys for source object
 * @returns {object} formatted object
 */
const chain = (obj, list) => {
  list.forEach((key) => {
    // e.g.
    // key: prop[0][0] -> chain({}, [key]) -> { prop: [ [ {} ] ] }
    // key: prop[0][]  -> chain({}, [key]) -> { prop: [ [] ] }
    // key: prop       -> chain({}, [key]) -> { prop: {} }

    const newKey = key.replace(/\[(\d+)]/g, '[].$1');
    // prop[0][0] -> prop[].0[].0 (key -> newKey)
    // prop[0][]  -> prop[].0[]   (key -> newKey)
    // prop       -> prop         (key -> newKey)

    const keyPieces = newKey.split('.');
    // prop[].0[].0 -> ['prop[]', '0[]', '0'] (newKey -> keyPieces)
    // prop[].0[]   -> ['prop[]', '0[]']      (newKey -> keyPieces)
    // prop         -> ['prop']               (newKey -> keyPieces)

    keyPieces.reduce((data, oldPiece) => {
      let newPiece = oldPiece;

      if (newPiece.includes('[]')) {
        // newPiece: 'prop[]' or '0[]' (data[newPiece] is array)

        newPiece = newPiece.slice(0, -2); // 'prop[]' -> 'prop', '0[]' -> '0'
        newPiece = strToNum(newPiece); // 'prop' -> 'prop', '0' -> 0

        if (!isArr(data[newPiece])) {
          data[newPiece] = [];
        }
      } else {
        // newPiece: 'prop' or '0' (data[newPiece] is object)

        newPiece = strToNum(newPiece); // 'prop' -> 'prop', '0' -> 0

        if (!isObj(data[newPiece])) {
          data[newPiece] = {};
        }
      }

      return data[newPiece];
    }, obj);
  });

  return obj;
};

export default chain;