Unwraps path keys into object

unwrapper.ts

/**
 * Converts object with flat structure, where key contains a full path to each leaf,
 * into regular POJO. 
 * 
 * @example
 * // input:
 * {
        'recipients[0].name': 'is required',
        'recipients[0].email': ['is required', 'invalid format'],
        'subject': 'is required',
        'senders.primary.phone': 'must be valid',
        'senders.secondary.addresses[0].country': 'is required'
 * }
 * // output:
 * {
        recipients: [{ name: 'is required', email: ['is required', 'invalid format'] }],
        subject: 'is required',
        senders: { 
            primary: { phone: 'must be valid' },
            secondary: { addresses: [{ country: 'is required' }] }
        }
 * }
 */
const unwrapPathKeys = (inputObj: Record<string, any>) => Object
    .keys(inputObj)
    .reduce(
        (outputObj, fullPath) => fullPath
            .split('.')
            .reduce((parent, pathKey, keyIndex, { length: pathSize }) => {
                const parsed = /^(?<key>[^\[]+)(?:\[(?<idx>\d+)\])?$/g.exec(pathKey);
                const cleanPathKey: string = parsed?.['groups']['key'] ?? pathKey;
                const arrayIndex = parseInt(parsed?.['groups']['idx'] ?? '-1');

                if (keyIndex + 1 === pathSize) {
                    // reached the last key in path
                    parent[cleanPathKey] = inputObj[fullPath];
                    // go back to root
                    return outputObj;
                } else if (!parent[cleanPathKey]) {
                    // parent has no such node - create
                    parent[cleanPathKey] = arrayIndex === -1 ? {} : [{}];
                } else if (arrayIndex && parent[cleanPathKey].length < arrayIndex + 1) {
                    // the array is already in parent
                    // we just need to add a new item to that array
                    parent[cleanPathKey].push({});
                }

                // progress deeper to child
                return arrayIndex === -1
                    ? parent[cleanPathKey]
                    : parent[cleanPathKey][arrayIndex];
            }, outputObj),
        {});