Deflatten flat object in JavaScript

const isNumericString = (digits) => {
  if (typeof digits !== 'string') {
    return false;
  }
  return digits.match(/^\d+$/);
};

const deflatten = (flat, separator = '.') => {

  const tree = {};

  for (const key in flat) {
    const value = flat[key];
    const subKeys = key.split(separator);
    let nested = tree;

    for (let i = 0, len = subKeys.length; i < len; i++) {
      const subKey = subKeys[i];
      const nextSubKey = subKeys[i + 1];
      const existing = nested[subKey];

      // Initialize next nesting level
      if (!existing) {
        nested[subKey] = isNumericString(nextSubKey) ? [] : {};
      }

      // Store value
      if (i === len - 1) {
        nested[subKey] = value
      }

      // Go deeper
      else {
        nested = nested[subKey];
      }
    }
  }

  return tree;
};

const flat = {
  'foo': 42,
  'bar.baz.qez.0.name': 'alice',
  'bar.baz.qez.1.name': 'bob',
  'bar.baz.qez.2.name': 'charlie',
};

console.log(
  JSON.stringify(deflatten(flat)),
);

// Result
// {"foo":42,"bar":{"baz":{"qez":[{"name":"alice"},{"name":"bob"},{"name":"charlie"}]}}}