Transform formData from a form element into Object literal.

formDataToObject.js

/**
 * `objFromFlatKeys` Convert the template string into a tree path and set the value into the referenced object path.
 *
 * @param {string} strPath - template string path to convert in nested object tree
 * @param {Object} prevobj - reference for the nested object result
 * @param {*} value - value to set at the end of the nested object tree
 *
 * @example
 * // define the target object
 * const obj = {};
 * // multiple call
 * objFromFlatKeys('root.item[]', obj, 'A');
 * objFromFlatKeys('root.item[]', obj, 'B');
 * console.log(obj);
 * // logs: { root: { item: { 0: 'A', 1: 'B' } } }
 *
 * @author João Neto <joaopintoneto@gmail.com>
 * @license MIT
 */
function objFromFlatKeys(strPath, prevobj, value) {
  const tokens = strPath
    .replace(/\[(.*?)\]/g, ".$1")
    .replace(/\.+/g, ".")
    .split(".");
  const lastTokenIndex = tokens.length - 1;

  tokens.forEach((key, i) => {
    let k = key || Object.keys(prevobj).length;
    prevobj[k] = i === lastTokenIndex ? value : prevobj[k] || {};
    prevobj = prevobj[k];
  });
}

/**
 * `formDataToObject` Transform formData from a form element into Object literal.
 *
 * @param {HTMLElement} formElement
 * @returns Object
 *
 * @example
 * // given the follow html
 * <form id="form">
 *   <input type="text" name="root.item[]" />
 *   <input type="text" name="root.item[]" />
 *   <button type="submit">submit</button>
 * </form>
 *
 * // when form is submitted
 * document.getElementById("form")
 *   .addEventListener("submit", function (e) {
 *     e.preventDefault();
 *     console.log(formDataToObject(this))
 *     // logs: { root: { item: { 0: 'A', 1: 'B' } } }
 *   });
 *
 * @author João Neto <joaopintoneto@gmail.com>
 * @license MIT
 */
function formDataToObject(formElement) {
  const formData = new FormData(formElement);
  const obj = {};

  [...formData.entries()].forEach(([key, value]) => {
    objFromFlatKeys(key, obj, value || "");
  });

  return obj;
}