Tiny form serialization helper

tiny-serialize.js

function serializeForm(form, urlEncode = false) {
  const formData = new FormData(form);

  if (urlEncode) {
    const searchParams = new URLSearchParams(formData)
    
    return searchParams.toString();
  }

  const serializedFormData = {};

  // The code below casts `<select multiple>` and checkboxes values to array
  // when more than one option is selected.
  for (const [key, value] of formData) {
    if (key in serializedFormData) {
      const serializedValue = serializedFormData[key];
      serializedFormData[key] = Array.isArray(serializedValue)
        ? [...serializedValue, value]
        : [serializedValue, value];

      continue;
    }

    serializedFormData[key] = value;
  }

  return serializedFormData;
};

document.getElementById("form").addEventListener("submit", event => {
  event.preventDefault();

  const formData = serializeForm(event.target);
  console.log(formData);
});