resolve Type error for jQuery for submitting json

The following error TypeError: 'append' called on an object that does not implement interface FormData. is being sent to the console log and refers to the line $.ajax({ as per script below.

The error is not clear to this reader, as upon consulting the documentation, there are at least 3 components with Type as well as settings which could set custom types.

      document.getElementById('startButton').addEventListener('click', () => {
        codeReader.decodeFromVideoDevice(selectedDeviceId, 'video', (result, err) => {
          if (result) {
            console.log(result)
            document.getElementById('result').textContent = result.text
let formData = new FormData();
let CodeParams = {
  code_data: result.text
};
formData.append("code_json_data", JSON.stringify(CodeParams));
$.ajax({
  url: "update",
  type: "post",
  data: formData
  });
              }
          if (err && !(err instanceof ZXing.NotFoundException)) {
            console.error(err)
            document.getElementById('result').textContent = err
          }
        })
        console.log(`Started continous decode from camera with id ${selectedDeviceId}`)
      })

What is incorrect with the declaration of FormData ?