fixin.me/app/javascript/controllers/readout_unit_controller.js

import { Controller } from "@hotwired/stimulus"
import { disableElement, enableElement } from "element_helpers"

export default class extends Controller {
  static targets = ["select", "button"]

  unitChanged() {
    if (this.selectTarget.value && this.selectTarget.value !== this.selectTarget.dataset.defaultUnitId) {
      enableElement(this.buttonTarget)
    } else {
      disableElement(this.buttonTarget)
    }
  }

  setDefault() {
    const select = this.selectTarget
    const form = document.createElement('form')
    form.action = this.buttonTarget.dataset.path
    form.method = 'post'
    form.dataset.turboStream = 'true'
    const methodInput = document.createElement('input')
    methodInput.type = 'hidden'; methodInput.name = '_method'; methodInput.value = 'patch'
    const unitInput = document.createElement('input')
    unitInput.type = 'hidden'; unitInput.name = 'quantity[default_unit_id]'; unitInput.value = select.value
    form.appendChild(methodInput)
    form.appendChild(unitInput)
    form.addEventListener('turbo:submit-end', event => {
      if (event.detail.success) {
        select.dataset.defaultUnitId = select.value
        this.unitChanged()
      }
      form.remove()
    })
    document.body.appendChild(form)
    form.requestSubmit()
  }
}