Maybe/app/javascript/controllers/money_field_controller.js

import { Controller } from "@hotwired/stimulus";
import { CurrenciesService } from "services/currencies_service";

// Connects to data-controller="money-field"
// when currency select change, update the input value with the correct placeholder and step
export default class extends Controller {
  static targets = ["amount", "currency"];

  handleCurrencyChange() {
    const selectedCurrency = event.target.value;
    this.updateAmount(selectedCurrency);
  }

  updateAmount(currency) {
    (new CurrenciesService).get(currency).then((data) => {
      this.amountTarget.placeholder = data.placeholder;
      this.amountTarget.step = data.step;
    });
  }
}