currency-converter.js

// index.js
ReactDOM.render(<App />, root);

// components/app.js
class App extends Component {
  render() {
    return (
      <>
        <h1>My App</h1>
        <ConverterForm />
        ...
      </>
    );
  }
}

// components/converter.js
class ConverterForm extends Component {
  state = {
    fromCurrency: 'USD', // put some default value here..
    toCurrency: 'GBP',
    amount: 100,
    result: null,
  };

  render() {
    const { fromCurrency, toCurrency, amount, result } = this.state;
    return (
      <form onSubmit={this.calculate}>
        <input value={fromCurrency} onChange={(evt) => this.setState({ fromCurrency: evt.target.value })} />
        <input value={amount} onChange={(evt) => this.setState({ amount: evt.target.value })} />
        <input value={toCurrency} onChange={(evt) => this.setState({ toCurrency: evt.target.value })} />
        {result !== null
          ? <p>Result: {result}</p>
          : null
        }
      </form>
    );
  }

  calculate = () => {
    const { fromCurrency, toCurrency, amount } = this.state;
    ConversionUtils.convert(fromCurrency, toCurrency, amount).then((result) => {
      this.setState({ result });
    });
  }
}

// utils/conversion.js
const ConversionUtils = {
  convert(fromCurrency, toCurrency, amount) {
    // Make an API call to get the conversion result.
  }
}