Form.tsx

import * as React from 'react';

interface FormState {
  name: string;
  message: string;
}

class Form extends React.Component<{}, FormState> {
  constructor(props: {}) {
    super(props);

    this.state = {
      name: '',
      message: ''
    };
  }

  handleInputChange = (event: React.FormEvent<HTMLInputElement>) => {
    this.setState({
      name: event.currentTarget.value,
      message: this.state.message
    });
  }

  handleTextAreaChange = (event: React.FormEvent<HTMLTextAreaElement>) => {
    this.setState({
      name: this.state.name,
      message: event.currentTarget.value
    });
  }

  handleSubmit = (event: React.FormEvent<HTMLFormElement>) => {
    event.preventDefault();
    console.log(this.state);
  }

  render() {
    return (
      <form onSubmit={this.handleSubmit}>
        <p>Nimi: <input type="text" name="name" placeholder="Matti Meikäläinen" onChange={this.handleInputChange} /></p>
        <p>Viesti: <textarea name="message" placeholder="Kirjoita viesti..." onChange={this.handleTextAreaChange} /></p>
        <button>Lähetä viesti</button>
      </form>
    );
  }
}

export default Form;