Forms

The default behaviour when submitting a form is form to post the result and move on to another page. Your markup usually looks like so:

<Form>
  <input type="text" name="name" id="name" />
  <button>Submit</button>
</Form>

Controlled components

In React however you tend to want to control Forms a bit more. You usually wants to:

  • verify a form is a valid before being submitted
  • give error message on input field before submitting so you can correct the answer

You can verify a forms validity by listening to onSubmit() method. That will give you the form as input and you are able to inspect and determine wether the forms data should be persisted. It will look like this:

class App extends Component {
  onSubmit = (ev) => {
    console.log('form', ev.target);

    const { name } = ev.target;
    // value
    console.log('name field', name);
    return false;
  }

  render() {
    return (
      <div className="App">
        <form onSubmit={this.onSubmit}>
          <input name="name" id="name" />
          <button>Save</button>
        </form>
      </div>
    );
  }
}

Single source of truth

The form itself maintains the state of all its input fields but you tend to want to control that and make React the single source of truth. We can do that by putting each element value in the state, like so:

class App extends Component {
  state = {
    firstname: void 0,
  }

  onSubmit = (ev) => {
    console.log('form', ev.target);
    return false;
  }

  handleChange = (ev) => {
    this.setState({
      firstname: ev.target.value,
    });
  }

  render() {
    return (
      <div className="App">
        <form onSubmit={this.onSubmit}>
          <div>
            <label>First name</label>
            <input name="firstname" id="firstname" value={this.state.firstname} onChange={this.handleChange} />
            {this.state.firstname}
          </div>
          <div>
            <button>Save</button>
          </div>
        </form>
      </div>
    );
  }
}

Above we are creating the method handleChange() that reacts every time the change event is triggered. We can see that we subscribe to change event when we connect the handleChange() method to the onChange, like so:

<input name="firstname" 
       id="firstname" 
       value={this.state.firstname} 
       onChange={this.handleChange} 
/>

Adding more input elements

So far we have seen how we can add an input element and hook up a method to onChange and stick the value of the element into the state object. Does this means we will have 20 different methods if we have 20 different inputs? No, we can solve this in an elegant way:

class App extends Component {
  state = {
    firstname: void 0,
    lastname: void 0,
  }

  onSubmit = (ev) => {
    console.log('form', ev.target);
    return false;
  }

  handleChange = (ev) => {
    this.setState({
      [ev.target.name]: ev.target.value,
    });
  }

  render() {
    return (
      <div className="App">
        <form onSubmit={this.onSubmit}>
          <div>
            <label>First name</label>
            <input name="firstname" id="firstname" value={this.state.firstname} onChange={this.handleChange} />
            {this.state.firstname}
          </div>
          <div>
            <label>Last name</label>
            <input name="lastname" id="lastname" value={this.state.lastname} onChange={this.handleChange} />
            {this.state.lastname}
          </div>
          <div>
            <button>Save</button>
          </div>
        </form>
      </div>
    );
  }
}

Let's zoom in on the interesting part:

handleChange = (ev) => {
  this.setState({
    [ev.target.name]: ev.target.value,
  });
}

We replaced setting a specific key by name, firstname in this case, to a more generic variant [ev.target.name]. This will enable us to hook up the handleChange() method to any input of the same type.

Radio buttons and check boxes

Radio button usually have the behaviour that we want one radio button out of many selected. To accomplish that we ensure all radio buttons that belong together have the same name. We therefore create a markup looking like this:

  <div>
    Man <input type="radio" name="weather" value="sunshine" onChange={this.handleChange} />
    Woman <input type="radio" name="weather" value="rain" onChange={this.handleChange} />
  </div>

As we can see above we can easily attach the handleChanged() method to the onChange.

What about check boxes? Well for checkboxes you can select several different ones, if you check a checkbox then a checkbox of the same group shouldn't be deselected, unlike a radio button. Checkboxes are a little bit different in nature as they require us to look at the property checked instead of value. Let's have a look at their markup:

<div>
    Yellow <input type="checkbox" name="yellow" id="yellow" value={this.state.yellow} onChange={this.handleChange} />
</div>
<div>
    Blue <input type="checkbox" name="blue" id="blue" value={this.state.yellow} onChange={this.handleChange} />
</div>
<div>
    Red <input type="checkbox" name="red" id="red" value={this.state.yellow} onChange={this.handleChange} />
</div>

As mentioned we need to look at the property checked, this means we need to alter the method handleChange() slightly, to this:

handleChange = (ev) => {
  const value = ev.target.type === 'checkbox' ? ev.target.checked : ev.target.value;

  this.setState({
    [ev.target.name]: value,
  });
}

From the above code we see that we inspect to see wether target.type is of type checkbox, if so we look at the checked property to find our value.

Select list

Select lists are quite easy, we can construct those using a select tag and many option tags, like so:

{this.state.products.length > 0 &&
<select name="product" onChange={this.handleChange}>
  {this.state.products.map(p => (
    <option selected={this.state.product === p} value={p}>{p}</option>
  ))}
  </select>
}

Above we simply repeat out a list of products and set the value property and the inner HTML of the option element. We also set the selected property if our this.state.product corresponds to the rendered out element.

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