I'm generating form field elements using a ponent template importing an array of data. I want to be able to hide some of these elements and show them when some other's element's criteria has been met; this is fairly mon in form fields, e.g. When you select item A, form field X appears, when you select item B, form field X is hidden.

I've read a fair bit on conditional rendering on the React docs site but these examples don't really work for what I'm doing although the Preventing Component from Rendering section is close.

I made a Codepen demoing my setup, what I want to do is show the second field if the criteria for the first field is met (in this example, the first field should have 5 characters entered). I've passed through a prop to set the initial hiding but how can I find that specific hidden element and unhide it?

// Field data
const fieldData = [{
    "type": "text",
    "label": "First",
    "name": "first",
    "placeholder": "Enter first name",
    "hidden": false
  }, {
    "type": "text",
    "label": "Surname",
    "name": "surname",
    "placeholder": "Enter surname",
    "hidden": true
  }];

  // Get form data
  class FormData extends React.Component {
    constructor(props) {
      super(props);
      this.state = {
        items: props.data
      };
    }

    render() {
      let els = this.state.items;

      return els.map((el, i) => {
          return <Input key={i} params={el} />
      });
    }
  }

  // Input builder
  class Input extends React.Component {
      constructor(props) {
          super(props);

          // Keep state value
          this.state = {
              value: '',
              valid: false,
              hidden: this.props.params.hidden
          };

          this.handleChange = this.handleChange.bind(this);
      }

      handleChange(e) {
          this.setState({
              value: e.target.value,
              valid: e.target.value.length < 5 ? false : true
          });
      }

      render() {
          // Element attributes
          const {type, label, name, placeholder, hidden} = this.props.params;
          const isValid = this.state.valid === true ? <span>Valid! Should show Surname field.</span> : <span>Not valid. Should hide Surname field.</span>;

          if (!hidden) {
            return (
            <div>
                {label ? <label htmlFor={name}>{label}</label> : null}
                <input
                    type={type}
                    name={name}
                    placeholder={placeholder || null}
                    value={this.state.value}
                    onChange={this.handleChange}
                />
                {isValid}
            </div>
            );
          } else {
            return null;
          }
      }
  }

  // App
  class App extends React.Component {

    render() {
      return (
        <div>
            <h1>Show/Hide test</h1>
            <p>What we want here is the surname to appear when firstname has a value (say, it has 5 characters) and hide surname when firstname doesn't.</p>
            <FormData data={fieldData} />
        </div>
      );
    }
  }


  ReactDOM.render(
      <form>
        <App />
      </form>,
      document.getElementById('app')
  );

I'm generating form field elements using a ponent template importing an array of data. I want to be able to hide some of these elements and show them when some other's element's criteria has been met; this is fairly mon in form fields, e.g. When you select item A, form field X appears, when you select item B, form field X is hidden.

I've read a fair bit on conditional rendering on the React docs site but these examples don't really work for what I'm doing although the Preventing Component from Rendering section is close.

I made a Codepen demoing my setup, what I want to do is show the second field if the criteria for the first field is met (in this example, the first field should have 5 characters entered). I've passed through a prop to set the initial hiding but how can I find that specific hidden element and unhide it?

// Field data
const fieldData = [{
    "type": "text",
    "label": "First",
    "name": "first",
    "placeholder": "Enter first name",
    "hidden": false
  }, {
    "type": "text",
    "label": "Surname",
    "name": "surname",
    "placeholder": "Enter surname",
    "hidden": true
  }];

  // Get form data
  class FormData extends React.Component {
    constructor(props) {
      super(props);
      this.state = {
        items: props.data
      };
    }

    render() {
      let els = this.state.items;

      return els.map((el, i) => {
          return <Input key={i} params={el} />
      });
    }
  }

  // Input builder
  class Input extends React.Component {
      constructor(props) {
          super(props);

          // Keep state value
          this.state = {
              value: '',
              valid: false,
              hidden: this.props.params.hidden
          };

          this.handleChange = this.handleChange.bind(this);
      }

      handleChange(e) {
          this.setState({
              value: e.target.value,
              valid: e.target.value.length < 5 ? false : true
          });
      }

      render() {
          // Element attributes
          const {type, label, name, placeholder, hidden} = this.props.params;
          const isValid = this.state.valid === true ? <span>Valid! Should show Surname field.</span> : <span>Not valid. Should hide Surname field.</span>;

          if (!hidden) {
            return (
            <div>
                {label ? <label htmlFor={name}>{label}</label> : null}
                <input
                    type={type}
                    name={name}
                    placeholder={placeholder || null}
                    value={this.state.value}
                    onChange={this.handleChange}
                />
                {isValid}
            </div>
            );
          } else {
            return null;
          }
      }
  }

  // App
  class App extends React.Component {

    render() {
      return (
        <div>
            <h1>Show/Hide test</h1>
            <p>What we want here is the surname to appear when firstname has a value (say, it has 5 characters) and hide surname when firstname doesn't.</p>
            <FormData data={fieldData} />
        </div>
      );
    }
  }


  ReactDOM.render(
      <form>
        <App />
      </form>,
      document.getElementById('app')
  );

• javascript
• html
• reactjs

• I would give the fields some unique ids and raise the state 1 ponent up, to the form, where you would have an object of filed states by ids, i.e. fieldStates: { '001': { value: '', valid: true, hidden: false }, '002': { ... } };. This way I'd be able to check different fields validity in one ponent and change their visibility. – Gleb Kostyunin Commented Dec 29, 2017 at 12:20
• This is what I was thinking about... would I use the key property in FormData to store this data? It's an acceptable pattern in React to send data back up to a parent ponent? – Paul Redmond Commented Dec 29, 2017 at 12:24
• I'm not sure i fully get how can you use the key property here... I'd store the fieldStates in the FormData state and define the handleInputChange handler in the FormData that i'd pass to Input ponents and in this handler I'd update the mon fieldStates state. Sorry I can not give a more detailed example, gotta run now! If i could be of help, when Im back, I'd love to explain in more detail. – Gleb Kostyunin Commented Dec 29, 2017 at 12:38
• No worries, thanks. I'm newish to React so still figuring out some of these concepts. – Paul Redmond Commented Dec 29, 2017 at 12:39

2 Answers 2

You can lift the state up so a parent of the Input will handle the state and validations.
You can conditionally invoke the "validator" of the surname property or any other property only if it exists and do a convention with yourself that a validate method will get the name of: propertNameValidator .
So for example when you do the loop over the inputs, you could check if there is a validate method named surnameValidator and invoke it against the hidden prop that you will pass the Input, if it is not exists then just pass false.

Here is a small example with your code:

// Field data
const fieldData = [
  {
    type: "text",
    label: "First",
    name: "first",
    placeholder: "Enter first name",
    hidden: false
  },
  {
    type: "text",
    label: "Surname",
    name: "surname",
    placeholder: "Enter surname",
    hidden: true
  }
];

// Get form data
class FormData extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      items: props.data.map(el => ({...el, value: ''})) // add the value property
    };
  }

  onInputChange = (inputId, value) => {
    const { items } = this.state;
    const nextState = items.map((item) => {
      if (inputId !== item.name) return item;
      return {
        ...item,
        value,
      }
    });
    this.setState({ items: nextState });
  }

  surnameValidator = () => {
    const { items } = this.state;
    const nameElement = items.find(item => item.name === 'first');
    return nameElement && nameElement.value.length >= 5
  }

  render() {
    let els = this.state.items;

    return (
      <div>
        {
          els.map((el, i) => {
            const validator = this[`${el.name}Validator`];
            return (
              <Input
                key={i}
                {...el}
                inputId={el.name}
                hidden={validator ? !validator() : el.hidden}
                onInputChange={this.onInputChange}
              />
            );
          })
        }
      </div>
    )
  }
}

// Input builder
class Input extends React.Component {

  handleChange = ({ target }) => {
    const { inputId, onInputChange } = this.props;
    onInputChange(inputId, target.value);
  }

  render() {
    // Element attributes
    const { type, label, name, placeholder, hidden, value } = this.props;
    return (
      <div>
        {
          hidden ? '' : (
            <div>
              {label ? <label htmlFor={name}>{label}</label> : null}
              <input
                type={type}
                name={name}
                placeholder={placeholder || null}
                value={value}
                onChange={this.handleChange}
              />
            </div>
          )
        }
      </div>
    );
  }
}

// App
class App extends React.Component {
  render() {
    return (
      <div>
        <h1>Show/Hide test</h1>
        <p>
          What we want here is the surname to appear when firstname has a value
          (say, it has 5 characters) and hide surname when firstname doesn't.
        </p>
        <FormData data={fieldData} />
      </div>
    );
  }
}

ReactDOM.render(
  <form>
    <App />
  </form>,
  document.getElementById("app")
);

<script src="https://cdnjs.cloudflare./ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare./ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="app"></div>

If you already created the element and you just want to hide / show it, then you can conditionally add or remove a CSS class that hides it.

<input className={`${!isValid && 'hide'}`} />

If I'm not mistaken, what you want is to conditionally show or hide an input element. See if this helps.

render() {
      // Element attributes
      const {type, label, name, placeholder, hidden} = this.props.params;
      const isValid = this.state.valid === true ? <span>Valid! Should show Surname field.</span> : <span>Not valid. Should hide Surname field.</span>;

      if (!hidden) {
        return (
        <div>
            {label ? <label htmlFor={name}>{label}</label> : null}
            <input
                type={type}
                name={name}
                placeholder={placeholder || null}
                value={this.state.value}
                onChange={this.handleChange}
            />
            {this.state.valid && <input placeholder="add surname" />}
        </div>
        );
      } else {
        return null;
      }
 }