React.js How to access to input value in child component

Question

I wrote simple todo app, but now I have troubles with accessing to value of input in child component for App (InputForm).

Maybe I need to rebuild structure or logic somehow to make it works ?

Here's my App component:

class App extends React.Component {
  constructor (){
      super();
      this.state = {
          items : []
      }
  }

  addTodo ( e ){
    e.preventDefault();

    let itemHeading = this.refs.todoInput.value; // TODO Access to input value
    let itemKey = Date.now();

    const items = this.state.items.slice();

    items.push({
      heading: itemHeading,
      key: itemKey
    })

    this.setState({items: items});
  }
  render() {
    return (
      <div className="app-container">
        <InputForm onSubmit={this.addTodo.bind(this)}></InputForm>
        <TodoItems entries={this.state.items} />
      </div>
    );
  }
}

Here's my InputForm component:

class InputForm extends React.Component {
  render (){
    return (
      <form onSubmit={this.props.onSubmit}>
        <input
          ref="todoInput"
          type="text"
          placeholder="Type your text here" />
        <button type="submit">Add to list</button>
      </form>
    )
  }
}

Thanks for help.

Answer 1

Convert the input to a controlled component, and update the state whenever the text in the input changes. When submit is clicked, send the value to the handler.

Remove the refs, as they should be used for stuff that requires direct access to the DOM. This is what react docs has to say about refs:

In the typical React dataflow, props are the only way that parent components interact with their children. To modify a child, you re-render it with new props. However, there are a few cases where you need to imperatively modify a child outside of the typical dataflow. The child to be modified could be an instance of a React component, or it could be a DOM element. For both of these cases, React provides an escape hatch.

This is a case that doesn't requires this escape hatch, and you should just use props.

Working demo with comments:

const TodoItems = ({ entries }) => (
  <ul>
    {entries.map(({ heading, key }) => (
      <li key={key}>{heading}</li>
    ))}
  </ul>
);

class App extends React.Component {
  state = {
    items : []
  }

  // addTodo will receive the needed value without refs
  addTodo = (heading) => heading !== '' && this.setState(({ items }) => ({
    items: items.concat({ // concat returns a new array
      heading,
      key: Date.now()
    })
  }));

  render() {
    return (
      <div className="app-container">
        <InputForm onSubmit={this.addTodo}></InputForm>
        <TodoItems entries={this.state.items} />
      </div>
    );
  }
}

class InputForm extends React.Component {
  state = {
    input: ''
  };

  // input change handler
  onInput = (e) => this.setState({
    input: e.target.value
  });

  // submit handler
  onSubmit = (e) => {
    e.preventDefault();
    this.props.onSubmit(this.state.input);
  }

  render (){
    return (
      <form onSubmit={this.onSubmit}>
        <input
          // use value and onChange so it will be a controlled component
          value={ this.state.value }
          onChange={ this.onInput }
          type="text"
          placeholder="Type your text here" />
        <button type="submit">Add to list</button>
      </form>
    )
  }
}

ReactDOM.render(
  <App />,
  demo
);

<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<div id="demo"></div>

Answer 2

The refs object you're trying to access is on the InputForm instance, so if you want to access that instance from App you'll have to add a ref on the InputForm as well. Then you can access it with this.refs.InputForm.refs.todoInput.

Having said that, it's best to do ref callbacks rather than ref string, as the ref string is planned to be removed from React. Also it is best advised not to over-use refs. Maybe you can hold the value in the parent's state via onChange event or so.

https://facebook.github.io/react/docs/refs-and-the-dom.html#the-ref-callback-attribute