react js , get siblings or parent value

React enforces parent-to-child unidirectional data flow. So, there's no simple way to access data from siblings.

But, if a child changes the state across the component, you probably want a state for keeping a track of that.

Sample code:

var FormBox = React.createClass({
  getInitialState: function () {
    return {
      textBox: 'hey, get me!'
    }
  },
  pressButton: function () {
    alert(this.state.textBox)
  },
  textChange: function (e) {
    this.setState({
      textBox: e.target.value
    })
  },
  render: function () {
    return (
      <form action='.'>
        <input type='text' placeholder={this.state.textBox} onChange={this.textChange}/>
        <button onClick={this.pressButton}> Get it </button>
      </form>
    )
  }
})

ReactDOM.render(<FormBox />, document.getElementById('root'))

JSBin demo: https://jsbin.com/koxujuwope/edit?js,output

Also, just a suggestion... You should move to ES2015

One way to accomplish this is using refs.

After building your component, you may find yourself wanting to "reach out" and invoke methods on component instances returned from render(). link to refs docs

Refs are basically the html elements / react instances you want to access. In this case, we want to get Input html element. We get input element by this.inputValue. Then read its value by usual js rule.

 var form = React.createClass({
       submit : function(e){
          e.preventDefault();
          console.log(this.inputValue.value);
       },
       render : function(){
       return (
          <form onSubmit={this.submit}>
             <input type="text" id="textbox" defaultValue="hey, get me"
               ref={ function(node){ this.inputValue = node }.bind(this) }
             />
             <button onClick={this.submit}>Get it</button>
          </form>
          );
       }
    });

here is a jsfiddle for your example, you can check the console for the input value.

Another way to accomplish the same thing is to make your input a controlled element. That means you assign input's value attribute to this.state.inputValue and you pass a onChange event handler as onChange={yourInputHandlerFunction}.

See the answer which explains this way of accomplishing the same thing.