You need to have your function in the componentDidMount
lifecycle since this is the function that is called when the DOM has loaded.
Make use of refs
to access the DOM element
<input type="submit" className="nameInput" id="name" value="cp-dev1" onClick={this.writeData} ref = "cpDev1"/>
componentDidMount: function(){
var name = React.findDOMNode(this.refs.cpDev1).value;
this.someOtherFunction(name);
}
See this answer for more info on How to access the dom element in React