getElementById in React

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

You may have to perform a diff and put document.getElementById('name') code inside a condition, in case your component is something like this:

// using the new hooks API
function Comp(props) {
  const { isLoading, data } = props;
  useEffect(() => {
    if (data) {
      var name = document.getElementById('name').value;
    }
  }, [data]) // this diff is necessary

  if (isLoading) return <div>isLoading</div>
  return (
    <div id='name'>Comp</div>
  );
}

If diff is not performed then, you will get null.