React, ES6 - getInitialState was defined on a plain JavaScript class

• getInitialState is not used in ES6 classes. Instead assign this.state in the constructor.
• propTypes should be a static class variable or assigned to the class, it should not be assigned to component instances.
• Member methods are not "auto-bound" in ES6 classes. For methods used as callbacks, either use class property initializers or assign bound instances in the constructor.

export default class RadioOther extends React.Component {

  static propTypes = {
    name: React.PropTypes.string.isRequired,
  };

  constructor(props) {
    super(props);
    this.state = {
      otherChecked: false,
    };
  }

  // Class property initializer. `this` will be the instance when
  // the function is called.
  onRadChange = () => {
    ...
  };

  ...

}

See more in the React's documentation about ES6 Classes: Converting a Function to a Class

Adding to Ross's answer.

You could also use the new ES6 arrow function on the onChange property

It is functionally equivalent to defining this.onRadChange = this.onRadChange.bind(this); in the constructor but is more concise in my opinion.

In your case the radio button will look like the below.

<input type="radio"
       ref="otherRadBtn"
       onChange={(e)=> this.onRadChange(e)}
       name={this.props.name}
       value="other"/>

Update

This "more concise" method is less efficient than the options mentioned in @Ross Allen's answer because it generates a new function each time the render() method is called

If you are using babel-plugin-transform-class-properties or babel-preset-stage-2 (or stage-1, or stage-0), you can use this syntax:

class RadioOther extends React.Component {

  static propTypes = {
    name: React.PropTypes.string,
    ...
  };

  state = {
      otherChecked: false,
  };

  onRadChange = () => {
    ...
  };

  ...

}