react-arrow.js

import React from 'react';  
let {Component, PropTypes} = React;

export default class CoolComponent extends Component {  
  handler = (e) => { 
    // Do somthing cool here
  }
  
  render() {
    return <div onClick={this.handler}></div>;
  }
}

react-constructor.js

import React from 'react';  
let {Component, PropTypes} = React;

export default class CoolComponent extends Component {  
  constructor(props) {
    super(props);
    this.handler = this.handler.bind(this);
  }
  
  handler(e) { 
    // Do somthing cool here
  }
  
  render() {
    return <div onClick={this.handler}></div>;
  }
}

react-prototype.js

import React from 'react';  
let {Component, PropTypes} = React;

export default class CoolComponent extends Component {  
  handler(e) { 
    // Do somthing cool here
  }
  
  render() {
    return <div onClick={this.handler.bind(this)}></div>;
  }
}