Let's take a look at a basic component that declares an event handler for the click event of an element:

import React, { Component } from 'react';

export default class MyButton extends Component {
  // The click event handler, there's nothing much
  // happening here other than a log of the event.
  onClick() {
    console.log('clicked');
  }

  // Renders a "<button>" element with the "onClick"
  // event handler set to the "onClick()" method of
  // this component.
  render() {
    return (
      <button onClick={this.onClick}>{this.props.children}</button>
    );
  }
}

The event handler function, this.onClick(), is passed to the onClick property of the <button> element. By looking at this markup, it's clear what code is going to run when the button is clicked.