Redux implementation, basically

index.js

const React = require("react");
const ReactDOM = require("react-dom");

const reducer = (state, action) => {
  switch (action) {
    case "INCREMENT":
      return state + 1;
    case "DECREMENT":
      return state - 1;
    default:
      return state;
  }
};

const Counter = counter =>
  <div>
    <button onClick={dispatch("DECREMENT")}>-</button>
    <h1>{counter}</h1>
    <button onClick={dispatch("INCREMENT")}>+</button>
  </div>;

const Main = props => Counter(props.state);

let actions = [];
const initialState = 0;
const dispatch = action => () => {
  actions.push(action);
  const state = actions.reduce(reducer, initialState);
  ReactDOM.render(<Main state={state} />, document.getElementById("root"));
};

dispatch(null)();