connect.js

// connect() is a function that injects Redux-related props into your component.
// You can inject data and callbacks that change that data by dispatching actions.
function connect(mapStateToProps, mapDispatchToProps) {
  // It lets us inject component as the last step so people can use it as a decorator.
  // Generally you don't need to worry about it.
  return function (WrappedComponent) {
    // It returns a component
    return class extends React.Component {
      render() {
        return (
          // that renders your component
          <WrappedComponent
            {/* with its props  */}
            {...this.props}
            {/* and additional props calculated from Redux store */}
            {...mapStateToProps(store.getState(), this.props)}
            {...mapDispatchToProps(store.dispatch, this.props)}
          />
        )
      }

      componentDidMount() {
        // it remembers to subscribe to the store so it doesn't miss updates
        this.unsubscribe = store.subscribe(this.handleChange.bind(this))
      }

      componentWillUnmount() {
        // and unsubscribe later
        this.unsubscribe()
      }

      handleChange() {
        // and whenever the store state changes, it re-renders.
        this.forceUpdate()
      }
    }
  }
}

// This is not the real implementation but a mental model.
// It skips the question of where we get the "store" from (answer: <Provider> puts it in React context)
// and it skips any performance optimizations (real connect() makes sure we don't re-render in vain).

// The purpose of connect() is that you don't have to think about
// subscribing to the store or perf optimizations yourself, and
// instead you can specify how to get props based on Redux store state:

const ConnectedCounter = connect(
  // Given Redux state, return props
  state => ({
    value: state.counter,
  }),
  // Given Redux dispatch, return callback props
  dispatch => ({
    onIncrement() {
      dispatch({ type: 'INCREMENT' })
    }
  })
)(Counter)