Create components whose prop changes map to a global side effect.

npm install --save react-side-effect

<script src="https://unpkg.com/react/umd/react.development.js" type="text/javascript"></script>
<script src="https://unpkg.com/react-side-effect/lib/index.umd.js" type="text/javascript"></script>

<script src="https://unpkg.com/react/umd/react.production.min.js" type="text/javascript"></script>
<script src="https://unpkg.com/react-side-effect/lib/index.umd.min.js" type="text/javascript"></script>

• Setting document.body.style.margin or background color depending on current screen;
• Firing Flux actions using declarative API depending on current screen;
• Some crazy stuff I haven't thought about.

It gathers current props across the whole tree before passing them to side effect. For example, this allows you to create <BodyStyle style> component like this:

// RootComponent.js
return (
  <BodyStyle style={{ backgroundColor: 'red' }}>
    {this.state.something ? <SomeComponent /> : <OtherComponent />}
  </BodyStyle>
);

// SomeComponent.js
return (
  <BodyStyle style={{ backgroundColor: this.state.color }}>
    <div>Choose color: <input valueLink={this.linkState('color')} /></div>
  </BodyStyle>
);

and let the effect handler merge style from different level of nesting with innermost winning:

import { Component, Children } from 'react';
import PropTypes from 'prop-types';
import withSideEffect from 'react-side-effect';

class BodyStyle extends Component {
  render() {
    return Children.only(this.props.children);
  }
}

BodyStyle.propTypes = {
  style: PropTypes.object.isRequired
};

function reducePropsToState(propsList) {
  var style = {};
  propsList.forEach(function (props) {
    Object.assign(style, props.style);
  });
  return style;
}

function handleStateChangeOnClient(style) {
  Object.assign(document.body.style, style);
}

export default withSideEffect(
  reducePropsToState,
  handleStateChangeOnClient
)(BodyStyle);

On the server, you’ll be able to call BodyStyle.peek() to get the current state, and BodyStyle.rewind() to reset for each next request. The handleStateChangeOnClient will only be called on the client.

A higher-order component that, when mounting, unmounting or receiving new props, calls reducePropsToState with props of each mounted instance. It is up to you to return some state aggregated from these props.

On the client, every time the returned component is (un)mounted or its props change, reducePropsToState will be called, and the recalculated state will be passed to handleStateChangeOnClient where you may use it to trigger a side effect.

On the server, handleStateChangeOnClient will not be called. You will still be able to call the static rewind() method on the returned component class to retrieve the current state after a renderToString() call. If you forget to call rewind() right after renderToString(), the internal instance stack will keep growing, resulting in a memory leak and incorrect information. You must call rewind() after every renderToString() call on the server.

For testing, you may use a static peek() method available on the returned component. It lets you get the current state without resetting the mounted instance stack. Don’t use it for anything other than testing.

Here's how to implement React Document Title (both client and server side) using React Side Effect:

import React, { Children, Component } from 'react';
import PropTypes from 'prop-types';
import withSideEffect from 'react-side-effect';

class DocumentTitle extends Component {
  render() {
    if (this.props.children) {
      return Children.only(this.props.children);
    } else {
      return null;
    }
  }
}

DocumentTitle.propTypes = {
  title: PropTypes.string.isRequired
};

function reducePropsToState(propsList) {
  var innermostProps = propsList[propsList.length - 1];
  if (innermostProps) {
    return innermostProps.title;
  }
}

function handleStateChangeOnClient(title) {
  document.title = title || '';
}

export default withSideEffect(
  reducePropsToState,
  handleStateChangeOnClient
)(DocumentTitle);