React Side Effect

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

Installation

npm install --save react-side-effect

Use Cases

• Setting document.style.overflow 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.

How's That Different from componentDidUpdate?

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:

var BodyStyle = createSideEffect(function handleChange(propsList) {
  var style = {};
  propsList.forEach(function (props) {
    Object.assign(style, props.style);
  });
  
  for (var key in style) {
    document.style[key] = style[key];
  }
});

API

createSideEffect: (onChange: Array<Props> -> (), mixin: Object?) -> ReactComponent

Returns a component that, when mounting, unmounting or receiving new props, calls onChange with props of each mounted instance. It's up to you to reduce them, use innermost values, or whatever you fancy.

Component will have a static dispose() method to clear the stack of mounted instances.
When rendering on server, you must call it after each request.

You can use optional second mixin parameter to specify propTypes, displayName or statics. It will be mixed into the generated component.

Usage

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

'use strict';

var React = require('react'),
    createSideEffect = require('react-side-effect');

/**
 * Extract title from a list of each mounted component's props.
 * We're interested in the innermost title, but for other use cases we might want to call `propList.reduce`.
 */
function extractTitle(propsList) {
  var innermostProps = propsList[propsList.length - 1];
  if (innermostProps) {
    return innermostProps.title;
  }
}

var _serverTitle = null;

/**
 * Generate a component that reacts to mounting, onmounting and prop changes by updating document title.
 */
var DocumentTitle = createSideEffect(function handleChange(propsList) {
  var title = extractTitle(propsList);

  if (typeof document !== 'undefined') {
    document.title = title || '';
  } else {
    _serverTitle = title || null;
  }
}, {
  displayName: 'DocumentTitle',

  propTypes: {
    title: React.PropTypes.string.isRequired
  },

  statics: {
    /**
     * Peek at current title (for tests).
     */
    peek: function () {
      return _serverTitle;
    },

    /**
     * Call this on server after each request to get current title.
     */
    rewind: function () {
      var title = _serverTitle;
      this.dispose();
      return title;
    }
  }
});

module.exports = DocumentTitle;