React unique ID via context. Every render of the Provider resets the counter.

reactUniqueId.js

import React, {Component, PropTypes, Children, createElement} from 'react'

const CONTEXT_NAME = '__uniqueIDFunction__'
const CONTEXT_TYPES = {}
CONTEXT_TYPES[CONTEXT_NAME] = PropTypes.func

function createUniqueIDFunction() {
  let counter = 0
  return function() {
    return ++counter
  }
}

class UniqueIDFunctionProvider extends Component {

  static childContextTypes = CONTEXT_TYPES;

  getChildContext() {
    const context = {}
    context[CONTEXT_NAME] = createUniqueIDFunction()
    return context
  }

  render() {
    return <div>{this.props.children}</div>
  }
}
export const Provider = UniqueIDFunctionProvider;

class UniqueIDFunctionReceiver extends Component {

  static contextTypes = CONTEXT_TYPES;

  getChildProps() {
    const idProps = this.constructor.uniqueIDToProps(this.context[CONTEXT_NAME])
    return {...idProps, ...this.props}
  }

  render() {
    return createElement(this.constructor.componentClass, this.getChildProps())
  }
}

export function connect(uniqueIDToProps) {
  return function(componentClass) {
    return class extends UniqueIDFunctionReceiver {
      static componentClass = componentClass;
      static uniqueIDToProps = uniqueIDToProps;
    }
  }
}

// EXAMPLE

// const Item = ({id}) => <div id={id}>My ID is <strong>{id}</strong></div>
// const ConnectedItem = connect(
//   (getUniqueId) => ({id: `item-${getUniqueId()}`})
// )(Item)
// 
// export class Example extends Component {
//   render() {
//     return (
//       <Provider>
//         <ConnectedItem />
//         <ConnectedItem />
//       </Provider>
//     )
//   }
// }