Prompt.js

import React from "react"
import PropTypes from "prop-types"
import { history as historyType } from "./PropTypes"

class Prompt extends React.Component {
  static propTypes = {
    when: PropTypes.bool,
    message: PropTypes.oneOfType([PropTypes.func, PropTypes.string]).isRequired
  }

  static contextTypes = {
    history: historyType.isRequired
  }

  static defaultProps = {
    when: true
  }

  enable(message) {
    if (this.unblock) this.unblock()

    this.unblock = this.context.history.block(message)
  }

  disable() {
    if (this.unblock) {
      this.unblock()
      this.unblock = null
    }
  }

  componentWillMount() {
    if (this.props.when) this.enable(this.props.message)
  }

  componentWillReceiveProps(nextProps) {
    if (nextProps.when) {
      if (!this.props.when || this.props.message !== nextProps.message)
        this.enable(nextProps.message)
    } else {
      this.disable()
    }
  }

  componentWillUnmount() {
    this.disable()
  }

  render() {
    return null
  }
}

export default Prompt