React router and this.props.children – how to pass state to this.props.children

This question boils down to, how do you pass props to children?

June 2018 answer

Today’s tech:

• React 16+
• React Router 4: react-router-dom
• Render Props from official docs

---

Assuming some stateful component:

import React from 'react'
import { BrowserRouter, Route } from 'react-router-dom'

// some component you made
import Title from './Title'

class App extends React.Component {
  // this.state
  state = { title: 'foo' }

  // this.render
  render() {
    return (
      <BrowserRouter>

        // when the url is `/test` run this Route's render function:
        <Route path="/:foobar" render={

          // argument is props passed from `<Route /`>
          routeProps => 

            // render Title component
            <Title 
              // pass this.state values
              title={this.state.title}

              // pass routeProps values (url stuff)
              page={routeProps.match.params.foobar} // "test"
            />

        } />

      </BrowserRouter>
    )
  }
}

This works because this.props.children is a function:

// "smart" component aka "container"
class App extends React.Component {
  state = { foo: 'bar' }
  render() {
    return this.props.children(this.state.foo)
  }
}

// "dumb" component aka "presentational"
const Title = () => (
  <App>
    {title => <h1>{title}</h1>}
  </App>
)

Example on codesandbox

My previous oldschool answer that I wouldn’t recommend anymore:

Using a couple of React helper methods you can add state, props and whatever else to this.props.children

render: function() {
  var children = React.Children.map(this.props.children, function (child) {
    return React.cloneElement(child, {
      foo: this.state.foo
    })
  })

  return <div>{children}</div>
}

Then your child component can access this via props, this.props.foo.