[React] Understand React.Children Utilities

The data contained in this.props.children is not always what you might expect. React provides React.children to allow for a more consistent development experience.

For example, you have an component:

class App extends React.Component {
  render(){
    return (
      <Parent>
        <div className="childA"></div>
        <div className="childB"></div>
      </Parent>
    )
  }
}

Inside parent component, you have two children.

So if you log out 'this.props.children', it should be an Array.

class Parent extends React.Component {
  render(){
    console.log(this.props.children) // Array
    return null
  }

But things happen when 'Parent' component has only one Child:

class App extends React.Component {
  render(){
    return (
      <Parent>
        <div className="childA"></div>
      </Parent>
    )
  }
}

When you log out 'this.props.children', it become an Object. So if you call '.map' on an Object, it will throw error.

What you can do for this is using 'React.Children.map':

let items = React.Children.map(this.props.children, (child) => console.log(child)); 

Even there is only one Child inside parent component, it will still convert it into an Array not a Object.

Other ways to do this such as:

let items = React.Children.forEach(this.props.children, (child) => console.log(child)); 

let items = React.Children.toArray(this.props.children)

If you only want Object which means just one Child, you can use:

let items = React.Children.only(this.props.children)

But if 'this.props.children' contains multi children, then it will throw an error.