react.dev/tips/07-children-props-type.md

id	title	layout	permalink	prev	next
children-props-type	Type of the Children props	tips	children-props-type.html	style-props-value-px.html	controlled-input-null-value.html

Usually, a component's children (this.props.children) is an array of components:

var GenericWrapper = React.createClass({
  componentDidMount: function() {
    console.log(Array.isArray(this.props.children)); // => true
  },

  render: function() {
    return <div />;
  }
});

ReactDOM.render(
  <GenericWrapper><span/><span/><span/></GenericWrapper>,
  mountNode
);

However, when there is only a single child, this.props.children will be the single child component itself without the array wrapper. This saves an array allocation.

var GenericWrapper = React.createClass({
  componentDidMount: function() {
    console.log(Array.isArray(this.props.children)); // => false

    // warning: yields 5 for length of the string 'hello', not 1 for the
    // length of the non-existent array wrapper!
    console.log(this.props.children.length);
  },

  render: function() {
    return <div />;
  }
});

ReactDOM.render(<GenericWrapper>hello</GenericWrapper>, mountNode);

To make this.props.children easy to deal with, we've provided the React.Children utilities.