react.dev/cookbook/07-children-prop-type.md

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

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

/** @jsx React.DOM */

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

React.renderComponent(
  <GenericWrapper><span/><span/><span/></GenericWrapper>, 
  mountNode
);

To save an extra array allocation, it returns the component itself without the array wrapper when there's only one child.

/** @jsx React.DOM */

var GenericWrapper = React.createClass({
  componentDidMount: function() {
    // **warning**: yields 5 for length of the string 'hello', not 1 for the
    // length of the non-existant array wrapper!
    console.log(this.props.children.length); 
  },
  render: function() {
    return <div />;
  }
});

React.renderComponent(<GenericWrapper>hello</GenericWrapper>, mountNode);