ex for entry 7

cookbook/07-children-prop-type.md

@@ -7,6 +7,41 @@ prev: style-prop-value-px.html
 next: controlled-input-null-value.html
 ---
 
-Usually, a component's `this.props.children` is an array of components. To save an extra array allocation, it returns the component itself when there's only one.
+Usually, a component's `this.props.children` is an array of components:
 
-This means accessing, for example, `this.props.children.length` might be misleading, as it could either be the `length` property of the array of children, or that of a single string component.
+```js
+/** @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.
+
+```js
+/** @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);
+```