React Express

Component API
Component API
Components can be configured upon instantiation by passing properties to the constructor - these properties are called props. props may be accessed from within the component's methods as this.props, in order to alter how the component is rendered and/or how it behaves. However, props must not be altered from within the component's methods.
A parent element may alter a child element's props at any time. The child element will generally re-render itself to reflect its new configuration parameters. A child component may decide not to re-render itself even though its configuration has changed, as determined by shouldComponentUpdate() (more on this in the Component Lifecycle API section).
You may set default values for props by assigning an object to the static class property defaultProps, where the keys are the prop names and the values are the default prop values.
You may also define the types of props that a component expects by assigning an object to the static class property propTypes, where the keys are the prop names and the values are prop types, from the prop-types npm module (formerly React.PropTypes). We won't be using PropTypes in our examples, but they're very useful once you start working with larger and more complex codebases.
Components may maintain their state internally within the object state. this.state may be accessed from within component methods. Unlike props, parent elements may not access a child's state, as it is intended to manage the child's internal state rather than external configuration.
Note that you should never directly assign to a specific key within the state object, e.g. = 'bar', but instead use the method this.setState().
this.setState(object newState)
Components may update their state by passing an object to the method this.setState(). Any keys in the object passed to the method will be merged into (and override any existing keys in) this.state.
The following example includes a Counter component that maintains the elapsed time internally as state.count. The App component renders the Counter component with two props: size and color. The App can easily render several Counter components with different sizes and colors.